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I have two 12V water pumps that I want to control with an Arduino via a relay. They are generics and I am unable to find a reliable datasheet:

Good Pumps with Rohs Motor:

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Goso GB37-530

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Is there anything I can measure to check if they already have a flyback diode, or to know if they need it?

TimWescott
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Carlos Garcia
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2 Answers2

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It’s not too easy. You could test it with a series diode + resistor (like 10K) + LED across the pump and flicker the power with a mechanical switch. If the LED flashes, there is no flyback diode.

There is no harm in adding an external diode in any case, and I strongly suspect these pumps do not contain an internal diode. If you are planning on PWM, an external Schottky diode would be better in any case (the pumps might not like the PWM though).

Spehro Pefhany
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Rohs Motor :-)

Rohs just means Restriction of Hazardous Substances Directive by the European Union. Guess they want to state there is no Hazardous Substances in the motor.

you need a multimeter, best with a diode test, resistance measurement modes are subject of too high voltage with chances to damage an diode (but work at least). then you need to measure in both directions. If there is a diode it will be indicated.

In case you have no diode test at hand, a battery, a resistor and a voltage meter might also get the job done. Just limit the forward current to some 1-3mA. cant remember any diode which can not handle such currents.

schnedan
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    Any diode in the motor is likely to have a 50 V reverse voltage rating. No multimeter resistance measurement would apply more that a few volts. I can't see how you could damage a diode with a resistance measurement test. Don't forget that the low resistance of the motor will be in parallel with the snubber so both resistance and diode test functions may not generate enough voltage to forward bias the diode. – Transistor Sep 08 '20 at 21:21
  • @Transistor yeah, the low resistance of the motor could be a problem. thats true. But for "I can't see how you could damage a diode with a resistance measurement test.", the answer is: in forward mode forcing a 0,7V diode with e.g. 3V might damage the diode. well at least a diode without an attached motor – schnedan Sep 08 '20 at 21:27
  • LOL, Rohs motor, I'm such a begginer. Thank you for the answer! I will try to figure out what diode I can add to the pumps. Thanks! – Carlos Garcia Sep 09 '20 at 18:43