Soft Start, was Re: [Amps] Zero Crossing circuit

[Tomm Aldridge]

Also NTC surge suppressors are a good choice for low to mid power soft 
start.  See 
http://www.ametherm.com/Inrush_Current/transformer_case_study.htm


Tomm Aldridge wrote:

> Soft starting with a phase angle controlled TRIAC works great for 
> capacitive input filter PS inrush limit. Designs for single and 3 phase 
> abound on the internet.  My biggest of this type was actually done on a 
> transformerless boost converter operating from 3 phase.  Used 3 diode - 
> SCR blocks and applied 100kHz drive (called picket fence) through 
> trigger transformers.  The drive was generated by a slow ramp modulated 
> phase angle controller timed for each phase.  Current sense would 
> prevent the charging of a shorted capacitor.
> 
> Less complex but as effective is a series resistor that is switched out 
> of the line with a voltage sensing relay.  The capacitor loads the 
> output while charging so the voltage on the primary is less than 
> necessary to fire the relay until the cap is pretty well charged.  At 
> 90VRMS or so, teh voltage sensing relay fires and shorts out the series 
> resistor applying full line to the  primary of the xfmr.
> 
> The most "elegant", but also most massive is a saturable reactor with 
> the control winding and a series R acting as the voltage sense.  This 
> with a hard switched voltage sense relay to shunt it out of the circuit, 
> makes a great and bullet proof current limiter.  A 3 phase 5kW unit 
> weighs in at 200 lbs or so but is a joy to observe and listen to.
> 
> jeff millar wrote:
> 
>> 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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