[HomeBrew] Need help: surge current resistors in bridge rectifier

Sun, 7 Mar 2004 16:32:24 -0500

In the last 20 or so years there is no need to use the RC network across
a series string of diodes as the manufacturing process is so much more
controlled and precise.
In fact studies have been published that contend that adding that
network can potentially cause problems.

We've come a long way since the 866's have beeen retired.

Carl
KM1H

----- Original Message ----- 
From: "Dick Foster" <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Sunday, March 07, 2004 12:22 AM
Subject: Re: [HomeBrew] Need help: surge current resistors in bridge
rectifier

> ** Please do NOT cross-post messages to multiple mailing lists on the
"To:" or "CC:" line of the e-mail message. **
>
> Cal,
>
> One of my old ARRL Handbooks (1969) gives a "rule of thumb" that the
peak
> surge current for a silicon diode can be approximated by 12 times its
DC
> current rating.  So, if your diodes are rated at 550 mA they should
handle a
> surge current of 6.6 A.  Roughly estimating your peak voltage at 1200
volts,
> then the current limiting resistance is R = V / I = 1200/6.6 = 182
ohms.
>
> Measure the DC resistance of the transformer secondary.  If it's say
200
> ohms or more, you are in fat city and need no limiting resistor.  If
it's
> less, subtract the secondary resistance from 200 (I'm giving it a
little
> safety margin) and put that resistance in one leg of the transformer
going
> to the bridge.  The power rating of that resistor should be estimated
(good
> 'ole I-squared R) and give it a safety factor of at least two.
>
> You realize that when you series electrolytic capacitors they should
be
> shunted by voltage equalizing resistors, right?  The value of the
resistors
> is 100 ohms per volt of the supply voltage, or about 120K-ohms in your
case.
> They should have a power rating of 2 watts (or 3 to be conservative).
>
> With 6KV diodes you certainly don't have to put any in series.  But if
you
> ever do need to series rectifier diodes, shunt each with a 470K
resistor and
> a 0.01 uF capacitor.
>
> Have fun and let us know how your project turns out.
>
> Very 73,
> Dick W5TA
>
> ----- Original Message ----- 
> From: <[email protected]>
> To: <[email protected]>
> Sent: Saturday, March 06, 2004 10:47 AM
> Subject: [HomeBrew] Need help: surge current resistors in bridge
rectifier
>
>
> > ** Please do NOT cross-post messages to multiple mailing lists on
the
> "To:" or "CC:" line of the e-mail message. **
> >
> >
> > Hi to all:
> > I am about to build my power supply for the high voltage section of
a low
> power amp. I have a Hammond 800 VCT 465 mA transformer. I have a set
of four
> 6kV, 550 mA diodes from a microwave oven I'd like to use on the ends
of the
> amp so I can full wave bridge rectify the transformer. The center tap
will
> be n/c. The output should be on the order of 1.414X800 VDC or about
1131 VDC
> across four 560 uF/400 VDC caps. However, experience has taught me
that the
> voltage will be higher than that.
> >
> > My question is that since this will be a capacitor input, I can
expect an
> initial surge current of high proportions due to the fact that for
some
> fraction of a second, the capacitors act like a short before they
start
> charging. I would like to put two series resistors on the input of the
> bridge. How value should I use for this power supply? Thanks.
> >
> > Cal N6KYR.
> >
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