Capacitor? Looks like a battery to me

OptiKen · Oct 24, 2016

I'm trying to resurrect a couple of old flash units whose capacitors have completely dried out. It appears that today's capacitors have little in common with those of yesteryear. A Canon flash I have uses a 22.5V battery and a 100uf / 25v capacitor that looks like a AA battery. When searching for a replacement, the capacitors I find are like the ones you find on circuit boards - little 6mm x 7mm units with two wires coming out of the bottom.

Are these interchangeable? If they are, is the long or the short wire the positive pole?

Leigh B · Oct 24, 2016

Electrolytic capacitors of current manufacture work fine in vintage equipment.

The value standards have changed. Use the closest modern value.
For example a modern 47mfd cap would replace a vintage 50mfd cap, modern 22mfd for old 20mfd, etc.
Flash electrolytics have values in tens or hundreds of microfarads, abbreviated mfd.

Your new cap should ALWAYS have a voltage rating equal to or greater than the original part, no exceptions.

The package with both leads coming out the bottom is called "radial".
A package with one lead out each end is called "axial". Either of these will work.

Polarity marking can be confusing.
Vintage parts normally had a plus sign next to one lead, maybe printed on the side or molded into the plastic end cap.

Modern practice is to identify the negative side with a bunch of minus signs running down one side next to the lead.

There are many surface mount packages, described as SMT. You do NOT want those, as they're quite small.

- Leigh

shutterfinger · Oct 24, 2016

+1 for post #2.

flavio81 · Oct 24, 2016

Modern capacitors are much smaller than capacitors of 40 or more years ago, for the same capacity (microfarads) and operating voltage!!

You should be able to use any modern capacitor, if it is polarized then you need to check polarity on installing.

AgX · Oct 24, 2016

I too have such battery-like capacitator in one consumer flash, but never found such type of capacitator in any other electronic device of the past.
What was the idea behind those quick-exchange capacitators?

Leigh B · Oct 24, 2016

AgX said:
I too have such battery-like capacitator in one consumer flash, but never found such type of capacitator in any other electronic device of the past.
What was the idea behind those quick-exchange capacitators?

Plug-in electrolytics were quite common in military equipment.

The reason is that electrolytics are and were very unreliable components.

Given that any particular one might fail, making them plug-in meant a user could replace them.
With solder-in packages, you have to dis-assemble the product to remove the bad one and install the new one.

- Leigh

Luckless · Oct 24, 2016

General safety reminder: Capacitors are NOT friendly pieces of technology, and should be handled with care after careful study of electronics and related safety.

The size power range you're likely to find in all but some of the largest high powered flash gear is unlikely to be able to outright kill a human, but they still pack more than enough punch to potentially stun and stagger you. An experience electrical engineer I know had an adventure in the hospital after accidentally shorting a small cap and bashing his head on the corner of his work bench when he then tripped.

And don't forget that capacitors can also build up a charge over time even if not plugged in. Always follow proper safety protocols with handling and discharging.

fstop · Oct 24, 2016

Luckless said:
And don't forget that capacitors can also build up a charge over time even if not plugged in. Always follow proper safety protocols with handling and discharging.

You want to explain that Sparky.

flavio81 · Oct 24, 2016

fstop said:
You want to explain that Sparky.

I think he means that capacitors can still hold a charge over time.

Luckless · Oct 24, 2016

fstop said:
You want to explain that Sparky.

Were you referring to explaining capacitors building up charge, or explaining proper safety protocols?

Due to general liability my explaining proper safety protocols here will be limited to "Go study and research the subject in a setting more suitable than a photography forum". I've had enough people come back yelling at me "Why didn't you warn me X could happen!? I did X, and Y - a very bad thing, happened" over the years, and have no desire to add to the list.

As for building up charge while not plugged in: There are a number of factors at play.

If you rapidly discharge a capacitor to zero and then stop discharging it, it can retain enough energy in the material to reform and build up a charge again.
Some designs may slowly pick up a charge through field effects and the like.
And yes, of course, a capacitor can hold a charge for ages. Just because one was sitting on a shelf for a decade doesn't mean it is dead. At the same time, just because you "fully discharged" the one you were working on the day before does not mean it is still fully discharged when you go and pick it up today.

Leigh B · Oct 24, 2016

Luckless said:
And don't forget that capacitors can also build up a charge over time even if not plugged in.
Always follow proper safety protocols with handling and discharging.

fstop said:
You want to explain that Sparky.

Self-charging is a very well-known and accepted phenomenon, with significant safety implications.

Large electrolytics (when stored) normally have a shorting wire connecting the two leads or posts to prevent this.

- Leigh

Prof_Pixel · Oct 24, 2016

Leigh B said:
Large electrolytics (when stored) normally have a shorting wire connecting the two leads or posts to prevent this.

Usually, it's a high resistance bleeder resistor.

Leigh B · Oct 24, 2016

Prof_Pixel said:
Usually, it's a high resistance bleeder resistor.

No, usually it's a wire. Resistors cost money.
Multiply that by thousands of caps in storage and the bux add up.

High-value resistors are used in products as bleeders to discharge caps and prevent them from re-charging spontaneously.

- Leigh

shutterfinger · Oct 24, 2016

A capacitor is two conductive plates separated by a dielectric.
Electric current is the movement of electrons from one atom to another. The fewer the electrons in the outer (valence) ring of an atom the better a conductor it is.
Air moving across the contacts can cause a charge to build up.

Prof_Pixel · Oct 24, 2016

Leigh B said:
caps in storage and the bux add up.

I missed the "in storage" part of your comment.. and was thinking "in use".

AgX · Oct 24, 2016

Luckless said:
The size power range you're likely to find in all but some of the largest high powered flash gear is unlikely to be able to outright kill a human, but they still pack more than enough punch to potentially stun and stagger you.

You mix up flashes.

The OP does not have it about an electronic flash with its high-voltage/high-capacity capacitator, but about the minuscule capacitators used in incandescant flash triggering.

fstop · Oct 25, 2016

flavio81 said:
I think he means that capacitors can still hold a charge over time.

I have a background in electronics, I wanted to see if he could explain it and he didn't.
Its called dielectric absorption.

flavio81 · Oct 25, 2016

fstop said:
I have a background in electronics, I wanted to see if he could explain it and he didn't.
Its called dielectric absorption.

Didn't know that. Amazing!!

Capacitor? Looks like a battery to me

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