Transformerless power supply

Hi,

You can 'sniff' a supply off the mains using capacitors, but you can only get a few mA at 12v before the Cs get too big.

A switched mode power supply is the obvious answer.

But for 1w you can get a potted transformer that is inherently short circuit proof and only about 1" cube, so a regular (simple, reliable) transformer, bridge, capacitor, regulator supply becomes quite attractive (not in that way boys)

Hi,

You've been a little vague when you say "minimal volume". Is this a circuit that you are building to go into an existing housing and that's why size is of concern? Is it height or width or depth that is an issue or all three?

I may have one solution for you. My company sells a slim line DIN rail mounting power supply that is only 22.5mm wide, it has been designed for when space in a cabinet is an issue.

Cheers

Rob

Digital Instrumentation

Hi Rob,

you are just up the road from me!

Nice looking kit, if I need ow't like that, I'll know where to come!

Del

Thanks Del,

I look forward to hearing from you. I have driven through Harlow from time to time.

Cheers

Rob

Hi Del and Rob.... next time you're passing by Dunstable (M1 junction 11) pop in and say hi!!!

Now about that small power supply.... Buy a ready made one for a few quid... a mobile phone charger has a tiny pcb inside and can be bought for peanuts, as its a switched mode design it has universal voltage inputs, dc as well, and it can supply at good efficiency up to 5 watts at 12 ish volts...

John.

thanks for the link, will forward it to eng's at my work site

You can use SCR and fire it with small angle and charge a capacitor. You can also use MOSFET and fire ON/OFF to desired voltage to charge a capacitor. The controller part is often a small IC with oscillator, voltage reference and feedback control. This all can sit in 1-2 cubic cm space. However such supplies are not recommended as they do not provide mains isolation. Best thing is to use a small transformer and make a high frequency switching power supply with isolation and opto coupler feedback control. This may fit in 1 cubic inch easily and can provide 100s of watts of power.

Thanx for your suggestions. They were quite usefull.

"Battery eliminators" plug into a 13A socket and occupy around 120cm³. Mobile telephone chargers spring to mind as a commonly-available example, and many extra-low-voltage gadgets are supplied with something similar.

Hi,

if your load is not fully encapsulated then safe isolation between line and load is a must. A very simple and small chip is the LNK500 from POWER INTEGRATIONS INC. Using this you get a wide range power supply working from 85 ... 265V AC with full isolation, very less and tiny parts, an output power of up to 3W and with a green 300mW No-Load Input Power.
Regards Uwe

Have you ever seen Music-Door-Bell. Mains rectified by a single diode, thru a capacitor to the load [a resistor is added in series according to load] Just go to market & buy one, look into & try yours Enjoy