Dc/DC SMPS question

Discussion in 'General Electronics Chat' started by DougB, Sep 11, 2010.

1. DougB Thread Starter New Member

Sep 11, 2010
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I have a 12 VDC input, +/- 15VDC output SMPS that I bought several years ago for a CCD camera kit I was building. The unit is a custom design, has no part numbers or markings, and currently the people that were responsible for its development are not available.

Can I damage this unit if I reverse the polarity of the DC input? It would be a marginally educated guess as to what is supposed to be +12 and what is to be -12 when I go to connect it.

The unit is sealed hermetically so I can not look inside .

2. R!f@@ AAC Fanatic!

Apr 2, 2009
8,793
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U will damage it if you reverse connect it in any way

3. eblc1388 Senior Member

Nov 28, 2008
1,542
102
Put a 330Ω to 1K resistor in series with the 12V supply and a voltmeter connected to the SMPS output.

A correct supply polarity would result in some sort of voltage output. If there is no output, reverse the supply polarity. Hopefully the resistor will limit the fault current against reverse polarity.

If you get a voltage output on the SMPS +15V side and -15V side continuously, albeit much less than +15V, then the polarity is correct. Mark them clearly so you don't have to go through the same process again.

4. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
If they didn't include an polarity protection input diode then at least you were ahead of trouble by having the good sense to wonder if the polarity of input could be a problem.

I would almost guarantee that one of the components nearest to the input connection will be a polarized Electorlytic capacitor. That will have one side marked as positive. From there by following the traces, preferably with a continuity checker function of a multimeter, you will be able to find the positive side of the input connections.

5. SgtWookie Expert

Jul 17, 2007
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Potato Pudding,
That's going to prove tough to do without X-ray vision. The whole supply is encapsulated in epoxy.

6. eblc1388 Senior Member

Nov 28, 2008
1,542
102
He probably own a pair of these.

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7. Potato Pudding Well-Known Member

Jun 11, 2010
684
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Ok. Hermetically Sealed. Yuck.

Oh well. Try a diode check -multimeter test - across the inputs to see if it does have polarity protection.

Keep in mind that the Diode check will normally work but only for a little while before the filter capacitor develops charge.

Also describe the input connection or provide a picture because they might have used a connector that is meant to indicate the polarity. Most do. If it has two unmarked screw lugs or similar then the supplier did not create a very professional product.

8. DougB Thread Starter New Member

Sep 11, 2010
24
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This supply was meant to be mounted on a PCB face/pin down thru the board so has 5 pins, 2 input and 3 for the output. Because this was a business venture for those that developed the camera kit there was no schematics provided. At that time I decided to purchase only the SMPS supply and not the PIC controlled temperature and power supply kit they were selling. Consequently no PCB to follow traces on. Only have some documents printed from the website years ago that is no longer up that has a picture of the top side of the PCB. From that picture I definitely know where the +/- 15 VDC output terminals are. From that picture I can also see where the post output filtering goes and also the values of the capacitors and chokes, which was another worry of mine. Because of the digital component of this camera the supply had to be very quiet. They claim to have the ripple down to 2 MV. My educated guess on where the + of the 12v went on the input side, is that looking at the board, I am assuming that it would be on the same side as the +15 output, but it would only be a guess. I had also hoped that the square and round pads at the connections would help identify the + and - but unless they identify the inputs and outputs differently on PCB's as standard practice I do not see any consistency here.

In any case thanks for your help and anymore you can give would be appreciated.

9. SgtWookie Expert

Jul 17, 2007
22,183
1,729
Have you tried the Wayback Machine?

http://www.archive.org

They captured many shots from web pages over the last couple of decades. You might find more on there.

10. Potato Pudding Well-Known Member

Jun 11, 2010
684
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Header pins to connect it to another circuit board. Not much convention there you can count on.

This just keeps getting better.

If you have the pictures and can post them then we can try to help from those.

Other wise you are going to have to try the resistor limited connection which will offer a little protection.

Worst case is it dies with a fizzle when powered through the limiting resistor.

If you were to power it direct from a 12 volt battery for example it could go off like a (small) bomb if you had the polarity wrong.

11. Potato Pudding Well-Known Member

Jun 11, 2010
684
92
Also be sure to have a load resistor across the outputs when you try it. A 1 K resistor will probably be fine.

12. marshallf3 Well-Known Member

Jul 26, 2010
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Good point, I often forget to include some sort of load resistor. Even when I'm designing one into a circuit I think it's a good idea to have a tiny load right at the output terminals, not only for insurance purposes but I think it might tend to help in reducing output noise.

Now there's another thing we may not have thought about here - they may have included internal load resistors on the outputs of this unknown device. If so, you'd be able to measure them between both the + & - outputs to the common terminal. Both would probably be the identical value and neither would be polarized.