Electronics > Beginners

How to make a small PSU cut off sharply when switched off?

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Chris Wilson:
Circuit is a PCB mount 0.5A 6V mains transformer into a FWB rectifier, then a 6800uF electrolytic then a fixed 5V regulator, finally a 4.7uF output electrolytic with a 0.1 uF ceramic in parallel. This is powered from the mains via one of the cheap Ebay RF switched mains socket adaptors. The output of the little PSU goes to a commercial 20 Amp solid state mains relay that switches the mains on and off to a huge Farnell 60V @ 50 Amp linear bench supply. This set up allows me to switch Farnell supply on and off from all around the house, which is handy given the usage I have for it.

My concern is will the zero crossing solid state relay "like" the fact that when I turn the RF controlled socket supplying the 5V supply its output decays quite slowly despite having 3 x 680 Ohm resistors across it. I am thinking the SSR Triac may be put into a semi conducting state, although it hasn't complained so far... Am I worrying needlessly? Is it even possible for the SSR Triac to go into semi conductance? 

If it needs addressing any ideas as to how to make the supply 5V output shut down snappily when the mains to the 6V transformer is turned off? The SSR turn ON lower threshold is 3.2V DC. Thanks.

bdunham7:
The SSR Triac is not going to 'semi' conduct, even though it is a semiconductor!  :-DD

However, if the sole purpose of your small circuit is to power the SSR control, then the primary filter capacitor is much larger than it needs to be.  If the SSR is like the ones I have, you can have up to 32V input, so there is no need for the primary filter cap or the regulator.  You don't even need a bridge rectifier technically, as just half-wave would do.  However, if you like, leave the bridge rectifier and one of the bleed resistors and everything should work fine.  You might need to change out that bleed resistor with a higher value, depending on what the output voltage settles at.

Cliff Matthews:
No need to worry since traics and scr's either conduct or not and stay that way until phase reversal (which in your case, is already happening 25 times a sec.) and there's no half on state way like a bjt.

Doctorandus_P:
There is probably no need to worry, as said before.

There is a slight chance that  your solid state relay is on for a few positive (or only negative) half cycles before it shuts of completely.
You can test this by putting a light load (indescesan bulb) or weird load (drilling machine, whatever) on the solid state relay, and a potentiometer in front of it, and then experiment a bit.

If your solid state relay does anything weird, you should build a schmitt trigger on the input signal to the solid state relay.
This can be made from a few discrete transistors and resistors, or with an opamp or even a ne555.

bob91343:
Why not use a power line operated relay with contacts in series with the output?

It's so simple that maybe nobody considered it.

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