power connector: polarity? (solved)

[DiTBho]

A few days ago I received a USB tool that needs to be powered via an external power supply, which unfortunately I don't have.

There is a 1.3mm connector, but I don't know the polarity:
1) positive in the center? ground on the external ring?
2) negative in the center? positive on the external ring?

There is a VGA connector, hence I know there are GND pins.
Unfortunately when I attach my multi-meter to one of the VGA GND, I get short-circuits on both the center pin and the external ring

Code: [Select]

Power
conn!
 ___
/   \
[ * ]
\_^_/
  | ^
  | |
  | \______________B is it the negative? (external ring)
  |
  \________________A is it the positive? (central point)


Check { pointB, VGA.GND } ----> the multimeter reads it as short circuit
Check {pointA, VGA.GND } ----> the multimeter reads it as short circuit

This makes me ... puzzled why PointA and PointB can't be shorted.


How can I figure it out the right polarity? 

[ebastler]

It is most common for these barrel connectors to use the outer sleeve for ground, and the inner conductor for the positive supply. But use of the opposite polarity is not unheard of, so it's a good idea to double-check.

Isn't the polarity labeled near the power jack? A symbol like "- C– +" indicates negative polarity on the outer sleeve, positive on the tip.

When you measured the connection of either terminal to VGA GND, what setting of your multimeter did you use? If you had it in "beep when connected" mode, I would suggest that you repeat the measurement in proper resistance (Ohms) mode. I would expect one of the two terminals to show very close to 0 Ohms to VGA GND, and the other some higher resistance value.

Or you could tell us the exact name and type of your gadget, and someone might be able to check its power supply needs. Which would also tell you the voltage it needs, besides the polarity...

[mariush]

The "socket" at the end of the cable should have a +  or - marking on one side.

If you plug that tip with the +  matching the corresponding + on the cable socket, then the polarity would be the correct one, the expected one ... usually outside ground, inside positive,  or bottom side of the plug ground, tip positive.

But... should be enough to test with a multimeter once and you'll remember.

[Feynman]

Be aware when probing around with a DMM that these barrel connectors often times have three terminals. In this case they have an internal switch that shorts two of the terminals, when no mating part is plugged in. When the mating part is plugged in, the switch opens. Also it can happen very easily that your DMM tip makes contact with the center connector AND the outer connector at the same time when you try to measure inside the connector (instead of at the pins).

[DiTBho]

Isn't the polarity labeled near the power jack? A symbol like "- C– +" indicates negative polarity on the outer sleeve, positive on the tip.

I have a PSU here with 1.3mm jack, there is a label on the plastic telling inner tip is +5V, the outer sleeve is ground.
But I didn't know how the PSU connector of USB-device is wired internally.

When you measured the connection of either terminal to VGA GND, what setting of your multimeter did you use?
If you had it in "beep when connected" mode, I would suggest that you repeat the measurement in proper resistance (Ohms) mode. I would expect one of the two terminals to show very close to 0 Ohms to VGA GND, and the other some higher resistance value.

Yes, "beep when connected" auto-scale, it's the default setting when you power-on the multi-meter. It takes too much to stabilize the value when I tried to measure the resistance between VGA.gnd and the inner tip, whose value floated between 0.1 Ohm, 1K, and finally more than 10M -> OL (open circuit).

I can manually set the read-time, "ohm meter" mode, and I can exclude the "integration" function (useful for TrueRMS measurements and similar) since I noticed that with 0.25s values were already stable.

I repeated the measurements with "manual-scale", "ohm meter", "no integration", and this time I got measurements that make sense.

- the outer sleeve has a resistance of 0.001 ohm to the VGA.gnd ---> it's ground
- the inner tips has a resistance of >10M to the VGA.gnd ---> it's Vcc

I suppose the previous floating reads were caused by an internal big capacitor or something between Vcc and ground.

I am using a new multi-meter, I don't yet know how it behaves on measurements, it's much more complex than the previous one I used before the new purchase.

Lesson learned:
1) don't use "integration", otherwise the measurement slowly converges towards the final value
2) don't use auto-scale unless it's really necessary, it makes matters worse since it takes longer and in the meanwhile it shows fluctuating readings
3) don't use the "beep when connected" mode, use the classic ohm-meter mode

[DiTBho]

Also it can happen very easily that your DMM tip makes contact with the center connector AND the outer connector at the same time when you try to measure inside the connector (instead of at the pins).

I am using a couple of special probes as thin as a pin