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I work in a lab, and I've been trying to replace a lost AC-DC power adapter. I got one recently that has the exact specs that I need, which are apparently pretty rare (15V, 400 mA). However, when I test the power adapter with a voltmeter, it's actually outputting ~22V, despite what it says on the plug.

Now, when I plug in the power adapter, the camera "turns on" because the green LED on it comes on. However, when I try to run the camera software, it behaves exactly as it does if I have the camera unplugged or turned off. So the software is not "seeing" it, even though it's plugged in, and plugged into the firewire port on the PC (yes, it uses firewire... ugh).

So now I don't know why the camera is not actually working. My hypothesis -- is it possible that the camera is not really working because the power adapter is bad, and that the specs are too far off from what it wants? Even though the LED light turns on, maybe the voltage/current is close enough to make something work, but not close enough for it to actually function properly?? Is this stupid?

I'm just not sure if this is a software or hardware problem...

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Modern power adapters tend to be electronically regulated and not vary much in output voltage with load changes. Older ones that used iron core transformers were often less stable in output voltage with Vout rising to a substantially higher than specified value on no load. If you are 'lucky' the camera load may have reduced the voltage enough to prevent damage and/or the camera designers may have had their wits about them - but no guaranteed on either count, alas.

The voltage supplied should ideally be close to correct - usually 14V to 16V or so would be OK. 22V is higher than may be safe and may have caused damage.

Current rating of the adapter needs to be equal or higher than what is specified. Higher will almost always do no harm if voltage is OK.

One other thing to check is polarity. Most adapters have +ve on the sleeve and -Ve on the center pin but a small number have the opposite polarity. It is almost always essential to use the correct polarity. A very very few devices* do not care but this is very rare. Reversed polarity at best will not work and at worst may destroy the equipment and/or the adapter.


  • Such as ones that I build :-).
    Within reason, plug something in and it will work :-).
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  • $\begingroup$ I would add to this: the OP will be much better off finding a replacement for the original supply (presumably he knows the model number of his camera), either from the mfr or eBay. Second best is to use a lab DC supply, which as you wrote will be properly regulated and guaranteed to supply sufficient current. $\endgroup$ – Carl Witthoft Jan 7 '16 at 12:31
  • $\begingroup$ I did verify polarity before plugging it in, with a multimeter in Ohm mode. The outer shell is ground, which is the same polarity as the adapter (the diagram on it is like the left one here: en.wikipedia.org/wiki/Polarity_symbols#/media/…). Stupidly, there's no polarity info on the camera itself, nor on the spec sheet. Also, I didn't mention it in my OP, but I had to cut off the barrel plug from the adapter and solder on a different one because the original plug didn't fit. $\endgroup$ – StormRyder Jan 8 '16 at 18:47
  • $\begingroup$ @Carl, the company no longer exists so I don't think I'll be able to find the original power supply anywhere... $\endgroup$ – StormRyder Jan 8 '16 at 18:51
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What Russell said.

In addition, the reason the LED lights but the device no longer appears to work may be because the LED is just wired to indicate power, and the rest of the device is now blown out due to the much higher than specified voltage you applied to it.

Power LEDs are sometimes just a LED and resistor connected in series across the input power. These LEDs don't need to be particularly bright, so are often run at less than their maximum current rating. Many common indicator LEDs are rated for 20 mA, but are still plenty bright enough at 10 mA to tell you the power is on. Figuring the green LED drops 2.1 V and is just connected to the supply with a resistor, your 22 V as apposed to 15 V would result in 54% more current thru the LED. If it was only run at half its maximum current normally, this wouldn't even be pushing it. Even if the LED was set to run at its maximum rating normally, 54% overcurrent for a typical indicator LED won't blow it out. It will be brighter than normal, and it will have a shorter lifetime than rated, but would most likely work fine for days.

The circuitry in the rest of the device apparently wasn't so forgiving, and now it's just a hunk of junk.

Next time get a regulated supply of the right voltage.

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  • $\begingroup$ Good advice, but I wouldn't toss the camera without at least trying it out w/ a proper supply. And now I'm wondering what sort of camera doesn't have a battery option. $\endgroup$ – Carl Witthoft Jan 8 '16 at 13:28
  • $\begingroup$ How would I know if the AC-DC adapter I'm buying is regulated or not? Sites like Amazon are pretty skimpy on technical details... $\endgroup$ – StormRyder Jan 8 '16 at 18:26
  • $\begingroup$ Stor: It should say "regulated" in the datasheet. If Amazon doesn't give you proper specs, then buy from someplace that does. Jameco has a wide selection of power supplies. Their wall warts were specifically divided into regulated and unregulated sections last I looked. $\endgroup$ – Olin Lathrop Jan 8 '16 at 18:45

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