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When i've driven coils in the past(such as mechanical relays) i've had to put a diode to protect it against back EMF once turned off. Do i need to do that when driving stepper motors with a ULN2803a?

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3 Answers

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Diodes are advisable.

If you had bothered to read the data sheet you would have seen that the chip includes clamp diodes!

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Why are they advisable? – Dean Dec 7 '10 at 16:06
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It's still a coil, which means inductance. Abruptly changing the current through the coil will still cause spikes, same as a normal motor. – AngryEE Dec 7 '10 at 16:10
Note: I have driven many steppers on breadboards without using diodes (usually because I forget!) and I've never fried my driving circuitry, BUT I ALWAYS include diodes in my final designs. – PICyourBrain Jan 10 '11 at 13:14
up vote 4 down vote

To answer your question why diodes are advisable:
coils don't like it when their current changes, and when switched off they'll try to keep the current flowing (energy coming from their magnetic field. Compare with the coil's dual, the capacitor, which will resist changes in voltage. There it's the electric field which has to be in-/decreased). Current flowing from V+ to the transistor's collector. You would think that this causes the same voltage drop as when switched on and that the collector voltage will be low.
The important thing is that the coil now doesn't work as a load but as a source, and in a source current flows from - to +. This means that the collector voltage will be higher than V+. And this is what the collector must be protected against. How? By shorting the higher voltage to V+ via the diode.
The 2803A has the diodes integrated.

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Does this mean that they should be used with inductors aswell? – Dean Jun 15 '11 at 16:17
@Dean - anything inductive will show this behavior, so yes, for relays, solenoids, motors,... – stevenvh Jun 15 '11 at 16:19
I didn't mean in that application I meant an inductor in a DC-DC converter. But I guess it would be but most diagrams don't show them as being needed. – Dean Jun 15 '11 at 16:22
@Dean - no, not for DC-DC converters! Their working principle uses exactly this property. If you would place the diode the converter wouldn't work. – stevenvh Jun 15 '11 at 16:49
up vote 3 down vote

If you stay well below the max rated current of 500mA , say 350mA max, the clamp diodes connected from COM to V+ should be ok, but beware of excess heat dissipation if using PWM mode then use external clamps. and common mode chokes for coil pairs.

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Why use common mode chokes for the coil pairs? – Dean Apr 10 '12 at 13:14

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