Mains to 12v DC in small space

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Why not just use a "wall wart" power supply?

On a sunny day (Thu, 21 May 2009 14:02:42 +0100) it happened "Midge" wrote in :

Yo ucan either use a very small transformer with bridge rectfier, or, as you are probably isolated from the mains because of that relay, a capacitor in series with a resistor feeding a brigde with a zener. the capacitor limits the current, the resistor the inrush current if you switch it on at the moment the mains is maximum.

--- R -- C ---- ------------------------ + ~ + | |

Thanks for the suggestion but the cct needs to be within an enclosure rather than outside of it.

Why not just use a "wall wart" power supply?

Thanks Jan - my aim was to try and get the entire circuit in a space the size of a match box. 100mA transformers seem to be roughly 30mm LxHxD so will be a bit too big.

The relay isn't isolating from the mains - it's controlling another circuit.

Midge.

bp5041a/

It appears from this post that you may not need to be isolated from the mains, yet in another response it sounds like you may be. It makes a big difference in size and complexity.

Do you need to be isolated from the mains?

If not, Jan's cap, resistor and bridge circuit may just be the way to go.

Midge, I suspect Jan's point was that the relay coil is isolated from the contacts, so if you pick the right relay you do not need another isolation barrier.

Regards Ian

Ahhh I see what you mean!

I have not used these, but they look like they will do the job.

don

Sorry guys - I'm obviously not explaining myself that well, and my electronics knowledge is a bit basic too which probably doesn't help! I'll try and keep this short.....

What I'm trying to achieve is to disconnect a secondary mains circuit a predetermined (and probably adjustable) period of time after the primary mains circuit is switched on.

I'd normally have made up a simple little circuit with a mini transformer/bridge/fixed regulator to power a timer to switch the second circuit with a relay. i.e. everything isolated and any high voltages reasonably enclosed if anyone happens to poke around trying to adjust the timer pot with the power on. The issue is simply the lack of space in this particular case.

So it looks like Jan's suggestion would certainly help with the issue of space + it looks like there might be other devices out there for me to check out too.

Thanks.