[sdiy] how to double Si diode voltage rating

[Roy J. Tellason]

On Monday 25 February 2008 20:20, 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?

Not likely.

> 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?

I've seen this sort of thing done *way* back when,  typically when you're 
dealing with way higher voltages.  A circuit that comes to mind is a power 
supply for a ham radio linear amplifier,  output measured in KV.  They also 
had a high-valued resistor _and_ a capacitor across each diode in the string.  
I've heard of some folks that had success with doing without those parts,  
but I'd guess that they were also using parts that were pretty well matched,  
out of the same manufacturing batch or similar.

> 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?

The problem you'll run into with trying to connect them in series for more 
voltage is how the voltage is split across the parts (the purpose of the 
extra parts mentioned above).  Trying to use devices in parallel for more 
current is going to run into the same problem -- how are you going to 
determine how current is shared between them?  Some transistor power amps use 
small-value resistors in each emitter for that purpose,  but...

I've read fairly recently that almost *no* rectifiers are being made any more 
at under 1000PIV,  and that the ones you buy that are labeled as such are the 
higher-rated parts simply labeled differently.

Rectifiers are cheap,  and in terms of what happens when something fails,  I'd 
personally tend to design any power supply so that it'd be the most 
over-rated part in there.  I was buying units rated at 1KV/2.5A (under 
various numbers) in bags of 25 all the way back in the early 1970s and using 
them to replace pretty much of what I was running into out there.  For 3A 
parts I use the 1N58xx series.  For higher currents,  I have an assortment of 
bridge rectifiers around,  with all sorts of ratings,  some going up to 25A.

I too have a whole mess of 1N400x parts on hand here,  mostly salvaged.  I'll 
use 'em for things like across a relay coil to protect the transistor,  and 
smiilar stuff,  and I suppose I'll use 'em all up eventually.  Or not,  it 
doesn't much matter.  I sure wouldn't use 'em for much else,  though,  
certainly not anything critical.

-- 
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