[Amps] Zero Crossing circuit

[jeff millar]

Ed...

I've used zero voltage switched solid state relays twice in 2KW 
amplifiers, one at 432 and one at 144 MHz.  The 432 amp included the 
normal step start circuit, but once I noticed that the zero voltage 
switching feature, I tried it without a step start in the 144 amp.  The 
circuit switched on with very little drama, the lights blinked about the 
same amount as when transmitting at full power.

One design note on high voltage power supplies suggested that extremely 
high surge currents can damage electrolytic capacitors.  These surge 
current maximizes when the turn on relay or switch happens to make 
contact at the peak of the line cycle.  The zero voltage switching 
function reduces the surge currents to approximately the same amount as 
what it takes to recharge the caps at the peak of the line cycle. 

These amps showed no signs of RF getting into the switches, including 
during operation at a big hilltop VHF/UHF multi-multi contest  group 
with 1.5 KW on all the bands.  But for HF amps with a lot more RF 
currents flowing around the shack, it's probably best to run the control 
leads through an EMI filter similar to what you find on a AC power inlet.

One caution, the off impedance of these switches doesn't look that good 
in the spec sheet, leading to the conclusion that an "off" power supply 
could have 10 to 100's of volts on the output.  I didn't see this, the 
voltage didn't rise above a volt or so, but maybe unit to unit variation 
or operation at high temperature could cause more leakage.

hope this helps,

jeff, wa1hco

Ed Stallman <n5blz1 at houston.rr.com> writes:

>>Wondering if anyone has used a zero crossing circuit on a high voltage
>>supply to reduce inrush current at startup? would this work for soft start
>>or would it just assist?
>>Ed N5BLZ
>>
>>
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