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@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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