Anyone know a reliable AC-DC standby powersupply? Transformerless? (Details in post)

Hey guys,

I'm building a project that would require a small powersource that supplies constant 12V to a relay that is switched via an optocoupler from another circuit. Technically something like the standby 5v in PC powersupplies.

I have been looking at transformerless solutions, the so called 'capacitor' power supply. (Like this: https://www.edn.com/design/power-management/4418393/AC-power-your-circuit-without-a-transformer) What I am not clear on, is whether such a design is suitable for being left 'on' for long periods of time if there is no load connected.

The idea for this part of my project is to switch AC mains with a relay driven by a standby powersupply, triggered by an external signal via an optocoupler. The board will carry mains voltage either way and is isolated from any other electronics via the optocoupler.

Would I be better off using a small transformer with simple diode-capacitor rectification instead? I'm not sure about how much power either solution would draw in it's standby state, and space is definitely an issue (hence why I'm thinking transformerless).

Could anyone share some insight on this for me?

What I need from the powersupply:
-The ability to be left on for weeks, months, maybe even years, mostly without a load
-100mA output at 12V (Technically only 45mA for the relay and whatever miniscule current the potocoupler's on-state may swallow, but I like to play it safe)


Thanks for any ideas guys

Circuitry that can provide 100 mA, is the same circuitry that will unavoidably draw some current when idle.

The schematic at EDN has a resistor network and a couple of zener diodes. A certain amount of wasted current passes through these continuously. But it's necessary for them to be there, to prevent the filter/smoothing capacitor (CF) from rising to 170V!

As for the question whether a transformer will draw greater idle current, it's a 50/50 chance.

In any case you want to maintain supply V, to keep your sensor in readiness.

BradtheRad said:

Circuitry that can provide 100 mA, is the same circuitry that will unavoidably draw some current when idle.

The schematic at EDN has a resistor network and a couple of zener diodes. A certain amount of wasted current passes through these continuously. But it's necessary for them to be there, to prevent the filter/smoothing capacitor (CF) from rising to 170V!

As for the question whether a transformer will draw greater idle current, it's a 50/50 chance.

In any case you want to maintain supply V, to keep your sensor in readiness.

Click to expand...

For a good reliability I would suggest to use a small rechargeable 12 V battery trickle-charged from a small low-cost plugin AC/DC 12 V power supply. Such combination will almost never fail even if the AC supply fails for an hour.
I am strongly against "transformerless" power supplies. Some are unsafe if their output is directly connected to AC line, switches generate too much noise and can fail with more probability than those with transformers.

Thanks for the replies guys. I realize that there's going to be current drawn either way, I just wanted to minimize it and save space as much as possible. Though due to some newly discovered facts I no longer need to minimize the size as much as I originally intended. I've decided to just slap on a small transformer and diode/cap rectify it. Simple and should be reliable enough in the long run.

It is ok to leave the circuit on.
It’s a standard capacitor
Power supply used in a lot of
Consumer items ie fridges and freezers.
It can even be made simpler than that.

The only thing you should do is as follows.

Do put a resistor in series with Cac (100R) to limit the serge current.
Do use a fuse in the ac line
Do double insulate you relay circuit.
Do use a suitable rated series capacitor. 400V AC

The transistor shown is working as a parallel regulator.
If you build it on Vera-board make the spacing
Between the high voltage components greater than 0.2inch
And cut away any un-used copper between the high voltage
Connections. Be-careful it can be dangerous.

And the reliability of the circuit is poor
The life expectancy of the main smoothing capacitor Cf is only
About 2000 hrs also you may find Cac fails quickly.
This type of power supply is the main reasons you
Why you see so many fridges and freezers on the Tip

It may best to use a 240V to 12V transformer and a bridge
And capacitor. or a cheap power adaptor.

Yeah the capacitor life expectancy is what had me worried as the caps in such a circuit seem to be worked much harder then in any transformer based solution. But since I gave up on space saving (the method I was going to use for a universal mount fell through as the power supplies it would have been mounted on seem to have different header spacings between different years of make) I'll just stick to a small pcb mount transformer. Even with this method the life expectancy of the filter cap after the bridge is questionable, though that's probably just paranoia from having thrown out so many caps made in the recent years.