Stacked diodes in Avalanche mode

[Grumpy]

When I stack diodes in series the avalanche voltage changes very little.  I suspect that I need to equalize the voltage across them.

I found a rule of thumb for replacing diodes with other diodes that says to multiply the PRV (peak reverse voltage rating) by 500 ohms and to use a 0.01 microfarad cap across each diode.

So for a diode that has a PRV of 800 x 500 ohm = 400k ohms for the resistor.

Is this going to leak too much to hold off the hv in a pulser?

I'll give it a shot.

EDIT:
found this

http://74.125.153.132/search?q=cache:0KXbm3b-LtAJ:www.voltagemultipliers.com/html/FAQs/FAQ_diodes.html+avalanche+series+diode+equalize+voltage&cd=1&hl=en&ct=clnk&gl=us

Question: When diodes are used in a bridge or string configuration is it necessary to place equalizing resistors across the diodes? Also, is it necessary to place capacitors in parallel with the equalizing resistors to reduce noise and high voltage spikes?

Answer: Balancing resistors and capacitors are generally not required when using VMI diodes. Resistors in parallel can be used to help balance reverse leakage current (Ir) and reverse voltage (Vrwm) of diodes connected in series. Capacitors connected in parallel to resistors/diodes can be used to balance voltages across diodes during transient conditions. VMI's diodes are well-balanced for Ir and reverse recovery time (Trr).

Many of VMI's diodes have up to twenty junctions in series. The series junctions operate under many kinds of applications, and all sorts of conditions without compensating resistors or capacitors with no problems.

EDIT 2:

found this better explanation:

http://74.125.153.132/search?q=cache:PvdmMWnzsQ4J:lists.contesting.com/pipermail/amps/1999-July/011928.html+avalanche+series+diode+equalize+voltage&cd=7&hl=en&ct=clnk&gl=us

will try it now...

[Grumpy]

disregard this issue

i had the current switch on the power supply for my dc converter turned to 50 ma instead of 500ma and the power supply was nose-diving at about 340v.

anyway, I tested several differnt types of diodes:

1n4002, 1n4004, avelanche all over the place with most over 1000v

1n5??? pwer diodes av at about 800v

1n2071's are about 800v

so, you have to test each one if you use them in an avalanche circuit and they hold voltage off.  If just for sharpening then it may not matter.

I have tested a few hundred 2N5551's and they all avalanche at about 240v.  Just reject any that do not go into full avalanche which in my case was about 15% of them.

[poynt99]

Thanks Grumps. That's some good info!



.99

[Grumpy]

Thanks

I'm now using a 120v neon bulb as an indicator when over 1kv.

the scope is what clued me in to the power supply.  PS went down like it was short but there was nothing on the scope, so could not be avalanche.

I am reluctant to build 1.5kv pulse circuits but what the hell...will make for a good show and tell.

[the_big_m_in_ok]

When I stack diodes in series the avalanche voltage changes very little.  I suspect that I need to equalize the voltage across them.

This experimenter is using resistors in series with his doides for a specialized application:

http://www.capturedlightning.org/hot-streamer/stk/tc/cascade.htm

I have seen commercial schematics with high value resistors paralleled to the diodes, but the theory of this is something I should study more.