Tool for measuring electrical resistance

Not to be confused with ohm metre

An ohmmeter is an electrical instrument that measures electrical resistance (the opposition offered by a circuit or component to the flow of electric current). Multimeters also function as ohmmeters when in resistance-measuring mode. An ohmmeter applies current to the circuit or component whose resistance is to be measured. It then measures the resulting voltage and calculates the resistance using Ohm’s law V = I R {\displaystyle V=IR} .

An ohmmeter should not be connected to a circuit or component that is carrying a current or is connected to a power source. Power should be disconnected before connecting the ohmmeter. Ohmmeters can be either connected in series or parallel based on requirements (whether resistance being measured is part of circuit or is a shunt resistance).

Micro-ohmmeters (microhmmeter or micro ohmmeter) make measurements of low resistance. Megohmmeters (also a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is the ohm (Ω).

Design evolution

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The first ohmmeters were based on a type of meter movement known as a 'ratiometer'.[1][2] These were similar to the galvanometer type movement encountered in later instruments, but instead of hairsprings to supply a restoring force they used conducting 'ligaments'. These provided no net rotational force to the movement. Also, the movement was wound with two coils. One was connected via a series resistor to the battery supply. The second was connected to the same battery supply via a second resistor and the resistor under test. The indication on the meter was proportional to the ratio of the currents through the two coils. This ratio was determined by the magnitude of the resistor under test. The advantages of this arrangement were twofold. First, the indication of the resistance was completely independent of the battery voltage (as long as it actually produced some voltage) and no zero adjustment was required. Second, although the resistance scale was non linear, the scale remained correct over the full deflection range. By interchanging the two coils a second range was provided. This scale was reversed compared to the first. A feature of this type of instrument was that it would continue to indicate a random resistance value once the test leads were disconnected (the action of which disconnected the battery from the movement). Ohmmeters of this type only ever measured resistance as they could not easily be incorporated into a multimeter design. Insulation testers that relied on a hand cranked generator operated on the same principle. This ensured that the indication was wholly independent of the voltage actually produced.

Subsequent designs of ohmmeter provided a small battery to apply a voltage to a resistance via a galvanometer to measure the current through the resistance (battery, galvanometer and resistance all connected in series). The scale of the galvanometer was marked in ohms, because the fixed voltage from the battery assured that as resistance is increased, the current through the meter (and hence deflection) would decrease. Ohmmeters form circuits by themselves, therefore they cannot be used within an assembled circuit. This design is much simpler and cheaper than the former design, and was simple to integrate into a multimeter design and consequently was by far the most common form of analogue ohmmeter. This type of ohmmeter suffers from two inherent disadvantages. First, the meter needs to be zeroed by shorting the measurement points together and performing an adjustment for zero ohms indication prior to each measurement. This is because as the battery voltage decreases with age, the series resistance in the meter needs to be reduced to maintain the zero indication at full deflection. Second, and consequent on the first, the actual deflection for any given resistor under test changes as the internal resistance is altered. It remains correct at the centre of the scale only, which is why such ohmmeter designs always quote the accuracy "at centre scale only".

A more accurate type of ohmmeter has an electronic circuit that passes a constant current (I) through the resistance, and another circuit that measures the voltage (V) across the resistance. These measurements are then digitized with an analog digital converter (adc) after which a microcontroller or microprocessor make the division of the current and voltage according to Ohm's law and then decode these to a display to offer the user a reading of the resistance value they're measuring at that instant. Since these type of meters already measure current, voltage and resistance all at once, these type of circuits are often used in digital multimeters.

Precision ohmmeters

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For high-precision measurements of very small resistances, the above types of meter are inadequate. This is partly because the change in deflection itself is small when the resistance measured is too small in proportion to the intrinsic resistance of the ohmmeter (which can be dealt with through current division), but mostly because the meter's reading is the sum of the resistance of the measuring leads, the contact resistances and the resistance being measured. To reduce this effect, a precision ohmmeter has four terminals, called Kelvin contacts. Two terminals carry the current from and to the meter, while the other two allow the meter to measure the voltage across the resistor. In this arrangement, the power source is connected in series with the resistance to be measured through the external pair of terminals, while the second pair connects in parallel with the galvanometer which measures the voltage drop. With this type of meter, any voltage drop due to the resistance of the first pair of leads and their contact resistances is ignored by the meter. This four terminal measurement technique is called Kelvin sensing, after William Thomson, Lord Kelvin, who invented the Kelvin bridge in 1861 to measure very low resistances. The Four-terminal sensing method can also be utilized to conduct accurate measurements of low resistances.

References

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1. ^http://www.g1jbg.co.uk/pdf/MeggerBK.pdf Archived 2012-03-15 at the Wayback Machine A pocket book on the use of Megger insulation and continuity testers.
2. ^

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   ][1] Illustration of type. Note the absence of any zero adjustment and the changed scale direction between ranges.

https://www.codrey.com/electrical/ohmmeter-working-and-types/

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Using Meters for Troubleshooting

Voltmeter

A voltmeter is designed to be used on a live circuit. When working on live circuits, all safety precautions and PPE (personal protective equipment) requirements should be adhered to.

A voltmeter is essentially a very high value of resistance. When the voltmeter leads are connected to two points in a circuit, that high value of resistance is connected in parallel with that point and, due to Kirchhoff’s Voltage Law, experiences the same value of voltage. By measuring how much current flows across the meter’s internal resistance, it can calculate the voltage value.

Voltmeters typically read zero volts when measuring between points of equal potential. Closed switches and connected wires are examples of components that, when energized, are of equal potential.

Voltmeters typically read line voltage when measuring between points of different potential. In a control circuit this is line voltage. In the power circuit, this is a phase-to-phase voltage, which is different from their phase-to-ground voltage.

Open switches and contacts are examples of components that a voltmeter would measure line voltage across.

When current flows through a circuit, it drops off voltage proportional to the resistance of the device it flows through. Since closed switches and contacts offer nearly zero resistance, there will be no voltage drop measured across them. In every branch of a control schematic, there must be only one load to limit the current, and this device will have line voltage dropped across it when the circuit is operating. The coils of motor starters, control relays, and timer relays are examples of loads that when energized would have full line voltage dropped across them. Pilot lights are another example of resistive loads that would experience full line voltage.

Ohmmeter

Disconnect circuit from power supply first! Before using an ohmmeter in a circuit, use a voltmeter to confirm that power is off and that there is zero potential difference between the two points you wish to measure.

An ohmmeter works by using an internal voltage source to push a small DC current through its leads. By measuring the value of current, it can display a calculated value of ohmic resistance. Because it has an internal voltage source, ohmmeters cannot ever be connected in live circuits as they could cause damage to equipment or injury to the operator.

When using an ohmmeter in a control circuit, there are three typical readings you can get:

1. Close-to-zero ohms: This means that the leads of the ohmmeter are connected across two points that are electrically common. The two terminals of a closed switch or contact would give an ohmmeter reading close to zero ohms.
2. Very-high-to-infinite ohms: An open in the circuit would allow zero current to flow and so would read as infinite ohms. The terminals of open switches and contacts would give very high ohmic readings.
3. Some ohms: An ohmmeter measuring across a load such as a pilot light would read a very high (approximately megaohms), but not infinite, value of ohms. This is one way of confirming that a pilot light is in good working condition. The terminals of a coil should have continuity and a low (approximately tens to hundreds of ohms), but not zero value of resistance. If a coil is shorted and reads zero ohms, it needs to be replaced.

When using an ohmmeter to test fuses, first confirm they are removed from the circuit. If a fuse is in good condition, it should give a close-to-zero reading of resistance. If the fuse has blown due to a fault, then it should behave as an open and give an infinite resistance reading.

definition

A device testing and measuring the potential difference (voltage) between two points. Leads are connected in parallel with the circuit, and the meters very high internal resistance will draw a small current which can be used to determine the level of voltage.

Can be digital or analogue and measure either AC or DC.

The opposition to the flow of current in an electric circuit, measured in ohms (Ω).

In electrical terms, refers to a connection where current has more than one path to flow.

Loads connected in parallel will experience the same potential difference (voltage), but may draw different values of current depending upon their individual resistance.

"The sum of the voltage rises must equal the sum of the voltage drops in a circuit."

In a series circuit, the total voltage at the source is equal to each of the individual voltage drops in any loads.

E total = E1 + E2 +E3...

Loads connected in a parallel circuit will experience the same potential difference.

E total = E1 = E2 = E3...

The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).

A device for making or breaking the connection in an electric circuit.

In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load. The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation.

In contrast to the control circuit, the power circuit provides the large values of voltage and current used by the motor itself. Must be equipped with overcurrent and overload protection, and horsepower-rated contacts in the control gear equal to the voltage and current ratings of the motor.

The conducting part of a switch that makes or breaks a circuit.

A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.

Used when additional auxiliary contacts are needed in a control circuit, a control relay is a magnetic contactor which is not designed for the energization of motors, and does not have built in overload protection.

A small lamp connected in the control circuit to indicate the status of a motor or other situation.

A device used to measure the resistance of a circuit. Ohmmeters must not be used on live circuits. Ohmmeters connect a small internal voltage source to the circuit that is being measured or tested,  and determine the value of resistance or continuity by measuring what value of current flows through the meter.

Can be either digital or analogue.

Referring to two or more points in a circuit which have no loads or switches between them and have no potential difference between them.

The unit used to measure electrical resistance (Ω). It takes one volt to push one amp through one ohm of resistance.

An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.