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Author Topic: Equalizing resistors needed for HV diode string?  (Read 2047 times)
N9JG
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« on: February 05, 2006, 11:25:57 AM »

Recently I have replaced the HV capacitor and diode strings in my L-4B. In addition to replacing the caps, the original equalizing resistors across the caps were each replaced with 100K, 2 watt ceramic composition resistors.

The original HV diode string along with its equalizing bridge resistors were removed. The original diodes were replaced with 1N5408 diodes, but following a source (which I can no longer locate) I did not replace the equalizing resistors. The source had stated that modern diodes do not require the diodes to be bridged with equalizing resistors.

The work was performed in August, 2005, and the amplifier has been working fine ever since. But I have this nagging feeling that the diodes do need to be bridged. Should I add equalizing resistors, or are my fears groundless?

John, N9JG
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W5HTW
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« Reply #1 on: February 05, 2006, 01:01:27 PM »

My own personal recommendation would be to add the resistors.  Probably modern diodes are sufficiently equal in specs, and sufficiently reliable, it may be only an extra step in psychological protection, but I think I'd do it.  It isn't expensive or difficult, so why not?  

Ed
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N9JG
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« Reply #2 on: February 05, 2006, 01:08:08 PM »

Thanks for your comments. On newsgroup  rec.radio.amateur.boatanchors, two different people gave almost identical responses, as shown below.

No need to use the equalizing resistors.  Modern diodes will "zener"
when reverse biased beyond their rated PIV.  This will distribute the
inverse voltage across all the diodes.  Just be sure to use enough
diodes in the string.
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HA5RXZ
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« Reply #3 on: February 05, 2006, 11:46:27 PM »

Add the resistors. They may not be needed with modern diodes (and I mean MAY) but with high voltages a belt and braces approach does no harm.

HA5RXZ
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N9JG
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« Reply #4 on: February 06, 2006, 06:15:58 AM »

I guess I should have done a bit more research before I posted my question. According to the 2005 ARRL Handbook (page 17.9) "shunting resistors are generally not needed across diodes in series rectifier strings. In fact, shunt resistors can actually create problems because they can produce a low-impedance source of damaging current to any diode that may have reached avalanche potential."

The analogy to "belt and braces" does not appear to hold.
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