What diode to use for 2d plotter stepper motors

[paulmorrishill]

Hi, I've been working on a 2d plotter that I have built from old printer parts and I've been having problems with the chip (ATMEGA168) resetting every time one of the stepper's coils is switched off

I think it may be back emf caused by the collapsing field so the solution may be to put a diode in parallel with each of the stepper's coils to eliminate the emf. However I am not really sure what type of diode to use with what ratings.

Each coil has 4.6Ohm resistance and I'm powering them with about 5 volt and my PCB supports a maximum of 1.5A per coil. The shortest amount of time that a coil will be on for is 700Microseconds. I've read that using a diode can increase switching off times, so I would like to know what kind of extra time using a diode would add, and if it's significant, any way to reduce this time.

Also if it helps I am using 4 ULN2003 Darlington array driver ICs to drive the steppers coils each coil has 3 Darlington pairs in parallel

Thank you in advance, Paul

 

[Diver300]

You should use a schottky diode, as they are quicker to turn on an off.

If you are powering your stepper motor coils from 5 V, they won't be turning on very fast. Many stepper motor circuits have much larger voltages so that the current rises faster.

A diode in parallel with the coil will stop back EMF but it will also slow down the turn-off. You can fit quite a large resistor, maybe 30 ohms, in series with the diode. That will give about 30 V of back EMF, but that it probably what the ULN2003 can stand. The resistor will make the current stop much more quickly.

 

[mneary]

The ULN2003 already contains diodes which will be adequate for most applications. It's probably as simple as connecting pin(s) 9 to +V. Post your schematic so we can confirm. If speed is an issue you might want a resistance in series with this pin 9 connection.

If you are powering your coils with 5V it must not be the same 5V that operates the microprocessor. It can come from the same place, but the micro's 5V needs extra filtering. They should not share the same regulator.