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Old 22nd April 2008, 12:57 PM   #1
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Default connecting a switch mode power supply

Using an smps to power an intermittent load. Let's say six times an hour for five minutes each. Supply is 2.0 amps and load is 1.0 amp (dual muffin fans).

What is best practice? Put the relay on the 120vac side, or leave the supply energized and put the relay on the 12vdc output side?
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Old 22nd April 2008, 03:42 PM   #2
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Never mind that. I'm just going to use a bigger supply and keep it energized since there are other things I want to run with it. I'll switch 12vdc to the the muffin fans.

But how do I use my multimeter to measure max current available from the SMPS? This is not a computer supply smps and the only thing I know about it is 12vdc.
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Old 22nd April 2008, 05:25 PM   #3
Minion is offline Minion  Canada
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To measure the current of your SMPS i believe you have to wire it up and put a Load on the output and measure the Current with your multimeter...I would Increase the load on the PSU and keep measureing till there is no current change when increaseing the load ,That is when you should be reading the max current....

I guess you could use some High wattage power resistors for a Load.....

Cheers
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Old 22nd April 2008, 11:03 PM   #4
TheMG is offline TheMG  Canada
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Default Re: connecting a switch mode power supply

Quote:
Originally posted by bluebeard
What is best practice? Put the relay on the 120vac side, or leave the supply energized and put the relay on the 12vdc output side?

Better to switch the output side than to switch the mains side. Repeatedly turning the SMPS on and off from the mains power could potentially wear it out faster, as there is an inrush of current each time you turn it on.
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Old 24th April 2008, 06:53 PM   #5
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A related question? I wonder if I can modify a small SMPS for speed/voltage control for small (1.0 to 2.0 amp) dc motors?

How hard is it to identify a spot on the board of a small SMPS to trim the output voltage? Just before the final stage transformer?

Any advice? I'm just playing with some SMPS supplies that I've collected. 50c each at most thrift stores. 'Wall warts' from consumer electronics.

No way I could come up with circuit diagrams for these I'm sure unless i generated them on my own. The boards themselves are pretty simple.
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Old 24th April 2008, 11:32 PM   #6
TheMG is offline TheMG  Canada
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Usually you can only adjust the output over a relatively small range unless the SMPS is specifically designed for variable voltage output.

If you want to control the speed of a motor, you're best off to use PWM. It can be constructed fairly easily, all you really need is a transistor (mosfet works best), a resistor, and a diode, then of course you need something to drive it, which can be either a microcontroller (such as a PIC) or analog circuitry (such as a circuit based around the LM555).
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