Re: [Amps] Zero Crossing circuit

[R.Measures <r@somis.org>]

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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