caps back to back

the classic arangement of doing that uses two diodes, one in series with each cap. this adds a few vulgarities to the circuit, and I wouldnt use it for audio!

Steve Roberts

Probably a silly question, but when you connect two electrolytic capacitors back to back to make a non-polarised capacitor does it matter if you connect the positive plates together or the negative plates? If so, why?

Thanks

-- DF

that can't be correct, one of the fews rules I know that is ALWAYS true,,,,,

a cap and a diode in series (with nothing else) is always wrong...

Mark

No it does not matter although I seem to always put the positives together. I would like to know why other responders think this will not work properly or will fail? I have done this many times with no problems in AC and DC circuits and as long as the voltage rating is not exceeded this will work the same as a NP cap. Jeff

the interesting question that we should get to is.....

if you place two caps back to back.....+ to + or - to -

is the value of the combination 1/2 each or not.....

i.e. take 2 10 uF caps back to back, is it 10uF or 5 uF

its not a simple question as it seems...

Mark

An aluminium electrolyte capacitor, depends on the forward bias, to keep the aluminium oxide layer on the plates. If you reverse bias such a capacitor, the oxide dissolves into the electrolyte. Depending on the current flowing, and the duration, this can cause the capacitor to go short circuit. You have probably 'got away' with it, because with AC, with the current limited by the other capacitor, there is not time for the layer to be completely destroyed. However you are 'living on borrowed time', especially if voltages are present for longer periods... It will also shorten the overall life of the capacitor.

Best Wishes

ive done this in a old tv years ago to replace a cap, i think i put the 2 negitves to the centre.

capacitors

connect

There are a couple of ways of connecting up polarized caps to make a bi-polar one:

1) Just put them in series back to back (doesn't matter which way). Usually an electrolytic cap will have a higher leakage in the reverse direction and act a bit like a diode (they used to have electrolytic rectifiers that relied on this action). This will tend to give a DC bias on the capacitor so both caps are biased in the correct direction. The effective capacitance will be that of two caps in series i.e. 5uF in your example. 2) As someone suggested you can augment the basic circuit with a couple of diodes to ensure that the caps don't get reverse biased - the diodes are not just put in series with the capacitors though, they are put across each cap in the appropriate direction - this improves the rectification to guarantee the bias on the capacitor. The effective capacitance will be the same as above.

kevin

I think the capacitance jumps to 10 uF any time a diode is forward biased, and falls to 5 uF any time both diodes are reverse biased. If a sine wave voltage is applied across the pair of capacitors, the diodes take turns being forward biased the first cycle, and then stay reverse biased most of the rest of the time, with just a brief swing into forward bias each cycle, to replace any charge lost to leakage.

Yes, this scheme is nonlinear at small swings. On the other hand, we generally use wet electrolytics as coupling capacitors only when their reactances are small enough to be negligible, because they're crappy otherwise--nonlinear and time dependent.

I usually prefer to use a big resistor to one of the power supplies from the middle of the capacitor string, to keep them both at a few volts of bias in the right polarity. That keeps their characteristics much more stable and makes them more linear, whereas the diode trick makes them less linear.

Usually one or other side of the cap string is low enough in impedance that this doesn't cause any loss problems.

Cheers,

Phil Hobbs

On 22 Nov 2005 20:51:51 -0800, "Kevin White" wroth:

When you use the diodes, two 10 uF caps in series will give you the equivalent capacitance of 10 uF, not 5 uF.

Jim

What happenes to the capacitance if the voltage applied is below the forward voltage threshold of the diodes?

Does the capacitance jump from 10 to 5 to 10 as the diodes conduct and then reverse bias?

Al

It doesn't make any difference. "Non-polar" caps that you can buy at premium prices are usually just two ordinary electrolytics in series. They _might_ be sort of "folded" together, where there's one cathode and two anodes, but you'd have to look that up.

If it's a critical signal path, you might consider that the negative (at least on the tubular 'lytics I've seen) is usually the outside foil, but I can't really imagine that making much difference at the frequencies you're probably talking about.

Have Fun! Rich

I put two 22uF/16V electrolytics (Mouser PN 555-16V22) back-to-back and in series with a 100 ohm, .1% resistor. I connected the plus terminals together for the first test and the minus terminals together for the second test. The test data show no difference between the configurations.

The sine wave oscillator (Wavetek model 142) was adjusted to provide 7.071 volts to the series combination. The Wavetek output was monitored with an HP

3400 meter. I used an HP 331A to measure the voltage across the 100 ohm resistor as well as the distortion. The voltage across the resistor was 5 volts when the frequency was 179.3 Hz (as measured with an HP5302A). The distortion was 0.4% (seems high to me, but, what the hell do *I* know?).

Other responders have told you that it makes no difference whether the positives or negatives are connected together. I confirm this. Also, other responders have said that the capacitance would be approximately half the single-unit value. Although mine seemed a bit low (especially considering that electrolytics usually run high in value), I will confirm this as well.

What I cannot say is whether the capacitors will be damaged after long-term use in this manner. I would agree with another of the responses in that it will most likely depend on your application. If you do this in a power supply filter, it may fail either soonly or eventually. In a low-power audio circuit it may be fine. As they say, YMMV.

Happy Thanksgiving to all.

John

if the centre point is charged past the maximum end-to-end voltage it is.

Bye. Jasen

Only while the capacitors are charging, the rest of the time (most of the time) its 5uf

Bye. Jasen

I agree that reverse biasing an electrolytic is a bad idea, but two back to back do not behave the same. What I have not tested is what the breakdown voltage actually is, if it is the sum of the two in series or is it a little more than the voltage rating of one of the caps but there is a strange diode like action going on that biases each correctly. This subject was discussed in sci elect repair about a year or two ago. Jeff

2 caps back to back to perform AC on DC caps gives you the voltage rating of one unit. since one cap will always leak with one polarity you then only have 1 cap that will not leak! thus the lowest voltage rating on either cap is what you want to use as your max AC Peak Voltage.

--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Just to beat a dead horse, adding diodes that are parallel to each cap, and which point towards the positive side helps to bias the caps more quickly. This may help out with situations where the system is turned on and off frequently. Putting a large negative bias across an electrolytic will cause problems over a period of time (although keeping a reasonable bias voltage across it undoes the effect over time). The diodes only conduct at the very top and bottom of the input cycle, keeping the voltage between between the caps high (or low, depending on if the + or - sides are connected).

I recently was building a boost SMPS and put a big smoothing cap in backwards by mistake. It works for a bit, but then starts leaking, more and more, while the current through it increases. The heat builds up due to current flow through the decreasing resistance, and it'll eventually pop (that is why they have pressure release elements in the case). Thankfully, I caught it before it blew. I've had that happen too, and there is an amazing amount of fiber inside of these things that gets all over the lab.

--
Regards,
  Bob Monsen

Mathematics compares the most diverse phenomena and discovers the secret 
analogies that unite them.
- Joseph Fourier