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[OM] Re: T20 charges but will not fire..(flash caps)

Subject: [OM] Re: T20 charges but will not fire..(flash caps)
From: Joe Gwinn <joegwinn@xxxxxxxxxxx>
Date: Wed, 16 Jun 2004 22:21:23 -0400
At 3:32 AM +0200 6/17/04, Listar wrote:
>
>Date: Tue, 15 Jun 2004 22:56:27 -0700 (PDT)
>From: Tim Hughes <timhughes@xxxxxxxx>
>Subject: [OM] Re: T20 charges but will not fire..(flash caps)
>
>Joe wrote:
>>>
>I bet that 20-ohm resistor was quite inductive, and not rated for 300
>volts, and it sparked over.  Right through the epoxy conformal coat.
><<
>Actually it was a non-inductive planar metal foil resistor on a 
>modest heatsink. If you calculate the pulse power rating it is not 
>surprising it "exploded". Current was likely >150A.  Pulse rating on 
>wirewounds is often much better than foil, although if they are 
>epoxy rather than vitreous enamel as in old style high power WW 
>resistors, the epoxy is damaged by the 50J pulse of heat. The 
>vitreous enamel can go to many times the temperature of epoxy. 
>Typically a 20 ohm,20W wire wound would have had a spaced wind on a 
>ceramic former, so might not have broken down coil to coil.

It didn't occur to me that you were not using a wirewound.  In a 
smaller resistor, I can well believe that the heat pulse alone would 
do it.  I used to play around with exploding wires -- who needs 
epoxy, when we can vaporize copper?

>But basically, it was not a smart thing to do!

But, dramatic...

The point I would make to people making their own flashcap discharger 
is that resistors have voltage ratings, not just power ratings, so it 
may be necessary to use two or more identical resistors in series to 
prevent sparkover.


>  >If you don't have a suitable resistor hook up a voltmeter across the
>>>I would add one thing:  Leave the shorting cliplead in place, as such
>capacitors are subject to dielectric absorption. 
><<
>The electrtolyte caps don't seem to suffer nearly as badly from 
>dielectric sorbtion as oil filled paper etc. Rebound voltage seems 
>more like 10-15V.

If you've measured it, I'm happy.   In large flash units, one still 
needs to be careful, even with electrolytic capacitors, from the 
warnings I've read in some service manuals.  I never felt like 
testing the issue, and just did as the manual said.


>If you do a full dump just before you switch off the power, start 
>voltage will be the much safer 50-80V tube voltage, which will 
>discharge much quicker.

Yes.  But with less drama.


>  >>
>Many flashes have a drain resistor across the flash capacitor,
>precisely to dissipate such absorbed charges.
><<
>Most small flashes like the T20 have such large native current draw 
>from the high voltage, this is not strictly necessary. The better 
>designs switch in the discharge resistor when power switch is 
>switched off.

Yes.


>One series of flashes I have investigated that are much more 
>efficient are some of the Nissin Potato masher style Flashes. These 
>really take a long time to self discharge, unlike the equivalent 
>Metz flashes.

Nissin?  I haven't bought a flash in quite a long time, and hadn't 
heard of them.  I guess I need to wear flash units out faster.


>The point and shoots are another thing all together, as they must 
>conserve flash power to make the Li batteries last.   Linear 
>Technology makes a rather nice IC for making efficient flashes for 
>digi/point and shoot cameras, although it only works really well 
>with higher voltage batteries. (5~6V) It regulates the output 
>voltage, without needing a resistive divider for feedback, which 
>means capacitor discharge can be very slow in a careful design.

All the better to bite the incautious.

Joe Gwinn

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