Voltage Drop Tests

PURPOSE OF THE VOLTAGE DROP TEST

Voltage Drop Tests are often used to find components or circuits which have excessive resistance. A voltage drop across closed contacts or wiring indicates excessive resistance.

CAUSE OF CIRCUIT VOLTAGE DROP

A voltage drop in a circuit is caused by a resistance when the circuit is in operation. Part of the available voltage is used by the resistance resulting in less available voltage for other loads (lights, motors, etc).

Unwanted high resistance can be caused by:

^ Undersized Wiring (single strand example)
^ Corrosion On Switch Contacts
^ Loose Wire Connections Or Splices.

EXAMPLE OF AN INCORRECT CIRCUIT TEST






Check the wire in the illustration. If an ohmmeter is used to measure resistance (circuit off), the single strand of wire still making contact would give a reading of 0 ohms. This would normally indicate a good circuit. But when the circuit operates, the wire will not be able to carry enough current and the bulb will be dim. Using a proper Voltage Drop test will show this wire to have high resistance.


EXAMPLES OF CORRECT CIRCUIT TESTS

Measuring Voltage Drop - Accumulated Method

1. Connect the voltmeter across the connector or part of the circuit you want to check. The positive lead of the voltmeter should be closer to power and the negative lead closer to ground.

2. Operate the circuit

3. The voltmeter will indicate how many volts are being used to "push" current through that part of the circuit.






NOTE: In the illustration there is an excessive 4.1 volt drop between the battery and the bulb.

Measuring Voltage Drop - Step By Step

A step by step voltage drop test can be used to identify a component or wire which is operating under too much resistance. It is most useful in isolating excessive drops in low voltage/current systems such as those in Computer Controlled Systems..

Circuit Inspection:

1. Connect the voltmeter as described in illustration, starting at the battery and working your way around the circuit.

2. An unusually large voltage drop will indicate a component or wire that needs to be repaired. As you can see in the illustration above, the poor connection causes a 4 volt drop.

The chart that follows illustrates some maximum allowable voltage drops. These values are given as a guideline, the exact value for each component may vary.

COMPONENT VOLTAGE DROP
Wire <.001 volts
Ground Connections Approx. 0.1 volts
Switch Contacts Approx. 0.3 volts
Starter Solenoids Approx. 0.5 volts