OP AMP PWM

Intro: OP AMP PWM

This is a really simple split power supply Op Amp PWM, only seven circuits. The large heat sink is necessary. The parts are easy to get too.

Step 1: Building the Circuit

As you can see it is small and simple. The pot should be at least 20k, and the more powerful transistor you use the better. A 1nF cap will produce 8 kHz oscillations a 150pF makes 75 kHz but the lower the capacitance the less reliable the circuit is. You can find your happy medium. =]    The yellow leads go to ground and the load. The LED's are for the diagnostic circuit.

Step 2: Keeping It Cool

Since it uses a power transistor to run a heavy load it is a very good idea to have a cooling system. As the graph shows just a heat sink helps very much and a fan isnt a must.

Step 3: Specs and Troubleshooting

Duty Cycle: 0-100%
Power: -12 +12V
Freq: .5 Hz - 1 MHz

Troubleshooting:
The red shows the changes to the circuit. The * is because the vale depends on the LED'S you use. I used 6V ones and only a 380 ohm. The 10 uF cap slows the circuit so you can see the cycle and pulse width. If its not oscillating check the feedback network and the pot. The LED's should not light at the same time but will appear to if to increase frequency.
 

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    5 Discussions

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    JingleJoe

    7 years ago on Introduction

    Just a few things about this circuit, why have you got two 10k resistors in paralel giving feedback to the inverting input? surely one 4k7 resistor is the closest equivalent?
    Additionally, doesn't the transistor need a base resistor? I know that using a lower power transistor without a resistor on the base, breaks it, is it not the same for high power transistors?

    1 reply

    A base resistor is needed to keep the base-emitter voltage from going too high.In this case the transistor is acting as a kind of emitter follower & maintains the .7 volts across the B/E junction by it's nature.Any "excess" voltage is across the motor.

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    pyrohaz

    7 years ago on Introduction

    If you put the motor between +v and the transistors collector, you would probably get less of a voltage drop :P put a reverse diode in parallel with the motor too to stop your tranny blowing :P Nice job :D

    1 reply
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    jazzzzzzpyrohaz

    Reply 7 years ago on Introduction

    The diode and hooking the motor to V+ and the collector (the way I set up audio amps) are both great ideas. Thank you