Completed Project Is it safe to use a 36 year old electrolytic capacitor?

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
I have a 25V 15000MFD electrolytic capacitor. The brand is Mallory, USA made. Its connected to a power supply that has never been used for 36 years until yesterday for first time when I powered it up. It is working fine. But a friend told me sometimes these old caps might blow up since its never been used this long.Is it true? Should I replace it?
 
Last edited:

wobbinc

Joined Aug 29, 2016
4
I think it's pretty unlikely to blow up on you tbh.... It's true that if you have an high voltage electrolytic capacitor then there is a process called reforming that is sometimes suggested. This normally relates to DC bus capacitors in large motor drive systems where the bus ranges from 310 to 600v normally but can spike much higher for short periods during regeneration (where a motor is commanded to stop).

I doubt you'd see any issues with a 25v rated cap though :)

HTH
Robin
 

AlbertHall

Joined Jun 4, 2014
12,345
By far the most likely time for it to go bang is when you first apply voltage after after a long period of storage. If it's working now it will be fine.
 

tcmtech

Joined Nov 4, 2013
2,867
I just turn things on and go from there. If it blows up right away or within a short time after it was either already bad or damn close to it and would have need replacing anyohow.
 

wayneh

Joined Sep 9, 2010
17,496
Its connected to a power supply that has never been used for 36 years until yesterday ...
Age is not nearly so important as hours of use, and particularly the thermal history. Since this power supply hasn't been used much, I wouldn't worry at all.
 

Kermit2

Joined Feb 5, 2010
4,162
Like a motor. Electrolytic caps have a life span given in hours of operation and modified negatively by increasing temperature.

They wear out!
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
I just turn things on and go from there. If it blows up right away or within a short time after it was either already bad or damn close to it and would have need replacing anyohow.
Luckily it didn't blow up.Not yet. But if it does its gonna be pretty nasty I guess considering its size.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
By far the most likely time for it to go bang is when you first apply voltage after after a long period of storage. If it's working now it will be fine.
Hope it goes fine. I unscrewed the cap from the supply and tried the method of forming the plates by gradually increasing the voltage till it reached rated voltage.
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
I think it's pretty unlikely to blow up on you tbh.... It's true that if you have an high voltage electrolytic capacitor then there is a process called reforming that is sometimes suggested. This normally relates to DC bus capacitors in large motor drive systems where the bus ranges from 310 to 600v normally but can spike much higher for short periods during regeneration (where a motor is commanded to stop).

I doubt you'd see any issues with a 25v rated cap though :)

HTH
Robin
I tried reforming the cap today since I had just powered the supply briefly to just check the output and powered it down immediately. Now it seems to be working good and no leakage or anything yet. But what kept me thinking is that does the cap still have 74000MFD?
 

Thread Starter

Rahulk70

Joined Dec 16, 2016
536
Like a motor. Electrolytic caps have a life span given in hours of operation and modified negatively by increasing temperature.

They wear out!
Yep I'm aware of that have seen a lot of them bulge and leak at the top in Electronic ballasts, Motherboards etc. But this Mallory cap since it was made in 1981 was used yesterday for the first time in a power supply.
 

ian field

Joined Oct 27, 2012
6,536
I have a 25V 74000MFD electrolytic capacitor. The brand is Mallory, USA made. Its connected to a power supply that has never been used for 36 years until yesterday for first time when I powered it up. It is working fine. But a friend told me sometimes these old caps might blow up since its never been used this long.Is it true? Should I replace it?
Make sure it isn't getting hot - the dielectric is an oxide layer formed on one of the plates, the caustic electrolyte very slowly etches it away. Leakage current in normal use is what maintains the thickness of the oxide layer. The thin oxide in a neglected capacitor can result in a dangerous amount of leakage current before you even reach the rated voltage - enough leakage can cause temperature and a head of steam that bursts the can.

If you need to re form the oxide; you need a resistor that limits the current under what causes heating and bring it up to rated voltage over about 24h.
 

Kermit2

Joined Feb 5, 2010
4,162
If you have a power supply that can do 10.0 volts.
A GOOD quality 1 k ohm resistor.
A volt meter, and a watch with a second hand.

Charge the cap through the resistor while monitoring the voltage on the cap.

When the voltage gets to 6.3V stop counting seconds.

If you counted to 74 then the cap is 74,000 uf. Understand the method?
 

hp1729

Joined Nov 23, 2015
2,304
I have a 25V 74000MFD electrolytic capacitor. The brand is Mallory, USA made. Its connected to a power supply that has never been used for 36 years until yesterday for first time when I powered it up. It is working fine. But a friend told me sometimes these old caps might blow up since its never been used this long.Is it true? Should I replace it?
Does it still hold a charge? Does it look okay with a meter? Check it n resistance scale with a meter. It should show a low resistance then increase as it charges up. Then switch to voltage and see if it holds the voltage from the ohms check.
 

Stuntman

Joined Mar 28, 2011
222
Just another thought... Remember, there are an awful lot of vintage electronics from that era still working away. As mentioned, check for heat, then know if you see a bulged can or smell a fishy odor, an electrolytic is probably giving up the ghost.

If caps simply went bad after 30 years, I'd be plumb out of test equipment! :eek:
 
If you have a power supply that can do 10.0 volts.
A GOOD quality 1 k ohm resistor.
A volt meter, and a watch with a second hand.

Charge the cap through the resistor while monitoring the voltage on the cap.

When the voltage gets to 6.3V stop counting seconds.

If you counted to 74 then the cap is 74,000 uf. Understand the method?
Awesome thanks for the tip I have some old caps that I have wanted to test.:)
A.H.W.
 
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