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When I stop my 12V wiper motor, then turn it back on within a second or so, my 12V 5A cheap power supply turns off for less than a second, and comes back on, then turning on the motor. What causes this?

The motor's stall current is 4A at 12V, if that's relevant. And I'm using a diode to protect a transistor connected to an Arduino which switches the motor on. Very infrequently, this problem does not occur and I'm able to switch the motor back on immediately after turning it off.

Anyone know what's going on??

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  • \$\begingroup\$ Is it the turning off that upsets the PSU, or the on-immediately-after-off? Is the motor still spinning when you restart it? \$\endgroup\$
    – tomnexus
    Mar 29, 2015 at 6:26
  • \$\begingroup\$ It will be a short circuit protection feature of the supply. Put a large capacitor or two from the motor + pin to the - pin or wherever your connection to the motor driver is. Capacitors of 1mF (1000uF) or more, and at least 2 of them should fix it. You may try with less first up, but it will probably take a few mF anyway. \$\endgroup\$
    – KyranF
    Mar 29, 2015 at 6:37

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There are a number of components you might need - which ones you need, and their values all really depend on the type of motor and its condition etc, and the PSU and its condition etc.

The PSU although cheap is probably actually quite good, which is why it cuts off - it is protecting itself against damaging transients.

The problem is that when the motor is cut off, because it has coils, it tries to keep the current going by generating a voltage inverted to the original, which the PSU won't like.

Also, the problem could be that when motors start up, often they consume a lot more power than when they are running. So you may need a slow-start circuit. Again, this gets complicated but the series resistor might work. (R2). Or you could have a switch that you use to bypass a slightly higher value resistor once the motor has started turning. (R1).

Explanation of the component are below. There are further / alternative measures as well, but they get increasingly complex. From inductors to semiconductor motor driver circuits.

A small series resistor (R2). This resistor may need experimenting with. But I would not make it more than about 0.47 ohms. And at 0.47 ohms it should be rated at 8 watts or more, but you can combine resistors in series, or parallel, or a mix, and use lower power values. For lower than 0.47 ohms it can have a lower power rating. For example, for 0.22 ohms, the power rating just has to be higher than 3.52W. This resistor will help the PSU deal with whatever the motor throws at it, as well as make the other components below more effective. However its value should be kept as low as possible, as it will reduce the power available to the motor. Plus it will get hot.

A suppression diode. There are further / alternative measures as well, but they get increasingly complex. From inductors to semiconductor motor driver circuits.

Smoothing capacitor (C1) The capacitor may need fine tuning - too big and it itself may cause the PSU to cut out as it tries to charge. Too small and it will not counter the problems the motor causes. Make sure you get the polarity right or it will literally explode.

Transient suppressing capacitor (C2). This can be a small capacitor - the important thing is just to have it as close to the motor terminals as physically possible. However, there's a good chance the motor already has these internally, if for example it is a hand-held mini drill - perhaps open it up and see. If it is a bare motor however, you should add one to its terminals.

The 3 ohm start-up resistor (R1) should be 12 watts or more. You may want to experiment a bit with this, perhaps between around 2 and 4 ohms - too large and the motor may not start, too small and it may not inhibit the initial draw enough.

I would try the solutions in the following order:

  1. Suppression diode (because it's cheap, and always a good idea with DC motors)
  2. C2 - same as above but for AC motors too - but it probably won't solve your problem
  3. R1 - Most important for start-up draw
  4. R2 - Will make all other components more effective at shielding the PSU, but will cause slight power loss to the motor.
  5. C1 - May itself cause problems if too big, but if you're still having problems at this point, it's worth a shot.

Instead of trial and error, the better way to do it would be through measurements and various calculations, and/or using a made-for-purpose intelligent motor controller, but this obviously can get rather involved.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Awesome, so are diodes alone not enough to prevent this? In most motor circuits I've seen I've only seen a diode connected in parallel to the motor... or does it just mean the diode I'm using isn't right for the job? \$\endgroup\$
    – wildwood
    Mar 29, 2015 at 17:55
  • \$\begingroup\$ A diode will only stop the reversing of polarity caused by switching off the motor. It will not stop the motor's excessive start-up current. Also, any silicon diode will only conduct when there is around a negative 0.7v across it. Hopefully your PSU won't fuss about that though. Also, as the motor makes and breaks contacts at however many RPM, the capacitor will smooth out little spikes. (Saying that, actually you might want a special, small-value, high-frequeny capacitor in parallel with the larger one. Again, all this needs playing around with really. Some components might not be needed) \$\endgroup\$
    – CL22
    Mar 30, 2015 at 7:42

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