[Amps] Zero Crossing circuit

[R.Measures]

Mark -- With a simple, current-limiting R / step-start DC relay=20
circuit, it does not matter at which part of the cycle the mains are=20
switched on.  The advantage of such a circuit is that it=20
rather-leisurely takes 60-cycles (one-second) or so to charge the=20
filter -C instead of a fast and furious 1/60 of a second.  For small=20
power supplies, the difference is small, but for QRO anode-supplies=20
it's not small at all.  For example, in my current project, the anode=20
PS filter stores about 4500J.  Charging this amount of Joules in 1/60=20
of a second would be a large order.  Instead, when the supply is=20
switched on, a 5=BD resistor is initially connected in series with the=20=

electric-mains to limit the worst-case inrush-current to around=20
50A-peak.  When the capacitor is charged to c. 66% of normal, a DC=20
relay shorts out the 5=BD resistor and normal operation can begin.  =
Since=20
1-second is about the length of time it takes for a thoriated-tungsten=20=

filament to reach operating temperature, a 1-second or so delay would=20
seem to be okay.
-  Another facet of this issue is what happens when one-half of a FWD=20
filter-C bank gets fully charged on the first half-cycle?  Since there=20=

is no charge on the other half of the filter, any external current that=20=

is drawn at this point forces reverse-current through the uncharged=20
half of the C-filter bank.  For electrolytic filter-capacitors, this is=20=

not good.  However, with a simple relay/resistor step-start circuit,=20
both capacitors are brought up more or less equally in small=20
increments, so there is no instant where one C is fully charged and the=20=

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