I've got a device that accepts power from either a 110/220VAC outlet or a 12V battery.
Problem is, it doesn't say which terminal is the positive one (for the
Is there a way to test which way is positive, without either taking apart the device, or playing a 50-50 heads/tails game where I could destroy the device (it's got some electronics onboard, and it's kinda expensive).
If there are microchips involved, use a multimeter with a diode checking function, which will clearly show a diode... anode to the negative connection, cathode to the positive connection.
Beware, a protected device may have a series diode, in which case you will see a low resistance in the correct direction, and open in the other.
...Jim Thompson
Shame on them for making such a beast! Yes, I'm serious. I consider that to be pretty unfriendly.
If you have access to a current-limited power supply, you can try running the voltage up slowly and notice if the current rises abruptly at too low a voltage. Set the current limit to some reasonably value for the device. I'm assuming/hoping here that it draws relativly little current in normal operation, so you don't have to set the current limit to destructive levels.
Some devices have diodes to protect things: either a shunt diode to draw big current through the diode (and maybe blow a fuse before the diode fails or a circuit board trace burns up) if things are backwards, or a series diode to allow power supply current only in one direction, or a diode bridge to allow the power to be connected either way without mattering.
If the device has terminals (like headphone jacks or line input jacks) that likely have something tied to ground, you could perhaps find if that's connected directly to one of the power supply pins. It's usual, though not guaranteed, that "ground" will be the negative terminal.
If the device has an included battery holder, you may be able to deduce the polarity from finding one end of the battery holder connected directly to one pin of the power connector.
And finally, if you can open the device up and peer into it, you may be able to trace out enough circuit that the polarity becomes obvious. It's common, for example, to have a large polarized capacitor across the power supply, and the polarity markings on it will immediately tell you which way to apply power.
I'm assuming here that you have access to some basic test equipment, like an ohm meter, and basic circuits knowledge--if not, get someone local to you to help.
Cheers, Tom
(snip)
Not sure of the O/P's exact configuration, but it was widespread practice for the likes of (Rockwell-) Collins and Yaesu to use multi-pin power receptacles on their HF transceivers with the AC line and DC +ve using the same input pin.
Yeah- and is that "110/220VAC outlet" power supplied to the device through a wall-wart type of supply with possibly oriented connector?
No, actually... it's got the power supply built-in.
That would have made it easy... ;)
It's a Respironics / REMStar CPAP machine, for those who are curious...
They sell a battery cable (with jump-start cables, no less) for about $50-100... geez.
Mike
Unless you're worried about voiding the warranty (and I think I read somewhere that those warranty voiding stickers don't hold up in court), it's probably easiest just to take it apart. You can almost always spot an electrolytic cap and a voltage regulator right next to the power input.
Otherwise, try using an ohmmeter to see if either pin is connected to any other metal on the chassis.
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