Nonpolar, bipolar, polar capacitors

[blowerman]

I must confess I haven't gone through any other posts on the subject but have been looking into this so forgive me if I'm covering old ground. I've just moved on to attempting to repair a board in my IRD 245 balancer and it has Sine and Cosine signal outputs that go to the main microprocessor board each via nonpolar capacitors rated 250uF @ 3 Volts. Someone had obviously had a go at this board before because one capacitor is the original capacitor from Richey of that rating and one was a 125uF @ 15 Volts but still a nonpolar capacitor. According to the schematic they should both be the same and there was a big problem with this board when I got the unit, getting on for twenty odd years ago, with vibration levels not being read around 90 degree 270 degree positions. Anyway the old board was replaced with a known working one but after recent success with the defunct display I'm inspired to try and repair this reference board and want to put all the components back as per original. So long story short I have found the whole bipolar, nonpolar thing very confusing when looking for a suitable capacitor. I am I right in thinking that if I get two 500uF 3 Volt polar electrolytics and connect the negative poles of each together I should end up with a nonpolar 250 uF capacitor with outside +ve poles??

Sorry that was long story for a short question!

 

[Nigel Goodwin]

Pretty well, and that's basically all that bi-polar electrolytic capacitors usually are.

 

[blowerman]

Cheers Nigel. It all got a bit confusing with terminology when sourcing the parts. Suffice to say this value of nonpolar capacitor seems no longer to be found hence looking for another way to achieve it. Fingers crossed!

 

[rjenkinsgb]

You could also add a very high value resistor from the junction point of the two caps to eg. the circuit 0V (if the input after the cap has some positive voltage on it).

1M or even 10M - just something to maintain a small bias voltage of the correct polarity on both caps in the pair.

That would not make much difference in audio applications, but for instrumentation it could help reduce leakage in the long term?

 

[Nigel Goodwin]

Cheers Nigel. It all got a bit confusing with terminology when sourcing the parts. Suffice to say this value of nonpolar capacitor seems no longer to be found hence looking for another way to achieve it. Fingers crossed!

I was quite bemused by the 3V rating?, I don't recall ever seeing capacitors that low?.

 

[MaxHeadRoom78]

am I right in thinking that if I get two 500uF 3 Volt polar electrolytics and connect the negative poles of each together I should end up with a nonpolar 250 uF capacitor with outside +ve poles??

This is how AC induction motor start, bi-polar, capacitors are constructed, it is a very demanding application, very high current etc.
Usually consist of dry electrolyte construction.

 

[blowerman]

Attached a pickie of the caps taken off. Now both outputs should be via 250uF caps @3 Volts in the schematic but as you see one had been replaced and presumably this would have affected the output signal? I note what is said about the high value resistor to ground. If this would be a good safeguard then I'll give it a go! I have five of these instruments all from the late 1970's early 1980's. I've ended up with five on the basis of safety in numbers as they are a mainstay of the work we do.

 

[ZipZapOuch]

I would avoid the resistor. I don't see the physics behind a resistor minimizing leakage. Anti-series wired electrolytic caps have been a standard for as long as I can remember to make a high-value, non-polar cap.

 

[blowerman]

Okey dokey. I’ve read all manner of conflicting ideas, in my quest for a solution, for making up nonpolar capacitors using additional diodes and all sorts. Problem is when you’re not an electronic whizz you never know what’s the best course of action. I don’t have that deeper understanding born of theory so reliant on the wisdom of others. Thanks for the wisdom!

 

[Tony Stewart]

3V caps won't cut it on +/-2.5V or more. If one reverse polarizes more than roughly 15%, maybe 10% for solid tantalum, it almost shorts out.

Get >=15V Caps

https://www.digikey.ca/en/products/detail/cornell-dubilier-illinois-capacitor/157BPA025M/5411201?s=N4IgjCBcoKwExVAYygMwIYBsDOBTANCAPZQDaIAzGAJwDs1AHCALqEAOALlCAMocBOASwB2AcxABfQjQbVEIFJAw4CxMiAAMLKSDgaALHOgK0WPIRKRyYGBoAEAVoBiLdl0ggAqsMEcA8qgAsrjo2ACu-LiShHr08orK5mpWujB2AGquIJzc3r4BwaERURKlQA

 

[blowerman]

According to the schematic the maximum output is 1.4 Volts peak to peak as a sine wave. These signals are then fed off to the filter boards, left and right where they are also getting transferred from analogue to digital before being sent to the microprocessor. Essentially they are the signals dictating the position of the imbalance from the reference input. At this level I think the 3 Volt spec is understandable and all the other units have those capacitors. I'm still intrigued as to what effect a different value of capacitance might have had on the output that someone had fitted with the 125uF @15 Volts capacitor. I presume there would be an effect on the lead or lag of the signal??

 

[ChrisP58]

According to the schematic the maximum output is 1.4 Volts peak to peak as a sine wave. These signals are then fed off to the filter boards, left and right where they are also getting transferred from analogue to digital before being sent to the microprocessor. Essentially they are the signals dictating the position of the imbalance from the reference input. At this level I think the 3 Volt spec is understandable and all the other units have those capacitors. I'm still intrigued as to what effect a different value of capacitance might have had on the output that someone had fitted with the 125uF @15 Volts capacitor. I presume there would be an effect on the lead or lag of the signal??

You don't choose the voltage rating of a coupling cap based on the AC voltage. It's purpose is to couple the AC voltage with no loss of it's value or timing (no lead/lag time difference). A properly sized coupling capacitor will have near zero AC volts across it.

But you typically have DC voltage that you're trying to block. The minimum voltage rating of the cap should be at least the max DC voltage plus the max P-P AC voltage.

 

[Tony Stewart]

The higher the voltage rating the better and perhaps much bigger, also depends on the ESR spec. Those old caps were possibly 10 to 20% tolerance. We don't know the tolerance on your unit

https://www.digikey.ca/en/products/detail/w%C3%BCrth-elektronik/865250157010/5727935

can you share the schematic?