I have a 1200mfd 250VDC cap that is a filter for a DC motor. The bridge rectifier test ok but the overload pops quickly (motor spins for a second). I tested the cap with an analog meter and it goes to zero but never charges. It's a $30 capacitor should I do a further test and if so what would it be or do I assume it is shorted and throw it away? Appreciate any help!
Can you disconnect the motor?
If the fuses still pop after that, it may be a defective capacitor.
Disconnect the capacitor as well and see how it goes. If fuses still pop, it is not the capacitor...
With capacitor connected, and motor disconnected, the capacitor should be able to hold the high voltage quite long.
Check this.
And take care as these voltages are potentially deadly.
Disconnect power and discharge the capacitor before you connect or disconnected anything.
Can you draw a circuit for exactly what you have please?
How is this motor being powered?
What is the AC voltage you are applying to the bridge rectifier?
Remember that an AC voltage of 240V is normally 240V RMS. This means it has a peak voltage of 1.414 times the RMS value. Also in practice you should never run a capacitor at a voltage higher than 80% of its rated voltage.
I hope he has 110VAC mains...
I also guess it is microF and not milliF...
That’s a damn physically large device.
Please confirm the value.
Yeah I'll try it without the motor and see what happens.
Yes 1200mfd probably should have wrote 1200ufd. Probably 2" diameter by 4" long.
Correct. The can says 1200MFD and I have always believed that M means micro.
That is correct it does. I think you read the M as milli.
No... I have nowhere near the experience as a lot of you people and really had never even heard of a capacitor value mentioned as millifarads. I read M and micro right away.
But you wrote mfd which is milli Farads
I tested a (used) 680uF cap with my analog meter set to the 1K resistance range, and the needle only went about 3/4 of the way to zero, and then began falling as it charged. If yours goes all the way to zero, and stays there, I think the cap is shorted. By the way, most analog meters have reversed polarity in resistance mode, meaning the positive lead is actually at ground. You would want to be mindful of this when testing electrolytics. You can test your analog meter with your digital meter to see what the polarity is.
My mistake. As I posted above I've never ran across millifarads. I did some googling and even saw M as megafarads. Confusing.
Great information. Thank you!
It's simply a bridge rectifier with the cap and motor parallel across the output. 120VAC
Yes but what is between the 120VAC mains and your bridge rectifier?
If the answer is nothing then stop this immediately as it is very dangerous, and, depending on where you live, illegal as well.
Ah yes sorry resettable fuse.
I'm still not sure that this is making sense. When you say that you tested the cap with an analog meter and it goes to zero, do you mean zero ohms on an ohm-meter? if so it is definitely b*****d and needs to be replaced. I assume that it's an electrolytic, make sure the replacement is connected the right way round and is properly rated for the voltage and duty. Sounds like this is one of those high-voltage DC motors that run direct from line. You are lucky that the old one didn't explode, aged 'lytics have a habit of doing that.
Yes zero ohms. I would expect a good capacitor to charge up and the meter needle to start rising.
It is not your fault it is just an old fashioned convention to use M for micro.
With capacitors you can almost always be sure it is in microFarad (or nano or pico) as milliFarad capacitors are almost always written as 47000 microFarads.
And even larger values are quite rare (especially at a rated voltage of 250V).
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