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Filter Capacitor Forming

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lewisP

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When changing power supply filter capacitors i heard its bad to turn on the full power because the Electrolytic filter capacitors haven't formed yet. Why do electrolytic capacitors need to be formed and what is the proper way of forming electrolytic filter capacitors. You use an AC variac and turn up the voltage in steps. How long between the AC voltage steps so the filter capacitors form right. If you don't form electrolytic filter capacitors how long do the filter capacitors last without forming them or what is the side effect of not forming filter capacitors?
 

Audioguru

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Electrolytic capacitors might need to be powered gradually if they have not been powered for many years because they need to be re-formed.
I think a capacitor that lost its "forming" draws a higher current than normal for a few minutes that heats it.

Don't you think the heating (caused by its higher current) will be reduced if its voltage is increased slowly in steps? Oh, you never learned Ohm's Law and have another post with your name as ericwatson?
 

lewisP

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FILTER CAPACITOR FORMING STEPS:
install a 100K 2 watt resistor between the rectifier circuit and the first filter cap. On a circuit that uses the full wave bridge circuit and bleeder resistors, the center tap must be disconnected from the first two caps and the bleeder resistors temporarily removed for the procedure. You would then hook a volt meter up across the 100K and monitor the voltage drop, which will start off high around 200V and then continuously drop as the caps form and charge. As soon as the voltage drop reaches 5V or less, the caps are formed.

If you don't form filter capacitors they won't last as long and the capacitors charge will be defective. It has nothing to do with either the capacitors are new or old. If you solder in brand new filter capacitors you still have to form the capacitors using a variac and slowly bring up the voltage to form the capacitors.
 

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It's hard to give a serious anwer to the previous question because it mixes guesses and operating instructions that have been valid 50 years ago. They have some relevance if you're dealing with the restoration of antique devices, otherwise ignore it.

Readers who stumble upon this discussion are invited to review other Edaboard threads focussed on the specified behaviour of modern electrolytic capacitors, e.g. this one https://www.edaboard.com/threads/317209/

Secondly review the application notes from major manufacturers. Some are quoted in the linked thread.
 

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