How to determine polarity of potential Famicom power supply

[SatoshiMatrix]

I have a small and compact DC 9v 1amp power supply that's from something or another but it does not list what polarity it is. It has a barrel plug which would fit a Famicom, Super Famicom or model 1 Genesis and I'd like to use it with one of those which I know needs a negative center barrel plug, the + ---C--- -

I have a multimeter, but it doesn't seem to help me find out this information either. Luckily, I can take it apart, and I've taken photos of the board.





I know that vcc lines are red and ground is black, but it seems the red line goes to the side of the large capacitor that has the stripe, meaning that's negative. odd. I simply want to know how to tell what polarity the adapter is. if its center pin positive, I should just be able to desolder the two barrel plug wires and swap them correct?

Thanks for any help.

[Movax12]

Easiest clue is to look at the LED. Your meter can't display +/-? As well it would be good to verify that you are getting enough amps or you may get unpredictable results. -> Nevermind I didn't see the 1 amp comment, that should be enough.

[infiniteneslives]

Check with a multimeter. That's the only real way to know, they could have messed up and put the cap in backwards who knows...

If you didn't have a multimeter you could sacrifice a LED to verify polarity. Don't sacrifice your FC trying to find out.

[SatoshiMatrix]

I have a digital multimeter, yes.
heres the multimeter I own.


How do I use it to determine polarity? What setting and where should I put the probes? Again, this has a barrel plug connector like a Famciom/SFC/MegaDrive.

[Movax12]

If you connect the red (+) and black (-) to a voltage source and it reads minus voltage (-) then you have the probes reversed. (Make sure it is on DC).

[SatoshiMatrix]

So should I have the adapter plugged into the wall and then test the two parts of the barrel plug - the inner part and the exterior using the DC meter (roughly at 5 o'clock on the multimeter just below 10A ?)

[Movax12]

So should I have the adapter plugged into the wall and then test the two parts of the barrel plug - the inner part and the exterior using the DC meter.

Yes, but the meter should be:


Move the probes down one spot each, and put the meter on DC 20.

[SatoshiMatrix]

should it be plugged into the wall when I do this?

[SatoshiMatrix]

nevermind, I guess it would only make sense to.

So here are the results

plugged into the wall with the multimeter adjusted as indicated, I put the black probe into the center of the barrel and the red probe on the outside part.

Got a reading of 9.34.

Then put the red probe into the center and the black probe along the edge.

Got a reading of -9.34.

Does this mean this adapter is in fact, center pin negative as needed for powering a Famicom?

[blargg]

The first reading, where you got a non-negative voltage, shows the polarity. Black in center = negative, red on outside part = positive. So it's negative tip, which was the way power adapters were until many years ago, then they started going to positive tip.

NOW, this assumes you have the black one plugged into common/gnd on the meter (on yours it's the lowermost plug COM).

[SatoshiMatrix]

black probe is in com
red is in V ohm mA

so then negative tip, famicom ready?

[blargg]

Do you have a battery? Put the probes on it and get a feel for how to interpret the meter, based on the markings on the battery. Then tell us how you interpret your readings on the coaxial power connector.

[SatoshiMatrix]

yup.

tried an AA battery with the same setup.

red lead on the positive end of the battery and black lead on negative end. Get a reading of 1.35. If I put the probes the other way around, I get -1.35.

[koitsu]

Check with a multimeter. That's the only real way to know, they could have messed up and put the cap in backwards who knows...

If you connect the red (+) and black (-) to a voltage source and it reads minus voltage (-) then you have the probes reversed.

The first reading, where you got a non-negative voltage, shows the polarity. Black in center = negative, red on outside part = positive. So it's negative tip, which was the way power adapters were until many years ago, then they started going to positive tip.

All I can take away from this is: "to determine the polarity of an AC adapter, you attach the the black probe (COM) to the centre pin/barrel, and the red probe (VΩmA) to the outer edge/shell. If the voltage shown on the multimeter is positive, it means the AC adapter is centre-negative. If the voltage shown on the multimeter is negative, it means the AC adapter is center-positive."

• Centre-negative
• Centre-positive

Thus it seems to me that there must be some universal standard for multimeters that says COM (on the MM) is always positive and VΩmA is always negative -- otherwise there'd be no way to determine the polarity of the adapter, no?

This sort of shit is why I hate EE. You guys really gotta lay it out for us idiots verbosely, like we're 8 year old children. Honest.

[Movax12]

Thus it seems to me that there must be some universal standard for multimeters that says COM (on the MM) is always positive and VΩmA is always negative

COM is negative. As in common, as in ground as in the ground symbol that is there.

The other 'port' is for measuring Volts, Ohms or milliamps. The topmost port is for high amps, since the normal port can't handle more than milliamps. You would have to have the meter inline as part of the circuit to measure amps. Really this is pretty basic stuff.