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Re: [Amps] Zero Crossing circuit

To: wb8jkr@juno.com
Subject: Re: [Amps] Zero Crossing circuit
From: R.Measures <r@somis.org>
Date: Thu, 18 Nov 2004 02:58:03 -0800
List-post: <mailto:amps@contesting.com>
Mark -- With a simple, current-limiting R / step-start DC relay circuit, it does not matter at which part of the cycle the mains are switched on. The advantage of such a circuit is that it rather-leisurely takes 60-cycles (one-second) or so to charge the filter -C instead of a fast and furious 1/60 of a second. For small power supplies, the difference is small, but for QRO anode-supplies it's not small at all. For example, in my current project, the anode PS filter stores about 4500J. Charging this amount of Joules in 1/60 of a second would be a large order. Instead, when the supply is switched on, a 5Ω resistor is initially connected in series with the electric-mains to limit the worst-case inrush-current to around 50A-peak. When the capacitor is charged to c. 66% of normal, a DC relay shorts out the 5Ω resistor and normal operation can begin. Since 1-second is about the length of time it takes for a thoriated-tungsten filament to reach operating temperature, a 1-second or so delay would seem to be okay.
- Another facet of this issue is what happens when one-half of a FWD filter-C bank gets fully charged on the first half-cycle? Since there is no charge on the other half of the filter, any external current that is drawn at this point forces reverse-current through the uncharged half of the C-filter bank. For electrolytic filter-capacitors, this is not good. However, with a simple relay/resistor step-start circuit, both capacitors are brought up more or less equally in small increments, so there is no instant where one C is fully charged and the other C is fully discharged.
- cheerz


On Nov 17, 2004, at 5:31 PM, wb8jkr@juno.com wrote:

 I think the acquired advantage with the zero
crossing is then there isn't a chance the power
supply will be turned on at the AC cycle peak.

Mark WB8JKR


On Wed, 17 Nov 2004 20:03:55 -0500 jeff millar <wa1hco@adelphia.net> writes:
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@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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Richard L. Measures, AG6K, 805.386.3734. www.somis.org


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