SCR as a PS switch

[rljones]

I have a question hopefully someone here can answer. I read the following regarding a manufacturer's modification of a power supply (PS) switch for their amplifier (which is no longer being produced):

"The power switch, which was previouly a triac in the primary side of the power transformer, has been replaced by [...] an SCR-based circuit, [which] is positioned in the power supply after the diode bridge. This provides for soft turn on and better long-term reliability."

This would seem like a good thing for a power supply for amplifiers, but how does one do this?

On a related topic, does anyone have experience using Sidactors across the primary of their transformers for surge protection?

[subwo1]

I don't know much about sidactors, but I would venture to suggest turning the SCR on and off with an opto-coupler.
The opto-coupler would have to have delayed turn on so that it must wait for the zero-crossing of the AC. The easiest way to do it is to use an opto-coupler with a built-in zero-crossing detector.

[R. McAnally]

Originally posted by rljones
"The power switch, which was previouly a triac in the primary side of the power transformer, has been replaced by [...] an SCR-based circuit, [which] is positioned in the power supply after the diode bridge. This provides for soft turn on and better long-term reliability."

Sound like they are using it to "gradually" apply rectified AC (pulsating DC) to the capacitors. SCR's have the ability to chop the beginning portions of the DC pulses, by an amount probably determined by some sort of controller circuit in that design.

Im with subw1 on keeping the control on the primary side. Using a triac or relay in combination with dropping resistors on the primary side would be a much more efficient way of applying a soft turn on, since current is less and the voltage drop of the triac would have less affect on overall rail voltage than an SCR in the secondary.

[paulb]

Originally posted by subwo1
...I would venture to suggest turning the SCR on and off with an opto-coupler.

Actually, you can't turn off an SCR directly, you must remove its drive and then wait for the current to drop close to 0 before it turns off. This may make it tricky to control.
I'm with R.McN, a switched-in resistor on the primary is much simpler and likely just as effective.

[vbd]

I suggest the SCR circuit was chosen because it is cheaper to manufacture than a conventional soft start !