[Amps] Zero Crossing circuit

Thu Nov 18 07:50:46 EST 2004

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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