Re: Photoflash Caps

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Original poster: "Jason by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Jimbo,

> The load is a 1mH coil and the repetition rate is 1khz The
> capacitance is 100 uF. I was using a low ESR tantalum from
> Panasonic, but after a couple weeks it developed significant DC
> current leakage, (and they are fairly pricey). The pulses were about
> 5uS wide.

> > >  You guys certainly seem to be the pulse cap experts. Am I
> > > incorrect in thinking a photoflash rated cap. should work if
> > > operated conservatively?
> > Not really - electrolytics tend to have a relatively high ESR.
> Well yes, but are not photoflash caps optimized for low ESR?

AFAIK photoflash caps are designed to send short duration pulses to a mainly
resistive (and slightly reactive) load, i.e. a flash tube. Although the
pulse itself is a high current one, the capacitor is not expected to
reproduce this pulse at any kind of rate faster than about 2Hz in small
hotshoe mount systems. The problem with dumping the energy from an
electrolytic through an inductive load is obviously ringing, which will kill
the cap. Without a large (and relatively fast) diode to protect your cap,
using an electrolytic flach cap will not work.

Lets work out the RMS current -

Rsurge = sqrt. L / C
Rsurge = sqrt. 0.000001 / 0.0001
Rsurge = 0.1 ohms

Ipeak = V / R
Ipeak = 15 / 0.1
Ipeak = 150A

IRMS = Ipk x Duty cycle
IRMS = 150 x 0.000005 (pulse width) x 1000 (rep. rate)
IRMS = 0.75A

0.75A is not too much to ask from a capacitor, provided that the capacitor
can provide the required surge current. Lets take a standard value of 0.3
ohms as the resistance of a standard low ESR tantalum (value from RS
components, for a 20V 100uF Low ESR tantalum)

Pdissapated = IRMS^2 x Resr
Pdissapated = 0.75^2 x 0.3
Pdissapated = 0.17W

This is not much to ask from a capacitor, and I would be surprised if it
would cause failure. The only option I can think of is to try the circuit
out with various different capacitors, and maybee different capacitor
arrangements, such as 2x 50uF capacitors in paralell to handle the current.
Maybee someone on the list can help you find a capacitor with a low enough
ESR to handle the current. My only last piece of advice is to make sure that
when you buy the cap, it can handle both the RMS and the peak currents.

I hope I have helped (and if I have got my math wrong here, someone slap my
wrist :),

Best Regards,

Jason
[UK Geek #1139 Rank G-2]
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