[colug-432] Polarity

[jep200404 at columbus.rr.com]

In the "ounce of prevention versus pound of cure" department: 

On Thu, 23 Feb 2012 14:23:52 -0600, Thomas Cranston <thomas.w.cranston at gmail.com> wrote:

> I need to know the polarity of my DLink DI-704. 

It's good that you pay attention to it. 

> I have the user manual it only states 5VDC. 
> Polarity is not indicated on the unit itself.

That's both unfortunate and common. 

That's why when I get a new device that does not mark the power 
requirements at the input connector, that I mark a little 
sticker with such and put it on the device by the power connector. 
I also put a sticker on the power supply that says which device 
the power supply is for. 

Do so now for _all_ such devices and power supplies within 
your realm. 

> Is there a way I can determine the polarity of the power inlet, using a VOM?

Not by using the DVM alone on the inlet alone. 

If one really knows what one is doing, one can open up the device, 
follow traces on the circuit board to components and deduce what 
the required polarity (if any) is. 

> Is the center post of the power input + or -?

Probably yes. :-)

> Is there a way I can determine the polarity of the power inlet, using a VOM?

_If_ one has a correct and working power supply for the device, 
then measure the voltage coming out of it. 
That will indicate the polarity. 
Some power supplies don't regulate well (or at all) with 
no load or a light load. In such cases, connect a load 
such as a 10 ohm 10 Watt resistor to the power supply 
just long enough (just a few seconds) to measure the polarity. 
Even better would be to test the power supply with a 
resistive load that pass the rated amps at the rated volts. 
For 2 Amps at 5 Volts, that resistor would be: 
(5 Volts)/(2 Amps) --> 2.5 ohms 
Power in that resistor would be: 

   P = V * I = (5 Volts) * (2 Amps) = 10 Watts

Be sure to use a resistor that can handle the power. 
Things can get very "warm" very quickly either way. 
Look for power resistors in discarded CRT TVs. 

One can also search the web for replacement power supplies, 
hoping that the polarity is mentioned. 
The merchant's information could be wrong. 

Good luck.