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When it comes to measuring resistance with a multimeter in an electrical circuit,is there a formula you can use to determine if resistance is within limits?At the moment I'm just guessing if resistance is acceptance compared with a new length of wire.Anyone can help that would be appreciated.
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What resistance are you trying to confirm?
If its wire then it should be way under 1/2 an ohm.
If its a component other than a lamp, it should be R = Voltage (12 or 240?) divided by the current it would normally take. (if measured in Watts then R = Voltage squared divider by the Watts)
If its a lamp or heater then the resistance will increase with heat so harder to estimate.
If it is a 240V motor etc then these do not work, as the AC is a bit more complicated when coils are involved.
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AS3008.1.1 gives you the AC and DC resistance (or impedance) of different wire sizes at varying temperatures. If you know the sqmm of the wire you can compare it to what the tables say it should be. |
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It would help if we knew what you are measuring. For DC circuits R (ohms) = E/I ( E= Voltage / I = Current) With a multi meter an injected voltage will source a known I (current) for a given resistance. The multimeter gives a calculated resistance from the sourced current draw. AC resistance is (better known as impedance in a AC circuit)Impedance Triangle - Is used to calculate Impedance when resistance (R), Inductance (L) and Capacitance (C) are all present in the circuit, and the total reactance (X) is the difference between the Inductive Reactance (XL) and Capacitive Reactance (XC). and includes the phase angle "theta". If you are measuring a piece of wire, then VD (voltage drop) is important. VD=IxR The gauge of wire is functional in the circuit. No use putting a 1.0 sq mm wire on a starter motor circuit. You are delving into the world of "secret electricians business" Edited by user Saturday, 5 May 2018 3:57:49 PM(UTC)
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Assuming it's a low tech car you're talking about the smallest cable size you will encounter will be 16 awg, and it will be a DC circuit so you need not worry about inductive reactance, formulas and all that other stuff.
1 foot of 16 awg copper cable is around 0.004 ohms. You won't be able to tell if it's correct or 1000 times higher in resistance than it should be with any commonplace 2 terminal multimeter/ohm meter. Measuring the resistance of an entire circuit will be more useful.
If you're a little more specific about what you need to measure a more useful answer can be given.
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Originally Posted by: hammered  When it comes to measuring resistance with a multimeter in an electrical circuit,is there a formula you can use to determine if resistance is within limits?At the moment I'm just guessing if resistance is acceptance compared with a new length of wire.Anyone can help that would be appreciated. From a Sparky's point of view, your question is rather ambiguous. I'm under the assumption you want to know how much power you can draw through a certain size cable providing it's not damaged or corroded. Edited by user Sunday, 6 May 2018 2:19:35 PM(UTC)
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Wow,I didn't know there is so much to it.It's actually a 6 volt lighting circuit on a motorbike.It's a dc current to allow charging of the battery.I have a flickering tail light occurring with the indicators on.First thing to check was the earths.I believe they're all good(resistance in 3mm wire about same as new wire about .6-7 ohms.Going by you guys info the problem must be on the power side.The indicator circuit is about 2.3 ohms.Guess I'll need to investigate this further.Thanks guys.
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It will be the regulator rectifier that needs replacing. My XR400 used to chew them up yearly. Same symptoms.
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Sorry,already replaced rectifier.One of the connectors broke off.
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You don’t need to be sorry. Was the regulator combined with the rectifier? The regulator is the bit that will be causing the erratic power surges through your lighting. If it was combined with the rectifier I suggest the new one may be rs as well. If you only replaced the rectifier then try a new regulator.
Do the lights get brighter when you rev the engine up??
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The regulator is separate.Checking battery voltage with the motor running,the regulator seems to be working normally.Mildly increases to a particular rpm then levels out.
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