Resistance is one of the most popular three measurements of any electrical system which are voltage, current and resistance. Resistance is just like its name, it,s actually opposition of current flow in circuit . Or you can say it as oppsition of medium from which current has to be flow. And the quality of material to pass current from it is called conductance. So metals or other materials with good conductance value means they have less resistance in them.
Its unit ohm is named on the great scientist and physicist george simon ohm who has credit to give the most important and practical formula of electrical calculations “ohm,s law”. Resistance could be defined by ohm,s law as,
V = IR or R = V/I
In which v represents voltage, I for curent and r for resistance. According to it resistance is inversely proportional to current, as highers the rsistance current will be lowred and as lowers the resistance current will increased.
How to Measure Resistance Using Multimeter
Although resistance is a real opposition of current flow but it becomes so important while designing electric systems or testing other parameters of devices and motors. In that sense finding actual value of ohms becomes so vital, it could be done by simple steps with help of a best digital multimeter or analog meter.
If you are not a profesional electrician we will strongly recommend you to first take necessary safety trainings to avoid any misfortune. Carefully read manual of your multimeter and follow it strictly. To measure resistance through multimeter apply following steps accordingly,
- First of all check power on multimeter and check battery status.
- Now put red test leads to related ohm measuring socket and black to COM socket which is for neutral or common lead.
- Now turn of the circuit where you have to measure resistance, if circuit has any sort of capacitor discharge it also.
- Next step is to set range for ohm readings if you have a manual range multimeter, otherwise set rotory selector switch to resistance and meter will automatically set range for ohms.
- Now connect leads to component or circuit under test and check readings on screen, always make a firm contact.
- If you want to measure low resistance set range to lowest ohm reading, there is also another option of relative mode in some advanced multimeters through which a more accurate readings could achieved.
- After test turn off multimeter to save battery.
How to Measure Resistance of a Wire
To find resistance of a wire through multimeter is not much different from other resistance measurements. But some points has to be kept in mind before test. You should sure that your multimeter has that close resolution which is required to test a specific wire. Other steps are same as before, like putting black probe to COM socket and Red banana probe to socket having ohm measurements.
Now we assume here that there is a piece of wire of guage 2.5 mm has bare wiring ends on both of its sides. Most of multimeters are manual range, so first set highest range of that multimeter. Now connect probes of meter to both ends of that wire and see readings on screen. If there is zero reading seen on meter that means range is high so lower that range step by step and check results on screen. In this process a range will come where you will see exact resistance of wire.
Many times we see 1 or OL sign on meter while testing resistance of wires or something else. That,s actually indicates that range is too low to measure resistance. In that case you have to higher the range periodically.
There,s also a manual process to check right range of resistance for convenience during test. For example you have a piece of copper wire, check resistivity of that wire according to guage table and its length also. Now apply this formula,
R = Resistivity*length/area
Through this one can get possible resistance value of a wire and set range according to it.
How to Measure Resistance in a Circuit
If you are a profesional electrician or electrical engineer then you must know that resistance can,t be measured on live circuits. Reason behind it is multimeter,s functionality to find resistance. Multimeters apply a known voltage to circuit to find possible current and resistance of that circuit. In presence of external voltage multimeter is unable to give accurate results.
So first step is to power off all types of supplies to that circuit, unplug batteries. We recommend to disconnect capacitor also If circuit has any. Now you can check resistance between any two selected points of circuit.
How to Read Ohms on a Digital Multimeter
General home use Manual range digital multimeters have separate ranges for various resistance measurements. It can be like 20 ohm, 200 ohms, 20 kilo ohms , 2 mega ohm and so on. Now if selector switch is at 200 ohm range or resistance and you are getting reading 87 on meter that,s 87 ohm.
Similarly if the range is set on 20 k ohm results shows that resistance magnitude is 0.86 or 0.90 that means resistance is 860 or 900 ohms. Because now range is in kilo ohm and we have to multiply result to 1000.
If the multimeter is displaying 1 or OL (over limit) that indicates resistance is higher in magnitude and you have to increase range to 200k or more now.
If the result you get is 0.00 or nearly to zero then you have to lower the resistance range because it indicates that resistance value is much lower then selected range. Decreasing range to 2k or 200 ohm can bring right result in this case.
Can You Measure Resistance in a Live Circuit?
It,s natural that this question comes in someone,s mind that why we can,t measure resistance in live circuits while we can test voltage and current. Also you can do an experiment to test resistance in live circuit and get no reading at all. It,s because of working principle of multimeters.
Multimeters apply a known refrence voltage or current to circuits and calculate resistance according to it. So when someone try to do it on live or circuit with some voltage, it effects multimeter procedure and you can,t get result.
What is Impedance?
Impedance(Z) is a more broader form of current,s opposition to analyze industrial circuits. It,s actually overall resistance of a circuit or device to current flow. It consists of two magnitudes, one is resistance and other is reactance.
Z = R + X
Here Z is impedance, R is to denote resistance and X is for reactance. Reactance is a part of circuit current,s opposition which varies according to frequency while resistance is a stationary part and not depends on frequency.