Hey gang, I need a 10mf @350v Non Polarized Electrolytic, can I make my own by tying two 5mf caps together at the negative ends and using the + end for the connections, I mean will this work for the long run or do I need to get a real NP cap?

My intent here is to save money and get this job done without waiting on a mail order cap, my local supply house doesn't carry any NP caps.

Intentions are good, the calculations and idea is bad.
Order the right part.

Yes It will work, but in series I think you would need 2-20 mf caps to get 10 mf
bill

Right the capacitance gets cut in half and it doesn't matter if you connect plus to plus or minus to minus. I expect a couple 22uF would be close enough.

Here's way more info than you probably want to read http://electronics.stackexchange.com/qu ... ectrolytic

I've seen at least on store selling non-polarized electrolytics---I'll bet you a cookie that they are made using regular electrolytics in series.....

Fine, where do I find a 350 volt NP Cap?

I have read about making NP caps that way, but was wondering, if it sees AC would that mean one cap is operating in reverse polarity mode (momentary)? if so I know from exp that they will read partially shorted in which case the series math would not be right (not to mention the effect of longevity on the cap life). Just wondering out loud. I suppose it would be somewhat safer if the voltage rating was WAY high if you had to do this.

as far as getting large NP caps, I think I got some like you need from parts express, if mouser does not have them. the audio guys use them in cross overs a lot. Heck you may be able to get one at rat shack for that matter (again since its an audio thing and they still cater to the thump thump crowd).

I recently reviewed the theory behind using two nonpolar caps to make a nonpolar cap. Bottom line is that it works fine, and does not expose either cap to reverse polarity operation.

A couple misconceptions I had at first: I thought that since one cap would be operating reverse polarity at any given moment, and would basically conduct, then the usual rule for caps in series wouldn't apply, since only one cap would be active at a time. This turns out not to be true. What really happens is that a DC bias voltage builds up within a few AC cycles on the central junction between the two caps, keeping both caps biased with a DC voltage that prevents either one from going into reverse polarity. That means both caps are active at all times, so the usual rules for caps in series applies. That is, if you want to make a 10 uF nonpolar cap, you use two 20 uF caps in series.

Another misconception I had was that it would be a good idea to put a diode across each cap to prevent it from seeing any reverse polarity. Based on the discussion above, you can see this is not necessary.

It will not be easy to find a 350 V nonpolar cap, so it is perfectly fine to make one as you propose. In fact, even in cases where you can find a nonpolar cap on the market, making a homemade version with two polarized caps in series back-to-back is still a reasonable choice.

that would be an interesting check to have a comparison at different frequencys to check reactance between the two.

Tom, did your research address this as well? that is how the two types compare at different frequency?

a fun experiment, made up NP cap form Pcaps, and reg NP store bought cap, dual trace scope, input freq and amplitude and output amplitude, then vary the freq up and down and compare.

I wonder what the diff is in construction at least as far as the NP electrolytic types (I assume they are electrolytic based on small size). could they be simply two P caps in a single package?

In the construction of electrolytic caps, there is an "anode" foil and a "cathode" foil which have different chemical treatments. In a normal polarized cap, one of each is used. In a nonpolar cap, two anode or two cathode foils are used. So the basic construction is different.

Exactly. Making NP caps like this is done all the time, and is certainly industry recognized.

http://www.parts-express.com/pe/pshowde ... -932&DID=7

regular film cap prob too big anyway.

Here's the theory direct from Cornell Dublier's application notes:

"If two, same-value, aluminum electrolytic capacitors
are connected in series, back-to-back with the positive
terminals or the negative terminals connected, the
resulting single capacitor is a non-polar capacitor with
half the capacitance.

The two capacitors rectify the applied voltage and act as
if they had been bypassed by diodes. When voltage is
applied, the correct-polarity capacitor gets the full voltage.

In non-polar aluminum electrolytic capacitors and
motor-start aluminum electrolytic capacitors a second
anode foil substitutes for the cathode foil to achieve
a non-polar capacitor in a single case."
REF:
http://electrochem.cwru.edu/encycl/misc ... pguide.pdf

Well thanks all, I went ahead and bought two 22@350 caps and will make one up for now.
That Jantzen cap is just right and cheaper than what I spent on the two 22's but the picture they have is for a 4.7 mf, probably just a stock photo.

Anyhow, this is just for an old somewhat beat TV so a home made one will be fine.

Just curious though, in the unlikely event one of the caps shorts then AC would hit the other one and make it go bang?

Yes, one short will end up causing a second. One of the two is going to explode in that case, if the circuit driving the capacitor has a lot of current available.

WARNING:
Remember that if your original cap was rated for 350 volts AC ... you'll need a pair of much higher DC-rated caps to make the pair into a NON-polar A.C. cap.

When connected back to back the capacitance-value is half ( 20+ 20 = 10uf) ...that's true ... however the voltage rating stays the same.

The voltage rating only doubles when two DC rated caps are in series neg end to pos end.
ie: 20uf @ 160v DC ...in series with another .... 20uf @ 160v DC the combination will = 10uf @ 320v DC

However ... it is different when making a NON-polar (AC) cap by placing two back to back DC-rated caps together. :
If you wish to make a NON-polar (AC) cap by placing two DC rated caps back to back...
..... you'll need to use DC rated caps of nearly double the AC desired volt usage.
You should de-rate the two DC-rated caps by about 40-50% for AC usage.

So if you DE-rate your DC-rated caps by only 40%
and if you are using those two 22uf 350vDC rated caps BACK to BACK ...
....the resultant (AC) non-polar cap will be = 11uf @ 210 v AC
... so this combination will not work safely for a 350vAC environment.

Therefore you should use DC rated caps rated at 600+ volts DC

Two capacitors of 22uf @ 600vDC back to back ( when de-rated by 40%)
.... will create a non-polar cap of 11uf @360v AC

I don't know how much or what type of voltage is across this cap, I checked it briefly and it was about 200 DC, I had cobbled up a temporary one a few nights ago out of some 250v caps and they didn't explode so I think the 350v caps will be OK, it's been sitting here playing for a half hour so far with no problems.

This set needs a full recap to be 100% but the CRT (24") isn't very good so it's not going to see much , if any, use for a while.

The peak AC voltage is 1.414 times the RMS value (what most AC voltmeters measure). To read an AC voltage in the presence of a DC voltage, put a capacitor of something like .05 uf or larger in series with one of the meter leads and switch your meter to AC. A scope will let you see how an AC and DC voltage combine, but if you just add the AC and DC voltages you should have a usable figure.