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