Wiring headlight relays

InhoudsopgaveIntroduction...........................................................................................................................................2

Relays.....................................................................................................................................................2

What's a "relay"?................................................................................................................................2

Why is this useful?..............................................................................................................................2

Relay Ratings and Configurations.......................................................................................................3

Typical Automotive Relay...................................................................................................................4

The wiring...............................................................................................................................................5

My modification.................................................................................................................................5

Why you really want to do this...........................................................................................................5

Safety tips and application data.........................................................................................................5

Technical Summary............................................................................................................................6

The gory details..................................................................................................................................6

Where to run the wires......................................................................................................................8

Wiring diagram...................................................................................................................................8

Parts list..............................................................................................................................................9

Selecting wire sizes.............................................................................................................................9

Wire capacity chart..............................................................................................................................10

Measuring Wire Capacity.................................................................................................................10

Stranded vs. Solid Wire....................................................................................................................11

Open Air vs. Bundles and/or Conduits..............................................................................................11

Wire Length......................................................................................................................................11

Duration of Usage.............................................................................................................................12

Electrical Calculations.......................................................................................................................12

Capacity Chart..................................................................................................................................13

The diagrams........................................................................................................................................14

Typical Automotive Relay.................................................................................................................14

Wiring diagram, this story................................................................................................................15

Wiring diagram, other story.............................................................................................................16

No relay........................................................................................................................................16

With relay.....................................................................................................................................17

IntroductionThis page is all about wiring relays to drive your headlights so they are brighter and/or you can use higher output bulbs (if desired) without the risk of overloading your existing headlight wiring. If you have no idea why you might even want to do that, let alone how to do it, read on. If you don't even know what a relay is, read the next part carefully.

Relays

What's a "relay"?It's a electrical device that functions something like a wired remote control switch. Instead of having the switch you push/flip/whatever do the work of supplying power to whatever you wanted it to, you have it control a relay which then does the real on/off switching work. That's it. It's really not very complicated, now is it?

A mechanical relay does this through the use of an electromagnet - a magnet that is only "on" when there's power running through it - that pulls a set of spring loaded contacts to make or break the connection and achieve the on-off effect. This is called the "coil" or trigger wire - the other wire coming out of the coil is connected to ground. Whenever you apply power to the other coil wire (the trigger), the relay is on. As soon as power to this trigger is turned off, the relay turns off. Simple, huh? There are also "solid state" relays that achieve the same effect through transistors. Either one functions the same way, the solid state stuff just has no moving parts to wear out, but they tend to be more expensive and not as readily available since the regular mechanical ones are inexpensively and readily available as very high quality, durable units.

Why is this useful?For one big reason - some devices use a lot of power and that means large wires and heavy duty contacts inside all of the switches and connectors are needed. And you want to use as little wire (in length/distance) as possible. It's more expensive and heavier that smaller low-power wires and it's harder to work with. If the wire develops a short, it's a much bigger problem - and the longer the wire involved, the more chances you have for something to go wrong.

Additionally, heavy-duty switches are large, cumbersome, and generally have a very poor "feel" to them. By "feel", I mean the tactile sensation you get from using the switch - is it a smooth silky operation with a nice delicate "click" to tell you what's happening, or is it more like Igor straining to flip a massive and cumbersome switch to turn on the power to bring Dr. Frankenstein's creature to life? You get the idea. It's easier and cheaper to make a low power switch in the quality you would expect in a fine automobile. And it will last longer. That's a good thing!

A relay alleviates this by using a single relatively small and low power wire to control the on-off of electrical flow. You mount the relay near the device it controls, and run a simple large power wire to

the relay. Then you run a small wire back to the switch. The switch you flip just supplies power to the relay coil and functions as a trigger - if the coil has power, the magnet energizes and the relay contacts move to make (or break - it can work both ways) the high power connection to your device.

Relay Ratings and ConfigurationsRelays are typically discussed in terms of several things.

* How much power the high power side can handle in Amps * The voltage and power type (AC or DC) the coil needs to operate * The number and type of contacts the relay has

The first thing, the power rating, is very simple - a relay is rated for it's capacity to handle power. That's what it's for, and that's what you hear most often. It will be described as a 20A, 30A, or whatever relay. This must be as big as or bigger than the maximum rating of the thing you want to control with the relay. It will be rated at some voltage as well, so be sure it all matches.

The second things, the coil voltage and type, is typically omitted when working in a known environment. All automotive relays use a 12V DC coil, so this information is implied if the relay is intended for use in a car. For use in other environments (home, industrial, etc.) there will be a rating on what the relay coil expects. Just match this to what the trigger wire will have in it. Note that the coil does not have to work on the same voltage as the voltage being sent over the high power contacts. There is no need to send high voltage to the small switch - the whole point is to use small wires and switches to control the relay, after all. The common case here is a doorbell in your house. The actual pushbutton outside is typically being fed 24V AC and it hooks up to a relay inside the doorbell chime unit to make the chime happen. The chime will often work on 120V AC (normal household electricity), so the relay controls this.

The third thing, the number and type of contacts, is used to control various things at once and control them by turning them on or by turning them off. This is described in the same way any other switch is described - by the number of poles and the number of throws. Most automotive are very simple and of the SPST or SPDT variety - read below to learn what that means.

The number of contacts (or poles), is the number of things that the relay can control at once. The relay is just an electromagnetically controlled switch, and you can have the same electromagnet flip a number of switches in unison the same way you could mount a bar across a number of different switch levers to force them to be switched in unison. A good example of this are the circuit breakers in your house. Some of them will be two breakers tied together with a bar so they switch together to make a a two pole switch out of two single pole switches. With a relay, this is very useful for making one switch control two different things - like turning on the parking lights and the dash lights in your car with one switch.

The number of throws is the number of distinct contacts you can send power to - the number of places you can "throw" the switch to. (Think of an old fashioned blade switch - like Igor used - to get the mental image here.) Typically, you have one input and one or more outputs. If you can connect the output to one thing, as in a simple on-off switch, you have a single throw. If you have two outputs, like in a power window switch where you have up and down, you have two throws. If it was

more like a dial switch where you could select from three or more things, like the fan speed control switch, then you would have three or more throws.

This is number of poles and number of throws is designated with a simple abbreviation like "SPST". This stands for Single Pole, Single Throw. Another popular one is "DPDT", which is for Double Pole, Double Throw. Beyond that they are usually designated with numbers, such as "3PDT" (Three Pole, Double Throw) or "SP4T" (Single Pole, Four Throw". By looking at this information, you can tell how the relay can switch things, and find out if it is right for your needs. As noted above, most automotive are very simple and of the SPST or SPDT variety - they can control one thing and switch it on or off, or apply power to one of two different things.

Lastly, the contacts in the switch or relay are described as "normally open" (NO) or "normally closed" (NC). This simply describes what the "at rest" state is. For a relay, that means if no power is applied to the coil/trigger wire. In the typical case where you want to turn something on, you use the "normally open" set of contacts so that when you apply power to the relay, the contacts close, and power is sent to the desired device. This is used for things like turning on your fog lights or things like that. In the case of wanting to turn something off, you use the "normally closed" set of contacts so that when you apply power to the relay, the contacts open and the power is no longer sent to the desired device. This is used for things like an emergency stop switch or other more unusual "control" cases. One example is in certain multiple relay electric fuel pump setups on fuel injected vehicles to control when the pump is on and to ensure it turns off in case of the engine stalling - this is used to reduce the risk of fire (due to the pump still pumping fuel) in case of an accident and a ruptured fuel line.

Typical Automotive RelayThis diagram shows a typical Bosch relay used in the automotive world. It is the closest thing to a universal relay standard in the automotive world, so you will see this type of diagram and/or contact numbering system often if you work on cars enough. This is especially true in the hot-rod or "aftermarket" arena where these style relays are often used to achieve custom or "trick" effects on a vehicle such as anything that opens or closes with a motor, or the "no door handles" look where the door is unlatched electrically instead of mechanically.

at the end of the document there is a bigger version

This relay is a SPDT relay with a single NO contact (terminal 87) and a single NC contact (terminal 87a). These relays usually have a small wiring diagram molded into the top of the relay and all of the contacts are clearly labeled on the relay so you can trace the wiring with ease. They also use a

standard size and configuration for their plug-in terminals so that you can get a standard molded plastic base with the proper wiring hookups in it. This enabled you to unplug all of the connections all at once for ease of servicing - such as if you need to test or replace the relay itself.

The wiring

My modificationI made this modification years ago on one of my cars after installing higher output headlights - I needed to solve the "flickering headlights" problem that resulted when I used my high beams. The new lights drew so much more power that the existing circuit breaker in the headlight switch was being overloaded. This was both annoying and downright unsafe - so I fixed it with this change. I later found out about the benefits for just getting brighter headlights courtesy of an article written by the Southern California GS chapter of one of the various Buick clubs I know of and get information from. (I can't find the name of the club, but I do have a photocopy of the article itself. If someone cares to remind me who I ought to be giving credit to for this, please remind me so I can update this page.) The voltage drop information and encouragement for me to create an easy to read wiring diagram comes from their article. There was lots of good info in there, but the hand-drawn wiring diagram was less than readable - even for someone like me who actually understands how this all works. :-) So, I decided to put this page up to host a better wiring diagram and explain it in my own way along with details for the high output headlights. Since I'd done this myself years before I'd ever read their article, and I'm giving them credit for some of the extra details, I don't feel like I'm ripping off their idea. :-)

Why you really want to do thisWhy would you care about doing this? For one (or both) of two basic reasons. The first is that you have installed high output headlights (off-road units, etc.) and you're having problems with your headlights "flickering" on and off again while you drive. The second reason is to simply improve the brightness of your existing headlights. This is because the factory wiring for the headlights has lots of long "just big enough" wires, and after many years of service, this leads to extra resistance in the wiring and at each connection. That resistance sucks up electrical energy that could be used to produce light at the headlights, so your lights are dimmer than they could be. To put this into perspective, a 10% drop in voltage between the battery and the headlight is not uncommon - and that can cause up to a 30% drop in light output! That's the difference between being able to see to stop in time and having an accident - so this is a very useful safety and drivability modification. The total cost is less than $50 and can be done in an afternoon by anyone who is even vaguely familiar with how to do simple wiring work. $50 to get up to 30% more light from your headlights is very much worth it. So read on and learn how to do this.

Safety tips and application dataThe standard set of safety disclaimers apply - this is for your information only and none of this should be attempted unless you are sure you know what you're doing. This is not guaranteed to be 100% correct and you should use common sense when attempting any repairs or modifications to your vehicle. It is not my fault if you fry yourself, anyone else, or your car. I did not tell you that you should do this - only that you could do this. It's up to you to determine if and how this information applies to your car.

On the subject of application information, this entire page is focused on vehicles that use a traditional "positive switched" headlight system like most older American cars. This is where power goes from the + battery terminal to the switch, then to the headlights, then to ground and back to the battery. Some import cars, particularly Toyota's from the early to mid 1980's use a really weird "negative switched" system that runs power direct to the headlights and puts the switch after the headlights in the wiring diagram. You can do the same relay trick in those systems, but several key wires are inverted, and you need to be really careful about what you do because most people have trouble thinking about the system working "backwards".

As a side note, these "reversed" systems are prone to strange behavior when a headlight burns out - things like having all of the headlights burn out at the same time are not uncommon with these systems. That said, you may want to think about doing the extra work to use the relay along with some extra wiring to invert the system so it works "correctly". It's more work, but it can be done. I'd do it if it were my car, but I do things that most folks never notice or care about, so take that recommendation with a (not so) small grain of salt.

Lastly, the physics purists who want to pester me about actual electron flow from negative to positive can save it. I know about this, but it's confusing to most people and not relevant to the discussion here. This entire page is written from the perspective of the traditional positive-to-negative power flow in an electrical circuit. If you know what that means, now you know. If this is gibberish to you, don't worry about it - it was just the elitist purists trying to confuse you. :-)

Technical SummaryThis one is pretty easy to conceptualize if you understand how a relay. You splice two 20A relays into the existing wiring harness right out near the headlights so one relay controls the low beams and one controls the high beams. Use the existing high and low beam wires coming from the firewall to trigger the relay, run a new high power feed (with a fuse!) direct from the battery, and hookup the existing high and low beam wires from the headlights to the "normally open" contact on the relays. The hardest part of all this is typically finding the right wires in the existing wiring harness and finding a place to mount the relays - the actual wiring is pretty easy. Any SPST/normally open relay will do, though most automotive relays are of the SPDT variety - just don't hook anything up to the "normally closed" contact on the relay (pretend it's not there) and you'll be fine.

Note that if you pick some really monster sized off-road headlights that draw more power than the ones that you can plausibly use "on the street", you must use higher amperage relays than what is mentioned here. You must also use the appropriate sized power wires and you may very well end up replacing all of the headlight wiring from the relays out to the headlights themselves - don't forget to upgrade to a larger headlight ground if you do this! See my Wire Capacity Chart for more details.

The gory detailsNow, for the rest of humanity that has no clue what I just said, here's a step-by-step list of what you need to do. You should read the entire list and understand it before you start this project. If you are knowledgeable in such things, you should be sure to solder all of your connections in addition to crimping them. This helps ensure that you will have a more secure and lower resistance connection that will not degrade over time.

1. Find the existing wires that go to your headlights in the wiring harness out near the headlights. There will typically be two wires - one for the low beams and one for the high beams. The headlights are usually wired together as part of the harness in a daisy chain fashion - if not, there could be four wires in the harness - one for the left low beam, one for the right low beam and one for the left high beam, one for the right high beam. If in doubt consult a factory wiring diagram for your car or break out the old multi-meter and do some testing and tracing of the wires in your harness to figure out what goes where.

2. Figure out where you will mount your relays - make sure it's reasonably safe/dry, out of the way, close to the existing wires you just found, has space for the relays, and is reasonably close to the headlights. Make sure it does not interfere with anything like closing the hood or getting to anything else you may need to service on the engine in the future.

3. Disconnect the negative battery terminal so you don't fry yourself or the car. :-)4. Make a careful note of which wires are for what and then cut the existing wires for the

headlights where you want to splice in the relays. If you pick a point in the harness after the wires split for the left/right side headlights, you'll have two wires to deal with. You can either tape off one of the low beam wires or connect them both together where you make the relay coil (trigger) connection. I prefer to connect them both together.

5. Hook up the existing low beam headlight wires coming from the firewall to the relay coil (trigger) as shown in the wiring diagram.

6. Repeat the same process for the existing high beam wires coming from the firewall.7. Hook up the existing low beam headlight wires that go from your "cut point" out towards the

headlights to the relay "normally open" output connection as shown in the wiring diagram. If you picked a point for the relays after the wires split for the left/right headlights and had two wires to deal with in the above steps, then you must connect both of the low beam wires together where you make the relay "normally open" output connection. If you do not do this - only one headlight will work.

8. Repeat the same process for the existing high beam wires that go from your "cut point" out towards the headlights.

9. Hook up the ground wire from the relay to a new ground or to the existing ground wire for the headlights. This wire only needs to be 16 gauge (it carries very little power) and it should be black.

10. Run a new red 12 gauge feed wire over to your battery or to any other place (such as the starter solenoid connections) that gets full battery voltage and can be easily connected to. Make sure you put a minimum 30A fuse or circuit breaker into this wire as close to the connection with the battery (or other wiring) as possible. Failure to install a fuse or circuit breaker in this wire will create a safety and fire hazard in your new wiring! This is the wire that will carry all of the power to run your headlights - make your connections securely and cleanly so that your headlights are as bright as possible.

11. Mount the relays and make any final connections. Make sure all connections are taped and/or covered in "heat shrink" tubing so they are watertight - stuff under the hood gets wet and electricity and water don't play well together.

12. Go back and double check all of your connections against the diagram. If this is your first time doing wiring work, go back and triple check it to be sure. Seriously.

13. Make sure the headlights are turned off!

14. Reconnect the negative battery terminal. Nothing should be out of the ordinary here. If stuff starts smoking or melting, disconnect the battery right away and figure out what you messed up and fix it before reconnecting it again.

15. Turn on the headlights and make sure they are on low beam and make sure 1) nothing is smoking/melting/burning, and 2) just the low beams are on. If any problems occur, turn off the headlights and unhook the battery immediately - then find and fix the problem.

16. Switch the headlights to high beam and make the same two checks as on the low beams.17. Clean up - you're done. Enjoy your brighter headlights.

Where to run the wiresNext, you need to choose a place to draw the power for the headlamps. The two most common choices are the alternator output (B+, BAT) terminal, or the battery positive post. Some cars with remote-mounted batteries or underhood fuse panels have underhood power points, and these can be a good selection as well. So, which is the best power point?

On cars with full-current ammeters (mostly pre-1976 Chrysler products) it is best to take power from the alternator output terminal, rather than at the battery Positive (+) terminal. This so that when everything is in its 'normal' state (ie, engine running, battery charged) then the power for the headlamps doesn't go thru the car's existing wiring at all. This is the wise way to do it on cars with full-current ammeters, because such gauges must carry *all* current for the entire car. Keeping heavy current loads out of this area reduces stress on the entire wiring system, and eliminates much voltage drop on the charging side of the wiring.

The vast majority of cars, however, do not have full-current ammeters, which makes it OK to take your choice, based on access and convenience, of the alternator or battery positive terminal (or power point terminals, on cars so equipped). These points are all electrically common, and any of them will serve equally well.

You may have heard that it's not good to take headlamp power from the alternator output because of "voltage spikes"; this is a myth. No voltage spikes are present in an electrical system with good voltage regulation, and any spikes that are present in a system with bad voltage regulation are present in equal magnitude across the entire system. If your charging system is "spiky", indicated by vehicle lamps that flash brighter and dimmer with the engine running at a steady speed, then you need to fix the problem that is causing the spikes!

Another consideration when tapping at the battery is the potential for corrosion. Keep those terminals clean-clean-clean, and once you've added the power wire to the positive battery cable, usually via a ring terminal, be sure to overspray the terminals with plastic-based spray made for the purpose.

Wiring diagramThe wiring diagram below shows what you need to end up wiring to make this work, so if you know how to read a wiring diagram and feel like "skipping ahead", just go click on the thumbnail for the wiring diagram and check it out in full size, full color glory. It's shown for a four headlight system - if you have a two headlight system on your car, pretend the two inner "high beam only" headlights aren't there and you'll be fine. The wire colors shown here represent a typical GM vehicle (the green and tan wires, along with some of the black wires) as well as the proper/correct/desired wire colors

to use on any new wiring you do (the red and some of the black wires). Also, this is shown as a typical "Bosch style" automotive relay with the connections numbered as such. If your relay is not numbered like this, then just identify the wires by function and go from there.

there is a bigger version at the end of this document

Parts listSee the bottom section of my All About Relays page to get some ideas for where to buy parts from. Whatever you do, just be sure to get relays that have a connector and wires with them along with a way to mount them. For reasons I am unable to fathom, some people produce and sell relay kits that have no way to mount the relay. (Go figure...) The circuit breaker can be obtained from your local parts store - they sell units that mount to any flat piece of metal and have simple screw-on terminals for the wires. (I think the one I used was an Echlin CB6339 based on what I looked up on NAPA's website...) The wire, tape/heat shrink, crimp on connectors, and screws to mount things can all be obtained locally at any decent auto parts store. If you know how to solder and opt to solder your connections, then you should already know where to get solder and a soldering iron, aka, your local Radio Shack. :-)

* Two 20A or greater mountable automotive relays with connectors * One 30A circuit breaker * 10' of red 12 gauge wire * Electrical tape and/or "heat shrink" tubing * Crimp on connectors as required * Screws/bolts to mount the relays and circuit breaker - some sheet metal screws should do the trick * Solder and soldering iron if you decide to solder your connections.

Selecting wire sizesUse only stranded wire, never solid (household type) wire, in automotive applications.

Wire gauge selection is crucial to the success of a circuit upgrade. Wire that is too small will create the voltage drop we are trying to avoid. On the other hand, wire that is of too large a gauge can cause mechanical difficulties due to its stiffness, particularly in pop-up ("hidden") headlamp systems.

The headlamp power circuit ought to use no less than 14-gauge (2.5 mm2) wire, with 12-gauge (4.0 mm2) being preferable. 10-gauge (5.2 mm2) can be used if bulbs of extremely high wattage are to be used, but it's usually overkill. Be sure to pick a kind that flexes easily if yours is a hidden-headlamp system.

Do not fail to use the large wire size on both sides of the headlamp circuit! Voltage drop occurs due to inadequate grounding, too! you will only sabotage your efforts if you run nice, big wires to the feed side of each headlamp, and leave the weepy little factory ground wires in place. Most factory headlamp circuits run the too-thin ground wires to the car body. This is an acceptable ground--barely--on a new car. As a car ages, corrosion and dirt build up and dramatically increase resistance between the car body and the ground side of the vehicle's electrical system. It takes little extra effort to run the new, large ground wires directly to the battery Negative (-) terminal or to the metal housing of the alternator, and this assures proper ground.

Wire capacity chart

Measuring Wire CapacityThe amount of power a wire can safely carry is related to how hot it can safely get. All wires have resistance, and as power flows through a wire that resistance causes heat - and it can be quite a bit of heat. The more power you put through a wire, the hotter it gets. Insulation breaks down as it gets hot, and at some point it will melt away leaving the wire exposed to whatever is around it - other wires, grounded metal, people, etc. The heat can even be enough to start a fire in the surrounding material in some cases. Electrical fires are nasty and tend to start in the hardest to reach places - where the most heat builds up back in dark corners and tight spaces. This is why using the right size wires is important for your safety and for safety of others using your wiring work.

In some respects, the capacity of a wire is actually best measured in watts, not amperage. Why? Because a watt is a unit or power that is a combination of amperage (volume), voltage (pressure), and resistance to the power flowing through that wire. Watts measure the amount of power (aka, heat) a wire can safely dissipate. However, most wire charts are done in amps. This is unfortunate because it means the wire chart is sort of assumed to be at a single voltage level. For most usage, this is fine because the chart has an assumed usage. As an example, charts for amperage ratings of of various sizes wires for 110V AC house current charts are popular and reasonably well-known. On the other hand, the amperage ratings are very different for common/typical 12V DC automotive usage. For example, a 12 gauge wire is commonly rated at 20A for 110V AC home usage, but in automotive 12V DC use 12 gauge wire is commonly used for circuits carrying 60A! A prime example would be the main charging wire from the alternator to the battery and out to the main electrical circuits of the car. I thought I had a satisfactory explanation posted here previously, but a few folks took aim at it and blew gaping holes in my understanding - without actually explaining what I was trying to understand or explain here. As of yet, I have not gotten a satisfactory explanation for this discrepancy. No one I've talked to as of yet has been able to explain it to me, but if you think you

know the magic answer. Maybe I'm missing something obvious. Maybe I'm just not understanding this as well I as think I am. Who knows... At any rate, the chart below reflects the difference in 110V AC vs. 12V DC usage, even though I'm still at a loss to explain the details.

Remember, if in doubt, it's always better to put in too big of a wire than too small of a wire.

Stranded vs. Solid WireThis one is a bit of a mind-boggler, but it's important. When electricity flows through a wire, it mostly flows on the surface of the wire, not through the middle. This effect is more pronounced on high frequency AC than it is on DC or low frequency AC. This means that a "wire" of a given size that made up of many smaller strands can carry more power than a solid wire - simply because the stranded wire has more surface area. This is one reason why battery cables in your car and welding cables are made up of many very fine strands of smaller wire - it allows them to safely carry more power with less of that power being dissipated as heat. However, this "skin" effect is not as pronounced in a typical 12V DC automotive application, and the wire and cable used there is stranded for flexibility reasons.

When looking at a chart or description of wire capacity, take note of whether it is referring to stranded or solid wire - some charts may not specify but instead assume a default based on the typical wiring used in a given application. For example, almost all automotive wiring is stranded while almost all home wiring is solid. For most applications, flexibility or the lack thereof will be more important, but for very high frequency AC applications, stranded wire might be a requirement.

Open Air vs. Bundles and/or ConduitsHeat is the primary determiner of the maximum amount of power any wire can carry, and the ability of that wire to dissipate that heat has a large impact on the final rating. Wires that are run in bundles (such as in a wiring harness or wiring conduit) cannot dissipate heat as easily as a single wire run in "open air", and as such must be "de-rated" to less than their maximum value to account for this. Also, wires that are run in areas that are unusually hot (such as in an attic or in an engine compartment) may need similar de-ratings. If both situations are encountered together (bundled wires in an unusually hot environment) then you need to de-rate for both factors and the capacity is further reduced.

In a car, almost all wiring is run in a bundle, and much of it runs near the engine. In a house, a lot of wiring typically runs through the attic, often in a bundle/group and sometimes in a conduit. Pay attention to this and size your wires appropriately.

Wire LengthSince all wires have resistance, the longer the wire, the greater the resistance. This means that for longer wiring runs you need to use a larger wire to compensate. This phenomenon is often referred to as "voltage drop", and for lower voltage automotive systems, the loss of 2V or even 1V can be significant. On longer wire runs, plan on using a larger size wire. There are specific voltage drop calculations that depend on the wire size in use, the length of the wire, the load applied, and the voltage in use. The National Electric Code has tons of charts for this. You'd be surprised at some of the voltage drops you can find just form the wiring in use, so experiment with the calculator a bit to see if it's worth going to the next highest size wire in your application. On automotive applications of

only 12V, losing a single volt of power in the wire is a whopping 8% loss, so it can be a big deal for voltage critical applications like your headlights where more voltage = more light.

Duration of UsageSome electrical loads are continuous for long periods of times (like a light in your house or the headlights on your car) and some are much more intermittent (like a garbage disposal in your house or the starter in your car). This affects the wire size used - the longer a wire is in use, the more heat it will tend to retain. A wire for something that is only used for short periods (like the starter in your car) does not need quite as large of a wire as something that will be in use for very long periods of time. This means that for long-duration uses, you must de-rate the wire even further and use a larger size.

Electrical CalculationsThere are four basic units of measurement for electricity:

Power, measured in Watts, commonly referred to as "P" Current, measured in Amps, commonly referred to as "I" Voltage, measured in Volts, commonly referred to as "V" Resistance, measured in Ohms, commonly referred to as "R"

There are a number of formulas that relate each of these four things - they all change in relationship to one another such that if you know any two you can calculate the other two. Lots of folks on the Internet have easy-to use calculators that allow you to do this online - http://www.sengpielaudio.com/calculator-ohm.htm is one. The formula wheel below was on their website and presents the info in a pretty easy to understand format.

Capacity ChartThis chart is a simple "max capacity" chart for a short wire run. Increase the wire size for long runs - for example the wires running to the back of a vehicle to power the taillights may need to be one size larger to account for the length.

Gauge 12V

22 5A

20 8A

18 10A

16 20A

14 40A

12 60A

10 100A

8 150A

6 ??A

4 ??A

2 ??A

1 ??A

0 ??A

Wire capacity chart

The diagrams

Typical Automotive Relay

Wiring diagram, this story

Wiring diagram, other story

No relay

With relay

BronnenVoor dit stuk heb ik de volgende pagina’s gebruikt:

http://www.rowand.net/ http://www.danielsternlighting.com/tech/relays/relays.html

Alle tekst is ook van deze pagina’s gecombineerd.