How to make a small PSU cut off sharply when switched off? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 16, 2019, 05:32:56 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2019, 06:49:46 pm
content: The SSR Triac is not going to 'semi' conduct, even though it is a semiconductor!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.

author: Cliff Matthews
author_rank: Unknown
timestamp: June 16, 2019, 06:52:51 pm
content: 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.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 16, 2019, 10:03:03 pm
content: 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.

author: bob91343
author_rank: Super Contributor
timestamp: June 16, 2019, 10:17:20 pm
content: Why not use a power line operated relay with contacts in series with the output?It's so simple that maybe nobody considered it.

author: klunkerbus
author_rank: Unknown
timestamp: June 17, 2019, 12:42:49 am
content: As a variation on the line operated relay thought, I've had a couple of projects over the years where I added a line operated relay to switch a substantial discharge load onto the DC output when the AC was off.