Photo flash caps for the power supply?

[Original Burnedfingers]

I scored some photo flash caps out of some junked disposable cameras today. Most are 120mfd/330volts. I was wondering about a series parallel string of them to give me 120mfd/660vdc.

Anyone have experience with them?

[DigitalJunkie]

I have used them a few times on the bench,for experiments/prototyping,and they work fine.

I don't really have any experience with them long-term though. I'd assume they'd hold up just fine. (barring heat,excessive ripple,etc.)

[astouffer]

They are not reliable and are not made to handle the continuous ripple current of normal caps. I used them for a while until one decided to explode and let all the funny smoke out. There wasn't much left except for some paper wadding. They don't go quietly either

Using them on prototypes only sounds like a wise choice.

[Original Burnedfingers]

quote:
They are not reliable and are not made to handle the continuous ripple current of normal caps. I used them for a while until one decided to explode and let all the funny smoke out. There wasn't much left except for some paper wadding. They don't go quietly either

How old were the caps that you had a problem with? How long did you run then before one blew? Did you replace the cap and experience more problems?

I can site a similar experience with standard caps as I had one blow also and it sounded like a shotgun blast with the assorted mess.

The reason I started the thread is because GSI's information on their Dynaco 70 driver board lists Photo Flash Caps as the type being used in the power supply. It also looks like they paralled them for higher voltage capabilities. I was planning a similar power supply.

[Conrad Hoffman]

Photoflash caps are designed to dump their entire high voltage charge into nearly a short circuit in about a millisecond. I'd have to believe the connections are heavy duty and the esr is good, but who knows if they have special chemistry that has some other drawback in "normal" use. I'd love to hear some definitive word from a cap company, as I seem to have a large number of them in my junk box that I'm leery of using.

[HollowState]

I've always been of the opinion that photoflash capacitors are a poor choice for low frequency ripple in regular power supplies. They were designed to operate from high frequency charging supplies that ran around 10KHz or more. When used in 120 cycle supplies, I believe the useful capacitance is much lower then you think.

Yes, I know there are people that have, and still use them in modifications or home brews. But I believe they go out on a limb when doing so. They may hold up for a while, but it's not good design. Stop and think it through. So much capacitance in such a small package. Something is being sacrificed because there is no free lunch as someone once said. Also, how many successful and respected manufactures use them in their amplifiers? I know of none.

Victor

[Karsten Sømand]

FWIW, Broskie says:

Quote: Photo Flash: "What about audio; how well do these capacitors work with tube equipment? Fairly well is the quick answer, if a few key provisions are followed. The photo flash capacitor´s large volumetric efficiency betrays that these capacitors cannot tolerate a steady and heavy use. Using them near their rated voltage or rated temperature is sure to shorten their life. So too using them in a power supply, as the main filter capacitor, is sure to kill them.

So what is good about photo-flash capacitors? These capacitors are quite suitable for low-ripple power supply use; say for the first stage of an amplifiers RC filter, after being fed by a high-ohmage voltage-dropping resistor. Another use is as the output coupling capacitor in an OTL amplifier. In fact, one OTL manufacturer found that they worked perfectly in that position". End of quote.

The above written is a partly description about PSU´s etc. from Broskies Janus Shunt Regulator User guide.

Kind regards

Karsten

[astouffer]

How old were the caps that you had a problem with? How long did you run then before one blew? Did you replace the cap and experience more problems?

The caps were a few years old to begin with and in use for maybe a year as well. Three 160uF 330v were connected in series and had 750 volts across them under normal conditions.

After it blew I bought some normal 100uF 350v caps from mouser and so far so good. Look at some of the sites selling electronic surplus. All Electronics was selling large 450v snap-in caps for $2 each which normally went for over $11.

[tubelab.com]

Anyone have experience with them?

I had an exploding experience with some of them a few years ago. I believe that some of these (particularly those found in disposable cameras) were designed based on cost alone. As stated they were never intended for continuous ripple current. Putting a few in parallel insures that there will be ample stored energy available to guarantee a big bang when one cap shorts.

I have used the surplus caps from All Electronics with good results. Most of the 450 volt surplus caps on the market today were intended for use in UPS's so they will work in tube amps. I have 4 in series working at 1250 volts in my 845 amp. Use equalizing resistors when putting caps in series.

If you need a good source of cheap caps that are intended for abuse, rob the caps from dead PC power supplies. They are made to connect to the power lines (through a rectifier of course) and are properly vented so that they will usually just spew their guts rather than explode when they fail. Beware some older caps from PC supplies (PC and XT vintage) they weren't all vented, and there are still pieces of one imbedded in my ceiling (Lexan is your friend when playing with line powered things). Look for lines etched in the aluminum on the top. This is where the can is supposed to gently split when the internal pressure exceeds the venting point.

[bear]

Broskie's quote mirrors my experience and understanding on them... don't use them in the first section of a filter after a rectifier, in simple terms. Otherwise they are quite good, just keep the ripple off them...

_-_-bear