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 to current flow in circuit. Or you can say it as the opposition of medium from which current has to flow. And the quality of material to pass current from it is called conductance. So metals or other materials with good conductance value mean they have less resistance in them.
Its unit ohm is named after 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 current and r for resistance. According to it, resistance is inversely proportional to current, as resistance increases current will be lowered, and as lowers the resistance current will be increased.
How to Measure Resistance with a Multimeter
Although resistance is real opposition to current flow it becomes so important when designing electric systems or testing other parameters of devices and motors. In that sense finding the actual value of ohms becomes so vital, that it could be done in simple steps with help of a best digital multimeter or analog meter.
If you are not a professional electrician we will strongly recommend you first take necessary safety training to avoid any misfortune. Carefully read the manual of your multimeter and follow it strictly. To measure resistance through multimeter apply the following steps accordingly,
- First of all check power on multimeter and check the 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 off 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 rotary 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 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 more accurate readings could achieve.
- After test turns off the multimeter to save battery.
How to Measure Resistance of a Wire
To find the resistance of a wire through a multimeter is not much different from other resistance measurements. But some points have to be kept in mind before test. You should be sure that your multimeter has that close resolution that is required to test a specific wire. Other steps are the 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 multimeters are manual range, so first set the 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 the meter that means range is high so lower that range step by step and check results on the screen. In this process, a range will come where you will see exact resistance of a wire.
Many times we see 1 or OL sign on meter while testing resistance of wires or something else. That,s actually indicates that the 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 the right range of resistance for convenience during test. For example, if you have a piece of copper wire, check the resistivity of that wire according to the 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 the range according to it.
How to Measure Resistance in a Circuit
If you are a professional electrician or electrical engineer then you must know that resistance can,t be measured on live circuits. The reason behind it is multimeter’s functionality to find resistance. Multimeters apply a known voltage to circuit to find the possible current and resistance of that circuit. In presence of an external voltage multimeter is unable to give accurate results.
So the first step is to power off all types of supplies to that circuit, and unplug batteries. We recommend disconnecting the capacitor also If the circuit has any. Now you can check resistance between any two selected points of the 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 ohms, 200 ohms, 20 kilo ohms , 2 mega ohm and so on. Now if the 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 show that resistance magnitude is 0.86 or 0.90 which means resistance is 860 or 900 ohms. Because now the 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 the resistance value is much lower than the selected range. Decreasing range to 2k or 200 ohm can bring the right result in this case.
Can You Measure Resistance in a Live Circuit?
It’s natural that this question comes to someone’s mind 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 a live circuit and get no reading at all. It’s because of the working principle of multimeters.
Multimeters apply a known reference voltage or current to circuits and calculate resistance according to it. So when someone tries to do it on live or circuit with some voltage, it affects multimeter procedure and you can’t get a result.
What is Impedance?
Impedance(Z) is a broader form of current’s opposition to analyzing industrial circuits. It’s actually the overall resistance of a circuit or device to current flow. It consists of two magnitudes, one is resistance, and the 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 does not depend on frequency.