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Diode bridge & capacitor circuit problem

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siongboon

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Hi,

I have did an experiment with diode bridge using 1N4007.
The output of the diode bridge is a 100uF 400V capacitor.
I did not have any other load on the circuit.

Then I turned on the 230Vac power supply.
I thought there will be a rectified Vdc output before I measure,
but I notice the voltage keeps on dropping from 200Vdc down to below 100Vdc.
The voltage keeps on dropping even if I turn off, then on.
The voltage never rise up again

Seems that my component is failing, I guess.
Would appreciate if anyone can advise me on my observation.

Attached is the circuit.

Thank you for your advise.
Siong Boon
 

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  • diode bridge.pdf
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The capacitor should develop a charge of 325 VDC.

The diodes would be exposed to 325 reverse VDC.

If no component has burned up then it's likely that two or more diodes have failed and gone open circuit. This may or may not have occured when the capacitor drew surge current as soon as you applied power.

Is the capacitor an old electrolytic? There's a chance its dielectric has deteriorated. It may or may not be usable. If it is leaky then power is traveling directly between the foils. This is unsafe. It could burst.

Do not hook up the 230 AC again until you know which components failed. Have some safeguards put up when you test it again, because such a high power level can cause components to explode.
 

The capacitor is a new electrolytic.
1N4007 has a very high reverse voltage.
I guess the current surge theory could be the cause.
It will make sense if the diodes were damaged during powering up.
The capacitor starts to discharge when I was doing the measurement.
When I switched on and off, nothing happens and the discharge continue.

Thank you very much.


BradtheRad said:
The capacitor should develop a charge of 325 VDC.

The diodes would be exposed to 325 reverse VDC.

If no component has burned up then it's likely that two or more diodes have failed and gone open circuit. This may or may not have occured when the capacitor drew surge current as soon as you applied power.

Is the capacitor an old electrolytic? There's a chance its dielectric has deteriorated. It may or may not be usable. If it is leaky then power is traveling directly between the foils. This is unsafe. It could burst.

Do not hook up the 230 AC again until you know which components failed. Have some safeguards put up when you test it again, because such a high power level can cause components to explode.
Click to expand...


---------- Post added at 08:08 ---------- Previous post was at 06:42 ----------

I have check on the diode, seems to be working ok.
When I pump in 12Vdc, I was expecting 12Vdc at the output.
However I only got about 4.3-4.5V.
This voltage remains even I increase my input voltage.

What is the issue?

Thanks.
Siong Boon
 

The diode's operating characteristics have changed in one or both: (A) forward resistance, (B) reverse volt rating.

The 1N4007 is rated at 1000V. Your schematic exposes each diode to 325V. So item B should not have changed, unless the PN junction has been stressed.

If A increases greatly, then that would explain the results you got by testing the diode individually.

If rev V rating goes down, it explains the results when you used the entire bridge.

Did any diode heat up, or smoke? That would be expected when something goes wrong at 230 VAC.

Each diode must be checked individually. In both directions. It is easy to test it with forward voltage. However to test it for high voltage you must subject it to high voltage. This is risky as you know.

The capacitor value is not so high that we would expect a great deal of surge current on power-up. However if it was exposed to the wrong polarity at any time, then it can be damaged.

As a test, you can try creating a half-wave supply with just one diode and the capacitor. The capacitor should acquire 325 VDC.

When you disconnect the diode, the capacitor should hold that charge, and only lose it slowly.

Discharge the capacitor each time before you test the next diode. Attach a safe load such as a small bulb rated for that volt level.

These are all hazardous things to do when you work with high voltage. If it were me I would hold a clear plastic shield between me and the components.
 

sure u your voltmeter is in dc mode check it out .
next take 2x 25w light bulb and JOIN THEM IN SERIES and use as load u can remove the capacitor temporary.
 

Ya. I am sure it is in DC mode.
Using bulb as load is a good idea.
I will try it out; this should test the diode, alive or not.

dselec said:
sure u your voltmeter is in dc mode check it out .
next take 2x 25w light bulb and JOIN THEM IN SERIES and use as load u can remove the capacitor temporary.
Click to expand...


---------- Post added at 01:52 ---------- Previous post was at 01:45 ----------

The component looks ok on the outside.
Yes, I did discharge the capacitor. I had the whole circuit
covered with a plastic bin in case the big capacitor explode.
I am also very scare of high voltage.
I shall do further test to verify again.

Thank you so much for the suggestion.

BradtheRad said:
The diode's operating characteristics have changed in one or both: (A) forward resistance, (B) reverse volt rating.

The 1N4007 is rated at 1000V. Your schematic exposes each diode to 325V. So item B should not have changed, unless the PN junction has been stressed.

If A increases greatly, then that would explain the results you got by testing the diode individually.

If rev V rating goes down, it explains the results when you used the entire bridge.

Did any diode heat up, or smoke? That would be expected when something goes wrong at 230 VAC.

Each diode must be checked individually. In both directions. It is easy to test it with forward voltage. However to test it for high voltage you must subject it to high voltage. This is risky as you know.

The capacitor value is not so high that we would expect a great deal of surge current on power-up. However if it was exposed to the wrong polarity at any time, then it can be damaged.

As a test, you can try creating a half-wave supply with just one diode and the capacitor. The capacitor should acquire 325 VDC.

When you disconnect the diode, the capacitor should hold that charge, and only lose it slowly.

Discharge the capacitor each time before you test the next diode. Attach a safe load such as a small bulb rated for that volt level.

These are all hazardous things to do when you work with high voltage. If it were me I would hold a clear plastic shield between me and the components.
Click to expand...
 

I also advice u to add as a permanent load for safety say 100k 3w or 50k 5w across capasitor
 

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