Hello!

I've got a question (or two) about storing capacitors properly. I could not find a definitive answer, so decided ask to the experts:

The "Back Story" is:
Where I used to work, we serviced the capacitor banks for power factor correction on the 13.2kv distribution system. We always stored the spare capacitors with grounded copper bus bar sections or bleeder coils bolted across the terminals. I never gave it much thought, assuming this was to keep the potential at zero while stored & handled. They were stored in the substations, so eddy currents were, indeed, strong. (A volt-tik could ring just by walking into the stations.)

At home, I have a few (much smaller by comparison) old Mallory capacitors labeled from 1750 MFD to 95,000 MFD. (15 to 140 volts). They have been stored in a container on a shelf and have not been touched for more than 10 years.

Today, I decided to test them. (Why?)
By instinct, before touching one, I put my alligator clamp resistor of 10k ohm 20 watt across the terminals. I got a spark and the resistor warmed noticeably in the few minutes I kept it clipped on. I tried the others with similar results on two of them. Knowing me, they were bled to zero before storing.

The big question is:
How did they get this charged?
The room is an unheated 'cold room' adjacent to my basement. The storage container is plastic, but sitting on a non-gorunded shelf (if any of that is pertinent). No terminals were touching anything in storage. Are the small eddy currents surrounding us able to charge these over time?

Question Two is:
Should I store these shorted with a wire (or resistor)? Will that harm them in the long term?

Thanks for your ideas!
Paul

 

Capacitor Soakage

Electrolytics should be ordered as needed if possible, not kept on hand, though ending up with some is inevitable. Storing them in the tape&reel, or whatever form you have them in, is usually fine. Keep away from high temp/humidity if possible.

 

Thanks for your reply, Thatoneguy

I should have thought about spoilage. Back in the olden days, was not rare to get a motor start capacitor from our parts department only to find it was no good. (Usually on a roof top job, at night in January.)

These particular capacitors were inherited. They are part of "I might need that someday" collection of treasures. (I need an intervention!)

I'll remember to order electrolytic capacitors as-needed. Sometimes, I do order the 10-pack when I only need one. The other 9 remain unused for who knows how long.

Thanks Again for helping,
Paul

 

It has been suggested that when using an electrolytic capacitor that has been in storage for years, connected a DC voltage to the capacitor and "restore" it before connecting it to your circuit. The voltage should be set very close to the capacitor's rated voltage and left on for a couple days. All other caps should be fine to use out of storage, and so they don't have a shelf life.

 

Thanks Brownout!
I will give that a try with one or two of those who show open (and no short to case) on the capacitance meter. It will be interesting to experiment with.

I still can't figure out how some of them charged themselves in storage. Crazy, huh?

 

Now, when I said the restoring voltage should be 'close', I mean at least 90% of the rated voltage. Never exceed the capacitor's rated voltage.

 

Also, if you're going to connect a capacitor to a power supply, either turn up the voltage slowly or use a resistor to limit initial current. Connecting directly can cause dangerous charging current, unless current limited.

I know I didn't need to tell you that, but other's might be reading.

 

Good Words!

I guess I should have mentioned current limiting in my reply as well. Sometimes we think, because we're working with lower-than-line voltages all is safe.

I very much appreciate how strongly this forum stresses safety.

Enjoy Today!
Paul