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I recently purchased a power brick (VM-80W12) and I've been trying to test it with the multimeter to determine the polarity. It however registers 0 Volts continusly (yes I plugged it in) I've heard that sometimes a power supply will only switch on when it get's a load resistance on it's output and I was wondering if that was true. If so then how do I go about testing the power brick?

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  • \$\begingroup\$ Connect 1 k $\Omega$ resistor to the leads and see if you get voltage drop across it. It should be enough to start the supply. \$\endgroup\$ – AndrejaKo May 22 '11 at 23:36
  • \$\begingroup\$ Is there any documentation as to why that is? I'd like to learn more. \$\endgroup\$ – user3045 May 22 '11 at 23:54
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    \$\begingroup\$ @kurtnelle Description says: VCT VM 80W12 Universal 12V AC / DC Converter Adapter ~ Input 110V or 220V / 240V AC - Output 12V DC 5.3 Amp Comes With Multiple Tips The part you're looking at is 12 V, 5.3 A. So you need a resistor that will give some current at 12 V, but it needs to be less than 5.3 A. You also need to take a look at resistor's power rating, so that resistor doesn't burn itself. I put 1 k$\Omega$ for no particular reason. It's a common value and the dissipated power should be 0.144 W, so you can use cheap 1/4 W resistors. \$\endgroup\$ – AndrejaKo May 23 '11 at 0:13
  • \$\begingroup\$ @kurtnelle For further reading, take a look here for the resistance value and here for power calculation. \$\endgroup\$ – AndrejaKo May 23 '11 at 0:15
  • \$\begingroup\$ @AndrejaKo: That should have been an answer, not a comment \$\endgroup\$ – endolith May 23 '11 at 14:31
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Try plugging in a 1W 288ohm resistor across the load.

Should do the trick and turn the power supply on!

EDIT: A 270 ohm resistor would be much easier to find

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    \$\begingroup\$ 288 $\Omega$ isn't a standard resistor value, so it's going to be hard to obtain. \$\endgroup\$ – AndrejaKo May 22 '11 at 23:40
  • \$\begingroup\$ I've always wondered what the standard resistor values were - Is there a formula or a list somewhere? \$\endgroup\$ – charliehorse55 May 23 '11 at 0:31
  • \$\begingroup\$ There's a way to get the numbers. First, the scale for the resistors is logarithmic. Scales are published by Electronic Industries Association and International Electrotechnical Commission. The name of the scale starts with E and is followed by a number. The number tells us how many steps there are in the scale. The scale goes from one to 100 to 1000 and if you need smaller or larger values, you multiply and divide the scale. \$\endgroup\$ – AndrejaKo May 23 '11 at 0:49
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    \$\begingroup\$ You can find some more information, tips and the scales themselves here and here. There are many other sites explaining this, so searching for standard resistor values table for example will probably give you interesting information. Do keep in mind that the tables aren't limiting and that manufacturers can make any type of resistor they want. It's just that these are "standard" and others are proprietary \$\endgroup\$ – AndrejaKo May 23 '11 at 0:51

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