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Old 4th August 2008, 07:35 PM   #1
pjpoes is offline pjpoes  United States
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Default Best way to switch high current supplies

I've not spent a lot of thought on this in the past, but am now thinking maybe its worth addressing. What is considered the best, or what are some of the best ways to switch higher current power supplies dealing with mains voltage. For example the supplies in amplifiers which not only have a lot of voltage but often a lot of current surging in at startup. I've noticed that a lot of manufactured amplifiers appear to use a varistor on the switch. Is this a good idea to absorb some of the surge?

Another issue, what about the switch throws. Double pole or single pole?

This might be a minor or unimportant issue but I always worry about having a noisy AC line running from the back of the amp to the front and back again. I've thought about using a relay for switching instead, but admittedly, this would offer probably little improvement with a lot of extra expense and complication.
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Old 5th August 2008, 10:15 PM   #2
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Default Re: Best way to switch high current supplies

pjpoes said.
What is considered the best, or what are some of the best ways to switch higher current power supplies dealing with mains voltage. For example the supplies in amplifiers which not only have a lot of voltage but often a lot of current surging in at startup.

Joe said: In my amps that have 120,000 uF caps, 60,000 per rail at 50 volts, I use a 27 ohm sand resistor in series between the diode bridge and the caps. It limits the surge current upon application of the 120 volts AC.

The lights don't dim and the transformer does not moan in pain. 5 seconds later a 10 amp relay contact closes and shorts out the resistor which now fully connects the bridge to the caps. Done deal.


pjpoes said;I've noticed that a lot of manufactured amplifiers appear to use a varistor on the switch. Is this a good idea to absorb some of the surge?

Joe said:I don't like the idea of any kind of resistance in the high voltage AC side of the transformer which includes varistors or voltage dependent resistors. I use 14 gauge wire from the wall socket all the way to the transformer inputs. Heavy duty on/off switches too, 20 amp I believe. Shortest possible wire path here makes me happy.

Another issue, what about the switch throws. Double pole or single pole?

Excellent question. In the future you might use some kind of balanced transformer power so a double pole switch ,heavy duty of course, would be most excellent.

This might be a minor or unimportant issue but I always worry about having a noisy AC line running from the back of the amp to the front and back again.

Me too and thats why I put the AC on/off switch on the rear panel for shortest wiring and lowest noise pickup and use a Furman PL-8 Series II line filter to switch all my audio stuff. The on/off switch is designed to be used all day long in Pro applications so you know they can stand up to abuse.


I've thought about using a relay for switching instead, but admittedly, this would offer probably little improvement with a lot of extra expense and complication.

Yeah, not a good idea. Better off with switch in back and use some kind of external switching like a Furman.

Later, Joe
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Old 8th August 2008, 11:13 AM   #3
AndrewT is offline AndrewT  Scotland
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switches generally don't mind/deteriorate while closing.

It's the opening that causes the worst damage. The arc across the gap as the back emf tries to keep the inductor current flowing that melts/erodes the contact surfaces. An RC snubber is usually used and adequate to reduce this to acceptable levels. It also reduces the EMI that we hear over our Audio systems.

The biggest problem caused by that starting surge is the blowing of fuses. The easy fix is put in far too big a fuse. The correct solution is to fit a correctly proportioned soft start device. If the charging of capacitors is a problem then a slow start device should be used for this as well.
Note, that soft start and slow start are two different jobs requiring different solutions.
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Old 8th August 2008, 11:40 PM   #4
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AndrewT said:

Note, that soft start and slow start are two different jobs requiring different solutions.

Hi Andrew, In the spirit of I don't know everything,and I don't have a closed mind so I am open to new thoughts and ideas could you please explain the difference between the two?

My scheme of having switched resistors on the downside of the transformer has been working well for about two years now but I am always open to improvements.

Thanx, Joe
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Old 9th August 2008, 08:23 AM   #5
AndrewT is offline AndrewT  Scotland
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soft start is the short delay time (100mS to 300mS) and resistor limited primary supply to get the transformer flux up ready to start operating.

slow start is the much longer delay time (1S to 10S) and Thermistor limited secondary circuit that reduces the peak current that flows through the rectifier when first charging the smoothing capacitors.
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