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Low Cost PS for 12 Volts, 250ma for a small DC motor

Hi All,

I'm looking for a means to inexpensively implement subject PS from a

240/120 mains supply without using a switch. Yep, I know that a SMPS would do the job but can the job be done at less cost?

What I've looked at so far:

1) Capacitive Transformer. It looks like it might work except that I end up needing large 22uF high voltage caps on the front end which are expensive. Needs a switch. 2) Zener supply. Way too much power dissipated in the resistors. Needs a switch. 3) CT transformer. Looks good but needs a switch.

Any bright ideas or is SMPS the solution??

Regards,

Fred

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Reply to
Fred
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Hi Joerg,

Thanks for the reply. There could well be a large quantity of these supplies needed.

Regards,

Fred

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Reply to
Fred

When you say, "without a switch", do you mean, without having to manually switch a toggle switch or slide switch?

I knew a guy once who made 120/240 battery chargers, and he used a 240V relay. At 120V line voltage, it didn't pull in, so the primaries were in parallel. At 240, the relay pulled in, and put the primaries in series. A relay _does_ do switching, but I've never heard a relay called a "switch". :-)

Other than that, probably an SMPS, sorry.

Good Luck! Rich

Reply to
Rich Grise

Hello Fred,

SMPS is usually the answer here. Unless you are facing a gazillion units per year I would look for off-the-shelf wall wart type supplies or small power bricks that are rated for wide input voltage (90V to 260V or so).

Regards, Joerg

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Reply to
Joerg

Why can't you just use a wall-wart, and what's this "switch" you keep referring to?

--
John Fields
Professional Circuit Designer
Reply to
John Fields

Hello Fred,

Look at power supplies like these:

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About $15 if you buy 100 and IIRC they even come with adapters for several countries.

There are many more companies that make such supplies. Astec etc.

Regards, Joerg

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Reply to
Joerg

ISTR that the famous legal firm, Dolby Labs had a product that had a suitably rated light bulb in series with the transformer primary for just this function. It helps if you have a constant power requirement

martin

Reply to
martin griffith

I read in sci.electronics.design that Fred wrote (in ) about 'Low Cost PS for 12 Volts, 250ma for a small DC motor', on Wed, 7 Sep 2005:

Dolby Labs made a laboratory noise-weighting filter, with lower power consumption certainly, that did 120/240 V by putting a filament lamp in series with the primary of the mains transformer. The primary voltage stayed at about 90 V irrespective of the input voltage.

--
Regards, John Woodgate, OOO - Own Opinions Only.
If everything has been designed, a god designed evolution by natural selection.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Reply to
John Woodgate 

--
I think you\'re right.  That completely evaded me.  Thanks. :-)
Reply to
John Fields

Ah, The light bulb, probably the worlds' simplest XOR gate

martin

Reply to
martin griffith

Hello John,

Guess he means the 120/240 selector. Anyway, wall wart or power brick is the way to go. Much easier during the agency approval.

Regards, Joerg

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Reply to
Joerg

He wants a 'universal' PSU without the need to change voltage taps. See "

240/120 mains supply without using a switch ".

For all practical purposes therefore, an SMPS.

Graham

Reply to
Pooh Bear

Probably about $6 or less if it comes from China.

Graham

Reply to
Pooh Bear

3 watts makes a simple switcher just about viable.

I googled "power adaptor china".

One interesting hit was this.

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There's *tons* of outfits making this stuff out there.

Graham

Reply to
Pooh Bear

Hello Again,

Thanks to everyone that has contributed to this thread. I do appreciate the input. To confirm a question back a post or two, yes, we are looking for a PS that adapts to 120/240 mains input.

Regards,

Fred

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Reply to
Fred

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