D C voltage

[chippie]

I have been given a small petrol driven generator which has no markings for the 12V polarity. I need to determine this. I was thinking of running it and putting a load (12V LAMP)through an ammeter . Will the way the needle points be a true indication of polarity dependent on which side of the ammeter it is wired through? Is there an easier way to determine this.?

 

[bryantee]

a good digital multimeter will show polarity, a minus sign in front of the reading for the - lead and nothing in front for the +.

 

[oldharry]

If polarity is not marked then the generator is probably outputting ac, not dc, and may cause terminal damage if connected to dc electronics. Again a DVM will answer this one as it will only give a dc reading if the supply is dc.

If it is ac, then you will need to add a rectifier, which is very simple to do. Try Maplins Electronics for a range of suitable low cost components.

 

[Trilogy]

It doesn't even need to be a digital meter. Even a cheap analogue volt meter will show polarity as long as your switch it to DC volts.

Nick

 

[david_bagshaw]

If you dont have a volt meter to hand

make a salt water solution, and dip the outlet wires of the generator in while it is running. (perhaps with a resistive load connected incase there is a sensitive voltage regulator that could be damaged by open circuit)

the -ve bubbles more.

David
www.yachtman.co.uk

 

[HaraldS]

If you have a multimeter, measure the voltage without load, it tells you the polarity.
If you just have an ammeter, connecting it like you suggest will work, and plus will be where it is marked on the ammeter if it shows positive current flow.

A small bulb and a diode are another possibility. If the two in series light the bulb, plus will be on the side opposite to the stripe (Cathode) on the diode.

 

[Guest]

A multimeter is a usful bit of kit to have around on a boat. Bought mine for £8 in B & Q last month.

 

[chippie]

Many thanks to those who replied. I posted because I dont have a multimeter, but agree that is a good thing to have so will get one soon. The ingenuity of the other methods appeal to me a lot.