Using a Multimeter

For the purpose of this discussion, I am only going to talk about analog multimeters and simple direct current (DC) circuits. Digital multimeters work in a similar fashion. Multimeters are multi-function meters. In the simplest form, a multimeter has the functions of a voltmeter (obviously for voltage), an ammeter (for Amps), and an ohmmeter for measuring resistance (in Ohms).

Voltage:

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Voltage is measured by a voltmeter by connecting across the portions of a circuit where you want to determine the difference in potential. Inside the meter is an electromagnet (coil of wire) that creates a magnetic field when current flows through it. This magnetic field is used to move a magnetic needle over a calibrated scale from which the voltage is read. The different ranges on a multimeter are created by switching resistors to reduce the amount of current flow far enough that the current drain by the meter has a minimal effect upon the circuit being measured. It is worth noting that what the little needle is indicating is the amount of current flow since this is what creates the magnetic field which moves the needle. Voltage is measured by placing the meter probes in parallel with the portion of the circuit being tested.

	o	Connecting the voltmeter leads to battery terminals shows how much voltage is present in the battery.
	o	There should be no voltage read across the terminals of a switch that is in the closed (or ON) position.

Current:

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Current can be read in either of two ways. A clamp can be placed over the insulated wire. This clamp senses the magnetic forces generated by electrical current any time if flows. The other method is by direct-reading.

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In a direct reading of current, you must break the circuit at any point and insert the Ammeter into the circuit in series with the other components.

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A very small resistor inside the Ammeter provides a voltage drop across a known resistance. This voltage drop is used to generate a very small current which, when fed into the same electromagnet as the voltmeter to deflect the needle indicating the current.

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This is more involved and intrusive than most other readings and care must be exercised in not passing excessive current through the meter to prevent damage.

Resistance:

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Resistance must only be read in a circuit with no power applied. The meter provides the current needed to measure the resistance and any other voltage present will not only skew the results, but probably damage the meter.

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Always check for voltage before selecting the resistance feature.

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As with the other functions, a current is passed through the resistor being tested and the voltage drop is measured.

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The meter has a known voltage source and the different resistance scales are each a different internal resistance within the meter which is placed in series with the resistor being tested.

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The percentage of Voltage drop across the resistor being tested versus the internal resistance applied when selecting the scale, indicates the value which is read on the scale in Ohms.

Not all readings with a multimeter need to be direct readings. Sometimes you want to measure current, but most Amp meters will blow a fuse if more than 10 Amps is routed through them. Any time you suspect the current might possibly be too high for your meter, there are alternatives.

Indirect Current Reading:
Sometimes you need to know a value when you are unable to take a direct reading. For instance, if you need to know the current in a circuit but are unable to shut down power to insert your test leads. You can determine current with a few voltage measurements of known data points. In our example, we have a supply voltage of 400 Volts DC and we have identified a resistor in the circuit that we know to be a 10 ohm resistor.

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First verify the supply voltage is 400 Volts DC.

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Then check the voltage drop across a component of known resistance (We are using a resistor in the circuit with a known value of 10 ohms). The reading is 50 volts.

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So we know that the other elements in the circuit present a voltage drop of 350 Volts because 400 - 50 = 350.

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The percentage of circuit resistance of our known component is 50 / 400 = 0.125 (or 12.5%)

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If 12.5% of the voltage drop of the overall circuit is within this one component, the rest of the resistance is 87.5% of the voltage drop of the whole circuit.

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87.5 / 12.5 = 7, so the rest of the resistance in the circuit is 7 times greater than our reference resistor of 10 ohms so that makes it 70 Ohms.

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Overall circuit resistance is 10 X 8 or 80 Ohms.

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The current flowing through the circuit is 400 Volts divided by 80 Ohms (or 5 Amps)