Smallest, cheapest isolated power supply

[cowana]

I want to build a small module to be connected inline with the power cable to my audio amplifier.

It will contain an MCU, and monitor the incoming audio signal. If there is a signal, it will switch on the amplifier with an SSR. If there has been no signal for 10 minutes, it will switch off the amplifier.

So, this project needs a power supply, with the following specifications:

Input voltage: 240v AC (UK mains)
Output voltage: between 3.5v and 5.5v DC
Output Current: <20mA (probably <5mA)
Must be isolated/floating, as it will be connected to audio ground

If it wasn't for the last condition, I would use a capacitor/resistor voltage divider and a zener diode to regulate.  However this wouldn't be isolated.

I could use a small transformer (such as http://uk.farnell.com/block/vb1-0-2-9/transformer-1va-2-x-9v/dp/1131623), single diode rectifier, reservoir capacitor and a zener diode for regulation - however this is quite a large solution and I'd like it to fit in a very small case.

Any other thoughts or suggestions I've missed would be appreciated!

Cheers,
Andy

[Chet T16]

Smash open a phone charger?

[madires]

I use those small Block transformers too for projects with low power consumption. With 22mm X 32mm X 27mm they are quite small, IMHO. And the best feature is that they run 24x7 for 20 years without any problem (home automation). I doubt a good quality mobile phone charger would be smaller or as reliable as the transformer. And the transformer isn't expensive either (some weeks ago I paid EUR 2,40 for 1.5W 6V).

[Zero999]

I agree a transformer is probably a more reasonable solution. An SMPS will have a higher leakage current and be more noisy so could be unsuitable for this.

[cowana]

Thanks for the responses.

I hadn't though of a phone charger - and for a project that doesn't require to be on 24/7/365, it's a good solution. However I'd be concerned about a chinese/cheap charger's long term reliability - and using a more expensive brand name one would cost significantly more than a transformer solution.

So, I think I'll go with the transformer solution - one of the 1W block transformers, single diode rectifier, bulk capacitor and zener regulation.

The audio signal will have a (schottky) rectifier, and then a LPF feeding into the uController.

Andy