[Amps] PIV requirement for identical, individual diodes

[Gary Schafer]

> -----Original Message-----
> From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com]
> On Behalf Of Jim Thomson
> Sent: Monday, April 23, 2012 6:48 AM
> To: amps at contesting.com
> Subject: [Amps] PIV requirement for identical, individual diodes
> 
> Date: Fri, 20 Apr 2012 13:39:14 -0400
> From: "Gary Schafer" <garyschafer at comcast.net>
> Subject: Re: [Amps] PIV requirement for identical, individual diodes
> used in a bridge rectifier configuration?
> 
> The PIV across each diode in a bridge will be the peak voltage of the
> transformer voltage.
> 
> In your case with a 1600 vac rms transformer the peak voltage will be
> 1.414
> x 1600 = 2262 volts peak. Each diode in the string must withstand that
> amount of reverse voltage. You need 1.5x to 2x safety margin so each
> diode
> should be rated at 3400 volts or so. You may get by with 3000 volts per
> diode but 4000 volts would be better.
> 
> Just draw a diode bridge circuit and you can easily figure out how much
> voltage is across each diode. Consider that at any given time one side
> of
> the transformer is at ground and the other side is connected to the
> filter
> capacitor. Then look at what diodes are reversed biased.
> 
> 73
> Gary K4FMX
> 


> ##  In the old days, they would use 2X  for a safety margin.  If you use
> MOVs
> across the incoming 240 vac line, typ they wont fully conduct in some
> cases till the
> peak line V is aprx double.    These days, makers of commercial diode
> bridge
> assys  will use  3X  for a safety factor.  Since your no load B+ will be
> 2262 vdc
> I would use a bare min of 5 x 6A10  diodes per leg..and pref  7 x diodes
> per leg.
> 6A10 diodes are dirt cheap anyway.   With 5-7 x 6A10 diodes  per leg,
> 20-28
> diodes in total,  you will never blow em up.  In that FWB  config the
> diodes only have
> a 50 %  duty cycle anyway..even with a key down cxr.   In the above
> config, the bridge
> is good  for 12 A  CCS,  5-7 kv piv  and a 400A surge rating.
> 
 
> later... Jim  VE7RF

Hi Jim,

But the filter capacitor is ALWAYS connected across the transformer with a
full wave bridge. Any spike from the power line will always be shunted by
the large filter capacitor. No way for a large spike to reverse bias any of
the diodes.

73
Gary  K4FMX