[Amps] Zero Crossing circuit

Thu Nov 18 11:09:22 EST 2004

 For what it's worth, I myself have a variac in line with the xfrmr. I don't
mind bringing up things slowly on my own. :-)
 73, CUL...

 Bob de K2KI
 k2ki at starc.org

> ----- Original Message ----- 
> From: "Will Matney" <craxd1 at ezwv.com>
> To: <amps at contesting.com>
> Sent: Thursday, November 18, 2004 7:50 AM
> Subject: Re: [Amps] Zero Crossing circuit
>
>
> > As Rich mentions, the reverse current on 1/2 of the filter c,
> > electrolytic capacitors act similar to rectifiers in their operation
> > (polarization). Reverse current high enough could cause the capacitors
> > to fail prematurely. Another thing is when starting a loaded transformer
> > up this size it's not to healthy on it either. Hence, transformer life
> > can be increased by the use of a step start circuit. This is due to the
> > magnetizing current which is directly proportional to the electrical
> > current. If you don't hear the big "thud" your better off when the
> > switch is flipped =)
> >
> > Will Matney
> >
> > R.Measures wrote:
> >
> > > 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 5� 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 5� 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 at 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 at 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 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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> > >>
> > >
> > > Richard L. Measures, AG6K, 805.386.3734. www.somis.org
> > >
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