Ohmmeters Analog And Digital

An ohmmeter is powered by an internal battery that applies a small voltage to a circuit or component and measures how much current flows through the circuit or component. It then displays the result as resistance. Ohmmeters are used for

checking continuity and for measuring the resistance of components. Zero resistance indicates a short while infinite resistance indicates an open in a circuit or device. A reading higher than the specification indicates a faulty component or a high resistance problem such as burnt contacts, corroded terminals or loose connections.

Ohmmeters, because they are self-powered, must never be connected to a powered circuit as this may blow a fuse in the meter and damage its battery. Unless the circuit being measured contains a diode, polarity (attaching the leads in a particular order) is inconsequential.

An analog ohmmeter should be calibrated regularly by connecting the two leads together and zeroing the meter with the adjust knob. This compensates for changes in the state of charge of the internal battery.

CAUTION: Analog ohmmeters may apply a higher voltage to a circuit than a digital ohmmeter, causing damage to solid state components.

Use analog ohmmeters with care. Digital meters, on the other hand, apply less voltage to a circuit, so damage is less likely.

Analog meters can also bias, or turn on, semi-conductors and change the circuit by allowing current to flow to other portions of the circuit.

Most digital meters have a low voltage setting which will not bias semiconductors and a higher voltage setting for testing semiconductors. The information displayed on a digital meter in the diode test function differs from one meter brand to another. Some digital meters will display a value which represents the perceived resistance of the diode in forward bias. Other meters will display the forward bias voltage drop of the diode.

Digital ohmmeters do have one limitation. Due to the small amount of current they pass through the device being tested, they cannot check some semiconductors in circuits, such as a clamping diode on a relay coil.

Many analog ohmmeters will, when switched to the ohm function, reverse the polarity of the test leads. In other words, the red lead may become negative and the black lead may become positive. The meter will function properly as long as you are aware of this and reverse the leads. This is especially important when working with diodes or transistors which are polarity sensitive and only allow current to flow from the positive to the negative end. To check for polarity reversal, set the ohmmeter in ohm function and connect its leads to the leads of a voltmeter (red to red, black to black). If the voltmeter shows a negative value, that particular ohmmeter reverses polarity in ohm function. Most digital meters do not reverse polarity.

You should note that ohmmeters do little good in low resistance, high current-carrying circuits such as starters. They cannot find points of high resistance because they only use a small amount of current from their internal batteries. In a large conductor (such as a battery cable), this current meets little resistance. A voltage drop test during circuit operation is much more effective at locating points of high resistance in this type of circuit.

Taken with permission from the Toyota Advanced Electrical Course#672,

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