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"Shunt" diodes with resistors? Answered

I'm making diode bridges to have a nice supply of DC. I'm making a high voltage bridge for a 7.5kv nst and a low voltage bridge for regular 120v wall outlet . My high voltage bridge im going to make with several 1N4007 but i read that they should be "shunted" with 10Mohm resistors im not sure what is meant by this. Does it mean resistor, diode, resistor, diode or does it mean diode and resistor, diode and resistor. The pic says it better than i do.  Also, do i need resistors for a bridge for 120v bridge?, im going to use the diodes from a fluorescent light power supply.


Jack A Lopez

Best Answer 8 years ago

Shunted means putting a 10 Mohm resistor in parallel with each diode, similar to the bottom diagram, of the two diagrams you have drawn.

The reason why you are doing this is because there is an assumption that for all the diodes in the two sides of the of the bridge that share one current, that somehow the voltage is going to divide equitably across each diode.  For example if you have a string of 10 diodes, then the peak reverse voltage across each diode is going to be (1/10)  times the  peak voltage of the AC supply.  

But I suspect that reversed biased diodes do not make good voltage dividers.  That is to say you cannot expect the same shared tiny reverse current to produce the exact same reverse voltage in each diode.  So that's what the resistors do.  They make a nice predictable voltage divider that puts 1/N th the peak reverse voltage on each diode. 

Don't forget the factor of 2^(1/2) when converting RMS voltages to peak voltages.  I think for 7500 VAC RMS this works out to a minimum of 24 1000 V diodes total.  That's 6 on each of 4 legs of the bridge.  I think.  It'd be a good idea to make sure you've got the math right before powering it up.

jaydenrJack A Lopez

Answer 8 years ago

Thanks, i have made a couple of spark gap tesla coils, which i seen can be much more crudely constructed than fine solid state electronics which im taking a liking to. any how, as an electronics novice i appreciate the help, and so do my diodes.