Replacing a capacitor with one of a lower voltage in an AC/DC converter

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Eddy67716

Joined Sep 28, 2020
21
I am going to overhaul the capacitors of my Yamaha Electone E45's power supply to extend its life. The only problems is that for the 38 volt regulator, it needs 2 55 volt 10000uf Capacitors. The store only had the same uf value in 50 volts.

I did some research and it says you needs to calculate an extra 20% of the voltage in case of surges. I got 38 * 0.2 = 7.6, 38 + 7.6 = 45.6.
50 volts is higher than 45, but is that safe or would it end up blowing a fuse, bursting one of the capacitors or worse?

Schematic for Power supply is attached below.
 

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MisterBill2

Joined Jan 23, 2018
7,325
Indeed, the capacitor is quite isolated from the 240 volts mains side of the transformer. For a 38 volt supply a 50 volt capacitor should have an adequate margin, 12 volts. So I see no problem with a 50 volt capacitor. If you can get 63 volt ones that fit, they would also be OK.
 

Reloadron

Joined Jan 15, 2015
5,667
Is there a way I can precisely measure the peak with a multi-meter?
With a quality meter measuring the RMS value of a true sine wave multiply what the RMS value is by 1.414.
If that 38V is an RMS voltage then the peak rectified voltage will be 53V.
So Albert took the 38 volts RMS and multiplied it by 1.414 to get 53 volts or actually 53.7 volts peak.

After 50 volts the normal next high rating is 63 volts which are pretty common. I would just get a few 63 working voltage rated capacitors and be done with it as suggested. Your drawing in post #5 does not reflect the whole power supply. It does not even include the transformer secondary or rectification prior to your filter caps. You also don't mention the lead type, for example axial or radial? Here is an example of a 10,000 uF Nichicon 63 WVDC radial lead capacitor. That is just a single example of one manufacturer.

Ron
 

MisterBill2

Joined Jan 23, 2018
7,325
Look at the circuit before starting the RMD versus peak volts cry!! It is rectified DC! The label on the drawing lists the DC voltage, NOT the transformer output voltage. In addition, with a constant load current the supply voltage will be a lot less than the peak voltage. And with a nominal 38 volt level, the 50 volt capacitor will be fine. 63 will work also. But avoid 45 and 40 volt caps, that is excessively close.
 
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