Gadget Lab Lecture 4: Powering your circuit…

Components…Then it’s Project Time

Dr. Cindy Harnett

ECE Dept., U of Louisville

Spring 2008

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.

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).

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

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

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.

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/

Component Quiz

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?