-----Original Message-----
From: Gary Schafer
Sent: Monday, April 23, 2012 7:36 AM
To: 'Jim Thomson' ; amps@contesting.com
Subject: RE: [Amps] PIV requirement for identical, individual diodes
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Jim Thomson
> Sent: Monday, April 23, 2012 6:48 AM
> To: amps@contesting.com
> Subject: [Amps] PIV requirement for identical, individual diodes
>
> Date: Fri, 20 Apr 2012 13:39:14 -0400
> From: "Gary Schafer" <garyschafer@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
## good point. A buddy at work looked after a broadcast station in the
70's. Every
couple of years, they would get this big transient-spike in off the
commercial AC power
line. It would take out the tubes in the PA ! So it got past the filter
caps. At one of the telco's
I worked at up north, we had the commercial AC 3 phase power go screwy on
us on day.
It went off-on-off-on-off-on..and all in just 5 secs. After the event,
and all power back on,
it took out about 8 x big dc to dc power supplies, and fried all sorts of
other stuff. And we had
really good Josyln giant MOVs from line to neutral on the main panel.
## al those dc-dc supplies ran off UPS too. The strings of batteries are
all -52 vdc. The dc-dc
inverters were -52vdc on the input at 7A...and put out + 5vdc @ 40A. The
strings of batteries were
charged up with 3 phase rectifiers. The batteries were fine, and so was the
large rectifiers. The DC-DC
supplies at the tail end of all this mess didn't survive. All 8 of em had
the 40A breaker on the output side
trip open...go figure.
## One would think that the filter cap in a HV supply would kill any
transient..and real quick like, even
with low values of filter C. Perhaps if the rise time of the transient or
spike is fast enough, the cap won't catch it.
Sometimes the hv cap will catch it, and sometimes it wont. On paper, the
cap should kill it asap.
Jim VE7RF
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