30-05-2020, 05:06 PM
Hi JohnP
The way I suggest for measuring low resistances (when you have limited resources) is this: Take a 6V car battery and a spare headlamp bulb. Wire these in series, and then wire the combination in series with the low resistance item you want to test. The bulb will light up to normal brilliance, and limit the current flow to something close to its rating, e.g a 36 watt bulb will draw 6 Amps at 6 Volts. Now, take your digital DC voltmeter and set it to its most sensitive range (or let it auto-range). Place its probes either side of the junction, stud, rivet, piece of wire, switch etc that you are trying to measure and note the voltage drop. With 100 milliohms and 6 Amps test current you will see 600 mV, with 10 milliohms you will see 60 mV etc. If the resistance under test is too high you will begin to lose accuracy as the current won't be the full amount, but for anything up to 100 milliohms you won't be far off.
A 12V battery will work just as well, and an old 55 Watt foglamp or somesuch will give you a test current of about 5 Amps.
If you have a DC ammeter you can always measure rather than estimate the test current, but this isn't supposed to be a lab experiment, just a way of finding a dodgy contact !
The way I suggest for measuring low resistances (when you have limited resources) is this: Take a 6V car battery and a spare headlamp bulb. Wire these in series, and then wire the combination in series with the low resistance item you want to test. The bulb will light up to normal brilliance, and limit the current flow to something close to its rating, e.g a 36 watt bulb will draw 6 Amps at 6 Volts. Now, take your digital DC voltmeter and set it to its most sensitive range (or let it auto-range). Place its probes either side of the junction, stud, rivet, piece of wire, switch etc that you are trying to measure and note the voltage drop. With 100 milliohms and 6 Amps test current you will see 600 mV, with 10 milliohms you will see 60 mV etc. If the resistance under test is too high you will begin to lose accuracy as the current won't be the full amount, but for anything up to 100 milliohms you won't be far off.
A 12V battery will work just as well, and an old 55 Watt foglamp or somesuch will give you a test current of about 5 Amps.
If you have a DC ammeter you can always measure rather than estimate the test current, but this isn't supposed to be a lab experiment, just a way of finding a dodgy contact !