VDC on the bulk capacitors

Thread Starter

Kava Canada

Joined Aug 20, 2023
63
Dear all friends,
I hope all of you are well.
what could be the reason for not building a good VDC on the bulk capacitors in the SMPS.
I had a Vertox power supply its working on 208VAC, When t supplied it by 204VAC the output on the bridge was 245VDC and that was the same on the bulk capacitors.
regarding the equation Vin x1.414=VDC on the bulk so it should be 288VDC ( I changed the bulk capacitors ) but unfortunately it still the same.
Kindly anyone know the reason.
Thanks in 20231013_075050[1].jpg20231013_075050[1].jpgmy friends.
 
Last edited:

Papabravo

Joined Feb 24, 2006
20,978
Some combinations of rectifier and capacitor do not produce the theoretical peak value of the AC waveform. Your measurement technique could also be suspect. The best wat to see what is going on would be a digital sampling oscilloscope. Some meters do not correctly render a DC value when presented with an AC waveform, which the output of a rectifier capacitor combination most definitely is.
 

MisterBill2

Joined Jan 23, 2018
17,723
The voltage on the first stage filters with a rectified mains input will be lless than the theory predicts for an open circuit because the load current discharges some of that charge. And there is always some current draw because even at no load the efficiency is less than 100%. And it does not matter anyway because the supply output is regulated and not proportional to the supplied voltage.
 

Thread Starter

Kava Canada

Joined Aug 20, 2023
63
Some combinations of rectifier and capacitor do not produce the theoretical peak value of the AC waveform. Your measurement technique could also be suspect. The best wat to see what is going on would be a digital sampling oscilloscope. Some meters do not correctly render a DC value when presented with an AC waveform, which the output of a rectifier capacitor combination most definitely is.
Make sense. thanks sir tomorrow I will try to measure it with the oscilloscope and I will recheck it.
 

Thread Starter

Kava Canada

Joined Aug 20, 2023
63
Some combinations of rectifier and capacitor do not produce the theoretical peak value of the AC waveform. Your measurement technique could also be suspect. The best wat to see what is going on would be a digital sampling oscilloscope. Some meters do not correctly render a DC value when presented with an AC waveform, which the output of a rectifier capacitor combination most definitely is.
Make sense. thanks sir tomorrow I will try to measure it with the oscilloscope and I will recheck it. and in your opinion do you see 245VDC as normal?
 

Papabravo

Joined Feb 24, 2006
20,978
Make sense. thanks sir tomorrow I will try to measure it with the oscilloscope and I will recheck it. and in your opinion do you see 245VDC as normal?
No, it seems low, but then I can't look at a schematic diagram -- because you failed to provide one. How do you expect us to help you if you can't or won't provide us with the most basic information?
 

MisterBill2

Joined Jan 23, 2018
17,723
If a scope is to be used, CAUTION because that point in the power supply is still connected to the mains input. AND neither positive nor negative is at the mains neutral level. So isolation must be provided. AND a scope is not going to help very much.

ANY LOADING AT ALL will drop the measured voltage below that theoretical peak voltage. That is most likely the reason.
 

Thread Starter

Kava Canada

Joined Aug 20, 2023
63
If a scope is to be used, CAUTION because that point in the power supply is still connected to the mains input. AND neither positive nor negative is at the mains neutral level. So isolation must be provided. AND a scope is not going to help very much.

ANY LOADING AT ALL will drop the measured voltage below that theoretical peak voltage. That is most likely the reason.
thanks
 

MisterBill2

Joined Jan 23, 2018
17,723
Once again I ask you to consider that the theoretical PEAK voltage will only be reached if there is NO LOAD drawing any current at all from the capacitor.
So my question is, did you disconnect the balance of the supply circuit from the capacitor?? I see a fan in the photos, was that fan running when you measured the voltage on the capacitors?? Elsewhere, in the system, did you consider the voltage drop in the incoming power leads filter? That could drop the supply voltage a bit.
In addition, is that supply rated to be 100% efficient? I doubt that. And every bit of power that is converted to heat comes from that bulk storage capacitor. Also, consider that some switching power supplies have a bleeder resistor across that capacitor to speed the voltage drop when the supply is switched off.
So certainly one will never see the theoretical peak voltage on the bulk capacitors on any switch mode power supply. So why consider that a variable to be checked? Was the supply not meeting the specifications in some aspect? What was the reason for measuring that voltage??
 
Top