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

Sat, 6 Mar 2004 23:22:20 -0600

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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