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Gadget Lab Lecture 4: Powering your circuit…
Components…Then it’s Project Time
Dr. Cindy Harnett
ECE Dept., U of Louisville
Spring 2008
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Powering your circuitTrend toward low power, low-voltage circuits… A few examples (with many
exceptions!)15V: stereo and other amplifiers 1970s-onward
12V-3V: 555 timer you used last week (some types)
5V: Transistor-transistor logic, USB
3.3V: wireless sensor networks
1.8V: “low voltage” computer memory (late 90s)
1.2 V: “ultra low voltage” memory.
But people want an easy-to-find battery, usually one or more 1.5 V batteries.
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Previously: Small DC MotorWhat can we do to drive the motor at 1.5V from a 6V battery? The voltage divider didn’t work because the motor’s resistance is too small. It shorts out R2. Also: voltage dividers are very “dissipative” --they draw a continuous current, wasting power.
Design new voltage divider based on the motor’s resistance…but note that the resistance changes over a cycle. Probably not a good approach.
Use a 1.5 V battery (but often need 6V elsewhere in the circuit)
Use a 1.5 V voltage regulator - a specialized chip that takes 6v and outputs 1.5V -- this could work OK and save power.
Use “pulse width modulation” -Drive the motor with a pulsed 6V signal that averages out to 1.5 V over time. A very common solution.
Use the voltage regulator with a “buffer chip” that does notpull down the output voltage (An op-amp or transistor).
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Look at voltage regulators
78L05
78L05 regulator
Turn 9V into 5V with very little lost power
5V output doesn’t change if load changes
9V5V
TO-92 style TO-220 style
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What about getting a high voltage from a low voltage battery?
•One method: the “boost converter”
•Voltage across an inductor depends on the rate of change of current: V=L di/dt•A switched signal induces a large voltage across the inductor.• Use diodes to control the direction of current, and a capacitor to store the high voltage.•The EL driver project uses this strategy to produce a 150V, 200 Hz AC signal from 3V DC.
Small inductors
Diodes
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High voltage from low voltage
• Commercially available DC HV converters: 100 to 10000V DC from 5-12 V DC Drives MEMS actuators,
piezoelectric devices, photomultiplier tubes, electrophoresis chips.
Small current (Power is conserved: P=IV)
Looking inside, there is a small inductor and some other components.
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Component identification
• Some websites help you learn to identify ICs and other components (links on BlackBoard)
• Good for identifying a “mystery component” from a salvaged board or a junk drawer.
• Teardowns- new insights??http://www.uchobby.com/index.php/2007/07/15/identifying-electronic-components/
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Component Quiz
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Projects• Work on the project of your choiceUsing solderless breadboard first, then soldering
if possible, is a good idea.“Troubleshooting” tips:1. Use resistance meter to check all pins are
really connected how you want2. Compare to a known-good circuit.3. Most parts are symmetrical, but sometimes
polarity is important. Is it plugged in backwards/upside down/not at all?