"Making" NP electros" question

[Aharon]

The FAQ states that to make a non polarized electro(NP) you should put 2 caps back to back connected by the neg side,fine,I've done that before but never occured to me to double the value cause they are not really in series but the FAQ said to double the value for example for a 1uF NP use two 2uF back to back.......is that accurate?,I ask because, for example, the PT-80 could be sensitive to the value of those 10 NP caps in the circuit and I could not find any 1uF film caps at my store.
Thanks
Aharon

[Marcos - Munky]

I think you need to double the value, because they are caps in series. Did you looked for 1uF polyester or ceramic caps?

[travissk]

Yep, the FAQ is accurate.

Caps and resistors are sort of opposites of each other -

Resistors in parallel decrease resistance: R1R2/(R1+R2)
Resistors in series add resistance: R1 + R2

Caps on the other hand are the other way around. If you put them in parallel the capacitance adds, C1+C2, but in series they divide.

So, to get a 1uF, you need 2 * 2uF's in series: C1C2/(C1+C2) = 2*2/2+2 = 1uF

Electronics is full of such "yin/yang" relationships

[Aharon]

Hi Munky and Travissk.Yes I looked for 1uF films inmy neighbourhood with no luck so I'm going to use electros back to back.

Travissk,does back to back count as series?
Aharon

[Travis]

I'll pretend you were talking to me.  Yes, two caps back top back are in series.  Your two 2uF caps will be worth 1uF when placed in series.

[Aharon]

Great,thanks a mill.
Aharon

[R.G.]

I've done that before but never occured to me to double the value cause they are not really in series but the FAQ said to double the value for example for a 1uF NP use two 2uF back to back.......is that accurate?,

You are correct. You *don't* double the value, as I have recently found out. It's only recently that I found an old text on making NP caps. What happens is that an electro cap in reverse conducts like a diode. So whatever capacitance exists is always in the section of the NP cap that's correctly polarized. Hence, there are never two sections actually in series.

Except... (you knew there would be one of these, right?)

There is a slight voltage region right around 0V where an aluminum cap does not forward conduct, similar in concept but not mechanism to the forward voltage needed to turn a diode on. In that region, both caps do act in series.

Yep, the value changes depending on signal level.

For that reason, it is a bad idea to use NP caps wherever you depend on the capacitance value to tune something. In fact, electro caps have bad enough tolerances and drift over time that I don't like using them anywhere if I have to depend on the value being correct. I use them when I just need enough capacitance, not a critical value.

This is one of those things that's hard to dig out. I thought otherwise until recently.

[Aharon]

Thanks RG.I instictively felt they were not in series but needed help making sure.
Do you think that all those 1uF film caps in the PT-80 are critical or could I just go with the back to back electros?.
Thanks again for any help
Aharon

[R.G.]

Do you think that all those 1uF film caps in the PT-80 are critical or could I just go with the back to back electros?.

I'll look at the schemo, but there's an easy test:
1. if you depend on the tolerance of the cap being 10% or better, don't use either an electro or an NP; use film or find another way if the value needed is too huge for film. Note that polypropylene can be had in values up to 20uF, albeit in huge packages.
2. if the value of capacitance absolutely requires an electro (polarized or NP) then calculate or measure the DC voltage across the cap. If the voltage ever becomes reversed for the largest signal that will pass that point, use NP. If it never becomes reversed, use polarized.

[Aharon]

Thanks.Here's the schem for a quick peek.

http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=80&op=page&SubMenu=

Aharon

[R.G.]

All of the 1uF caps that have one end going either to ground or to a resistor to ground *except that 1uF from the emitter of the transistor* can be made polar, I think. The ones that have both ends tied to some voltage above ground I can't tell. I would use 1uF film on those instead of 1uF NP if there's room on the board.

[Aharon]

That helps a lot ,great ,thank you again RG.
Aharon