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Author Topic: Another capacitor question  (Read 2377 times)
NO2A
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« on: September 17, 2017, 05:19:30 PM »

I've been replacing several old electrolytic capacitors in some of of my old AM radios. I've read where ideally you should use a cap at about 2/3 of its rated limit. For a 6v radio,a 10 or 16v cap would be ideal. One forum I had read said if you choose too high a voltage, say 25,35,50v the cap might not "form" properly,and provide the correct capacitance. Sometimes the lower voltage caps may not be available,so I'll use the higher voltage ones. What I'm asking is, should I use the lower voltage caps, or is it not that important. ? Am I worrying about nothing? Thanks.
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N2CV
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« Reply #1 on: September 17, 2017, 05:49:29 PM »

From what i recall from school back in the 80s, the voltage listed is the maximum range the capacitor can handle.  This is generally 2x the operating voltage across the circuit.  Going beyond 2x the circuit voltage will not alter the capacitance.  With that said, using a higher voltage rated capacitor 2x, 3, 4x will not alter the function of the circuit.  Just don't change the capacitance value as the reactance may alter the way the circuit functions.

Someone correct me if I am wrong.  I have not really studied this since my college days at Univ of Maryland ...GO TERPS.

73,
Al
N2CV
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W9IQ
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« Reply #2 on: September 17, 2017, 06:06:13 PM »

It is considered a good design practice to stay below the rated WV. This will also extend the life of the capacitor - as will running it below its rated temperature. Forming voltages are rarely an issue.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
NO2A
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« Reply #3 on: September 17, 2017, 06:11:35 PM »

Thanks very much!
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G3RZP
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« Reply #4 on: September 18, 2017, 03:37:24 AM »

With high voltage electrolytic capacitors, they gradually 'lose' the ability to withstand the rated voltage e.g. running a 450 volt capacitor at 200 volts, it will eventually become a 200 or so volt capacitor, and to be run at 350, would need reforming. Low voltage caps aren't quite as bad at doing that.

Running a 12 volt capacitor on say 25 volts however is definitely an approach to shorten the capacitor life: if it was something large like 220,000 mFd, when it did die and if you had your face above it, it could shorten yours too!
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KC8KTN
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« Reply #5 on: September 18, 2017, 07:59:53 AM »

If Radio Shack was still like it used to be Capacitor Questions would be a thing of the past..Just Saying . Keeping it 100.....73ss...  P.S. I miss the Radio Shack  of the Olden Days.. You have ?s we have Answers...

https://www.youtube.com/watch?v=GitK6i3HQdU

https://www.youtube.com/watch?v=jmED7uETsQo
« Last Edit: September 18, 2017, 08:16:19 AM by KC8KTN » Logged
NO2A
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« Reply #6 on: September 18, 2017, 03:18:29 PM »

I do use Radio Shack in a pinch. The clerk is well versed in electronics,and had some old (used)parts in the back he said I was welcome to. They have mostly radial caps,with some axial,which is usually what I need for these old radios. But they'll charge you  $1.94 for a  .20 cent part. An interesting thing I've found is that the physically bigger the cap,the longer it will last.  That explains the longevity of these parts. Thanks for the forming info, that was my biggest concern.
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N8EKT
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« Reply #7 on: September 18, 2017, 03:46:13 PM »

I've been repairing electronics for nearly 40 years and I have ALWAYS used higher voltage rating replacement capacitors and have NEVER encountered a problem in doing so

If it's a 10 volt I use can 16

If it's a 16, I use a 25

If its a 35, I use a 50

As long as physical dimensions allow
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W9IQ
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« Reply #8 on: September 18, 2017, 04:06:05 PM »

That is a very good practice. The physics support your experience.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KH6AQ
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« Reply #9 on: September 18, 2017, 10:39:44 PM »

I thought only 70 Radio Shack stores were still open with none of them in Florida. Is your local store still there?
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NO2A
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« Reply #10 on: September 19, 2017, 12:53:09 AM »

I thought only 70 Radio Shack stores were still open with none of them in Florida. Is your local store still there?

We used to have a Radio Shack right up the street. Now there are only two,north in Defuniak Springs, and west in Navarre. Both are a good distance from here. I don't know about the rest of Florida.   
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K1ZJH
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« Reply #11 on: September 21, 2017, 06:57:00 PM »

I don't believe modern electrolytic caps have a problem operating well below their rated voltage.

Peter
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G3RZP
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« Reply #12 on: September 23, 2017, 02:36:32 PM »

Pete,

Quote
I don't believe modern electrolytic caps have a problem operating well below their rated voltage.

They don't have problem until and unless you the have a need to run them at their rated voltage without reforming. If you aren't recycling them into some other equipment, it isn't a problem - ever.
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