# PWM control of PS?

Discussion in 'General Electronics Chat' started by MikeA, Sep 29, 2013.

1. ### MikeA Thread Starter Active Member

Jan 20, 2013
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I have one of these LED power supplies that provides 36v @ 3A and would like to somehow control the output brightness of the LED connected to it.

So I thought I can connect a simple PWM circuit between the power supply and the LED, but when I measure the ouput of the power supply it's 36v DC, and 64v AC between ground and (-) and 64v AC between ground and (+).

Is this a non-insulated power supply? Can PWM work with it?

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2. ### #12 Expert

Nov 30, 2010
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You will probably find that the AC voltage is at a rather high impedance. Try attaching a resistor from the negative end of the output to earth ground with 18K, 1/2 watt and see if the voltage collapses. Then check the positive end of the output to earth ground and see if the voltage is still there.

3. ### MikeA Thread Starter Active Member

Jan 20, 2013
184
23
With 20k resistor (+) read 4.5v AC, (-) read 1.5v AC. So you are onto something.

If I were to make a simple PWM circuit with a PIC controller driven from another DC source, what precautions should I take given there is this AC current there?

4. ### shteii01 AAC Fanatic!

Feb 19, 2010
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Last edited: Sep 29, 2013
5. ### #12 Expert

Nov 30, 2010
18,076
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Math shows .005 uf between the 120 volt power line and the output. Try reversing the power cord connections.
I don't like this at all! That supply should be better than what you are measuring.

6. ### MikeA Thread Starter Active Member

Jan 20, 2013
184
23
You mean swap hot-neutral? The power supply is not marked, but there is a fuse on board, so I assumed that's the hot side.

7. ### #12 Expert

Nov 30, 2010
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Yes. Try swapping the power cord and see if the AC leak is only on one side of the power line.

killivolt likes this.
8. ### MikeA Thread Starter Active Member

Jan 20, 2013
184
23
With hot-neutral swapped I'm getting 20v AC without resistor, and 0.4v AC on both (+) and (-) with 20k resistor.

9. ### #12 Expert

Nov 30, 2010
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.4 volts AC compared to where?

10. ### MikeA Thread Starter Active Member

Jan 20, 2013
184
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0.4v AC relative to ground.

I just looked closer at the picture of the power supply I attached, mine is not identical, but component locations are very close, probably same schematic. In the picture I see "AC IN" and "AC N". Maybe "N" stands for neutral? Mine has to markings as to which is neutral, but even in the picture, the fuse is on "AC N".

11. ### #12 Expert

Nov 30, 2010
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Check the voltages of the output, AC and DC. Out+ to out-

You're down to 20 microamps, which is low enough that it might be as simple as humidity in the board.
or, it might be a leakage in a transformer. Can't tell from here, but it's in the range that it won't kill you on a personal level.
I'm thinking you can connect one of the output wires to earth ground and proceed as if nothing is wrong.

Last edited: Sep 29, 2013
12. ### MikeA Thread Starter Active Member

Jan 20, 2013
184
23
0.018v AC between (+) and (-) and 0.35v AC to ground. 31v DC (+) to (-). Should I keep neutral-hot in this "reversed" configuration? No idea why they decided to fuse the neutral on the circuit board.

13. ### #12 Expert

Nov 30, 2010
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I would do what is safest. If you want to put the power lines back where they were and try earth grounding one lead of the output, that's up to you. Blowing the fuse on the "hot" side is a safety consideration. I think you are chasing an error in the device, bad isolation. Again, my opinion, find the real problem or throw it in the trash and get one that is really isolated from input to output.

killivolt likes this.
14. ### MikeA Thread Starter Active Member

Jan 20, 2013
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23
What do my numbers indicate though? Given that with the hot going to the fused side there is slightly higher AC leakage than when the neutral is connected to the fused side?

This is indeed probably not the best made PS, but there are millions of them made. I saw them in all kinds of Chinese made LED lighting products installed as outside flood lights all over the world.

15. ### Alec_t Expert

Sep 17, 2013
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Dangerous practice. If the fuse blows the circuit (or part of it) is still at mains potential.

16. ### #12 Expert

Nov 30, 2010
18,076
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They indicate that the "isolated" power supply is not as isolated as you would expect.