Using dc capacitors in an ac circuit

Bob-J-H · Sep 7, 2020

Is there any reason to not use dc caps in an ac motor starting circuit. I am talking about wiring them back to back creating a non-polarized capacitor. I did it that way and it worked fine. But I haven't done a long term test. They are attractive because they pack a lot of mfds in a small package, and I have a lot of them. I am talking electrolytic caps.

Phil in Montana · Sep 7, 2020

Dont you will let the smoke out just before they explode...Phil

drcoelho · Sep 7, 2020

Agreed, you don't want to be using a DC capacitor in an AC circuit, DC capacitor (electrolytic) will fail if presented with the wrong polarity.

Bob-J-H · Sep 7, 2020

When connected positive to positive or negative to negative, (2 or more) You create a non polarized capacitor. It will work. I want to know how it works long term.

jz79 · Sep 7, 2020

you get half capacity when you connect them like that, and when one of them will start to fail, the other one will see inverted voltage and will fail soon afterwards, not worth the risk

9100 · Sep 7, 2020

I have been told that AC capacitors are two back to back lke you have, but that is just hearsay. Remember that AC capacitors come in two basic flavors, electrolytic and oil filled, not counting paper, polycarbonate, teflon, etc. You can use either for short time motor starting but for continuous running. Oil filled can be used for starting but are so much bigger for a given capacity that they rarely are.

In intermittent use, yours will probably work bu I would get the right ones on the grounds that they are not all that expensive vs cleaning up the mess if they fail.

Bill

JST · Sep 7, 2020

Electrolytic AC caps are exactly that way. The technique is old as the hills.

I would not swear that the electrolyte etc is exactly the same for AC types and DC types. It would not surprise me to find there are subtle differences.

thermite · Sep 8, 2020

Bob-J-H said:
Is there any reason to not use dc caps in an ac motor starting circuit. I am talking about wiring them back to back creating a non-polarized capacitor. I did it that way and it worked fine. But I haven't done a long term test. They are attractive because they pack a lot of mfds in a small package, and I have a lot of them. I am talking electrolytic caps.

Back-to-backing covers the polarity issue, but you still lack ripple tolerance. They can be OK, but only "briefly". Starting-only, so long as switched-out is not bad.

Fail to switch-out, over the longer term, left in-circuit they'll degrade, then fail. "Longer term" can be dreadfully short. They can be messy when they fail.

Proper AC rated ones that spec high ripple are not that expensive.

HVAC and food cold storage/supermart display reefer industries use them by the railcar load, their suppliers as good a source as any for competitive pricing.

Yes, the AC ones will be bulkier, any given capacitance.

Xin Loy!

Bob-J-H · Sep 8, 2020

thermite said:
Back-to-backing covers the polarity issue, but you still lack ripple tolerance. They can be OK, but only "briefly". Starting-only, so long as switched-out is not bad.

Fail to switch-out, over the longer term, left in-circuit they'll degrade, then fail. "Longer term" can be dreadfully short. They can be messy when they fail.

Proper AC rated ones that spec high ripple are not that expensive.

HVAC and food cold storage/supermart display reefer industries use them by the railcar load, their suppliers as good a source as any for competitive pricing.

Yes, the AC ones will be bulkier, any given capacitance.

Xin Loy!

Yea This would only be for motor starting, I know better than to leave electrolytics in the circuit longer than 3 seconds. Thanks everyone for your input. Anyone else feel free to chime in. (Xin Loy) I haven't heard that term in 50 years.

JST · Sep 8, 2020

Ripple current, basically AC current through the capacitor, is a function of the ESR, equivalent series resistance. Bigger parts often have better ripple ratings, but tall thin are the best, maximum surface area.

AC starting capacitors are OK with a very high current for their size, but ONLY for a VERY short time. They are designed to be severely overloaded, and the start service does that. Which is why they are good for a total lifetime use time measured in MINUTES. Often a few thousand starts. (1/2 second per start, gives you 120 starts per minute of life, so 5000 starts is 42 minutes of total life.

One start cannot exceed a few seconds, or you get into overheating and destruction. "A few" might be three, or possibly 5 seconds at the outside, but will likely cut life more than the total time indicates. And there may be a limit on starts per hour.

It's not so much their ripple tolerance, it is the limited "on time". They just need to stay below their maximum temp long enough to start the motor.

sfriedberg · Sep 8, 2020

Having literally exploded a DC electrolytic capacitor (if that house still exists, there're probably still spray marks on the basement ceiling) in my youth, I would not assume that you can make a satisfactory AC capacitor out of a back-to-back pair of DC caps. Since my little episode took less than 10 seconds, even the limited "on time" in a starting application is not going to give you (well, your caps) a satisfactory lifetime.

JST · Sep 8, 2020

Well, the back-to-back DC caps has been a staple item for decades. Some HIFI amplifiers used them to "decouple" the output from the speaker, and it is also common for circuits inside amplifiers and other such.

The starting types are optimized for that particular use, although they obviously are a variety of back-to-back setup. While correctly selected DC parts would likely work fine for just as long, there is no percentage in trying to do it, simply because the purpose made parts are not expensive, work well, and are designed to do the job. The manufacturers control all the variables of size, foil thickness, electrolyte, packaging, plus they do the testing to verify the design and manufacturing.

Why try to reinvent the wheel without being in control of any of the variables?

Doozer · Sep 16, 2020

Learning for me question....
If you hook up 2 opposing DC electrolytic caps to AC
do you need a diode in series with each one, so each one takes
half the ac wave ???

-Doozer

JST · Sep 16, 2020

Doozer said:
Learning for me question....
If you hook up 2 opposing DC electrolytic caps to AC
do you need a diode in series with each one, so each one takes
half the ac wave ???

-Doozer

No you do not.

DaveKamp · Sep 21, 2020

Back-to-back (and belly-to-belly!) on DC electrolytics is most commonly found it small-signal circumstances, where the POWER is low... it's simply being used to decouple stages in a signal chain. The capacitance is employed as a TIMING element (bandpass of a filter) rather than power transmission. Using them for power applications puts the electrolyte and foil in a really abusive relationship which becomes a really short-notice confetti-funeral-party...

JST · Sep 22, 2020

DaveKamp said:
Back-to-back (and belly-to-belly!) on DC electrolytics is most commonly found it small-signal circumstances, where the POWER is low... it's simply being used to decouple stages in a signal chain. The capacitance is employed as a TIMING element (bandpass of a filter) rather than power transmission. Using them for power applications puts the electrolyte and foil in a really abusive relationship which becomes a really short-notice confetti-funeral-party...

The type of capacitor makes a huge difference. Signal type capacitors are OK to make "assembled non-polar" capacitors from, for signal purposes, not good for power purposes.

Power supply types, with sufficient ripple current rating, are fine for assembling power type non-polar capacitors. Been done for decades.

However, the starting capacitor is a special-purpose part, designed to do that job, which it does well. You will not be able to assemble a better one, and will not be able to assemble a sufficiently good one that fits the same (typically rather tight) location, out of stock parts.

Just buy one, unless you are at the North Pole, and have no choice.

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