Electrical Circuit Voltage Measurements

Electrical and electronic topics for small boats
jimh
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Electrical Circuit Voltage Measurements

Postby jimh » Wed May 25, 2016 11:03 am

In making diagnosis of electrical problems, one often needs to measure the electrical voltage in a circuit. I present some advice on how to measure electrical voltage in typical boat electrical power distribution circuits.

Here is a sketch of the typical boat electrical power distribution circuit. Note that this sketch or circuit diagram follows the conventions of electrical drawings:

--the positive circuit is drawn along the top

--the negative circuit is drawn along the bottom

--the voltage source is drawn to the left

--the circuit flow is left to right for positive current

Image

In the circuit diagram I have included callouts with alphabetic notation so particular points in the circuit can be easily referenced. Now I will explain how to make voltage measurements.

BATTERY VOLTAGE

To read the battery voltage, connect the voltmeter as follows:

--connect the negative lead of the voltmeter to Z;

--connect the positive lead of the voltmeter to A.

The voltmeter will then read battery terminal voltage. Measuring the battery voltage directly at the battery terminals (A and Z) gives the actual battery voltage. If you want to measure the supply voltage available to a device, you can measure the supply voltage at the device. For example, in the diagram above, measure the supply voltage available to the device by connecting the voltmeter leads to K (+) and U (-). The voltmeter will measure the supply voltage at the device.

LOCATING CIRCUIT DISCONTINUITY IN POSITIVE BRANCH

If the circuit appears to have a discontinuity, you can locate the point in the circuit at which current flow stops in the positive circuit branch as follows:

--connect the negative lead of the voltmeter to Z;

--using the probe for the positive lead of the voltmeter, connect the probe in sequence to A-B-C-D-E-F-G-H-J-K circuit points. You should read the battery voltage (or very close to the battery voltage) at each of these points. If you read no voltage or a very significantly reduced voltage, you have discovered the first point downstream of the circuit discontinuity.

LOCATING CIRCUIT DISCONTINUITY IN NEGATIVE BRANCH

If the circuit appears to have a discontinuity, you can locate the point in the circuit at which current flow stops in the negative circuit branch as follows:

--connect the positive lead of the voltmeter to A;

--using the probe for the negative lead of the voltmeter, connect the probe in sequence to Z-Y-X-W-V-U circuit points. You should read the battery voltage (or very close to the battery voltage) at each of these points. If you read no voltage or a very significantly reduced voltage, you have discovered the first point upstream of the circuit discontinuity.