[sdiy] how to double Si diode voltage rating

mcb, inc. mcbinc at panix.com
Tue Feb 26 03:04:01 CET 2008


On Mon, 25 Feb 2008, anthony wrote:

> If I wanted to use some Si diodes in a bridge to increase their voltage, 
> like say I wanted to use 200V BY251's in a bridge with a 300VAC secondary, 
> would putting 2 in series suffice to double the voltage they can handle? 
> That is, would I use 8 diodes, 2 in series in each part of the bridge? Or 
> would using these diodes in a bridge, singley give a bridge that can handle 
> 300VAC? When in doubt, is it better just to use more?

When I've seen this done it's been with high-value resistor in
parallel with each diode.  When reversed biased, there's no
guarantee that the voltage drop across the diodes will be
equal.  However, I don't have a rule-of-thumb for selecting
the value, I'd probably base it on reverse current of the diodes
and a largish multiplier.  (I think there was also a small-value
capacitor in there as well but perhaps I'm mistaken there...)

> If I wanted to increase current carrying capability would I put the diodes 
> in parallel? Like if I wanted to use 1N4007's (since I have a ton) and I 
> needed 2Amps?

That's generally okay but add some margin for variation in
characteristics, even in the same lot.  If you end up needing
three, four or more, stop.  One of them is going to be a weak
link...

--
Monty Brandenberg


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