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capacitor between power and ground?

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New Member
hey guys
so i just bought these pumps from china for this project i am working on
i found them initially because i opened a bp cuff and saw them in

the pumps they sent are just with empty nodes to solder on which is fine
but i was wondering, because the pumps in the bp cuffs have a capacitor between the power & ground
why did they do that?
and what capacitor size should i use, does it matter?

i ask because the pump worked before, and now is seeming to be shorting my project when connected
is this part of the problem?



New Member
Are you talking about an AC driven motor?

If yes, the capacitor is used for phase shift to operate a three phase motor using single phase and neutral.

Without the cap the motor would hum only and finally burn up.


New Member
check the pumps ratings..........3v dc supply??yr 5v supply may have kiled it


New Member
Yes, it is a DC driven pump motor.

Normally they are fitted with a cap across the motor to reduce back EMF. If that cap is shorted (either caused by "monday production" or any other reason) the power supply will drop to its minimum if it is current limiting, since a shorted cap is a dead short for the power supply.

A quick check for the capacitor: Connect it to a multimeter using the lowest resistance setting and observe the values indicated on the display. They should start at a low reading (overflow due to short) and increase readout until the cap is fully charged to the voltage source of the multimeter and stay there. Then reverse the probes and observe again. This time the reversely charged cap should indicate a dead short which should disapppear quickly when charge current approaches zero because of reversed charging.
(Hope you understand the procedure - German English :))

I didn't dig into details about those pumps, but I guess the manufacturer won't use an unnecessary cap for them.

Replace the cap by one with the same capacitance and you'r e done. Use a voltage rating 8 to 10 times the nominal operating voltage of the pump motor.

Most decoupling caps are rated 63 or 100VDC, so it's fine to use one of those.

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