Checking Amperage

Electrical Components

  • Electrical components in a circuit must draw a specific amount of current to operate properly.

  • Measuring the current flow in a circuit helps determine if an electrical device is functioning correctly.

  • Low current flow (low amperage reading): Indicates higher than normal resistance in the circuit.

  • High current flow (high amperage reading): Indicates lower than normal resistance in the circuit.


Amperage Measurements in Series and Parallel Circuits

  • Series Circuit:

    • The same amount of current flows through every component in the circuit.

    • Amperage is equal at all points in a series circuit.

    • If one component opens (fails), the entire circuit stops conducting current.

    • Example: If the total current is 2 amps, every component in the circuit carries 2 amps.

  • Parallel Circuit:

    • The total circuit current is the sum of the currents through each branch.

    • Each branch receives a portion of the total current based on its resistance.

    • Branches with lower resistance draw more current.

    • The total current measured at the power source equals the combined current of all branches.

    • Example: If one branch draws 2 amps and another draws 3 amps, the total current is 5 amps.


Reasons to Check Circuits for Amperage

  • Cranks Slowly

  • Battery Dies

  • Never place the leads of an ammeter across the battery or a load; it will either open the fuse or destroy the DMM.


Measuring Current Flow

  • There are two methods:

    • Use an inductive clamp-on meter for high current flow

    • Use a DMM for low current flow

  • Don't use the DMM for measuring high current flow, or you risk damaging it


Steps to Measuring Amperage

Step 1

Determine the amount of current that can be expected in the circuit.

Step 2

Set the mode selector switch to the correct DC amperage setting (milliamps or amps).

Step 3

Connect the black test lead to the common (COM), or negative, input jack. 

Step 4

After connecting the black test lead:

  • Connect the red test lead to either the amps or the milliamp or microamp port

  • Always use the highest rated amp port first

Step 5

After connecting the red test lead:

  • Prepare a fused jumper wire

  • Make sure the fuse has a rating that is less than the maximum allowable current

Step 6

Open the circuit at a point between the power supply and the load.

Step 7

Next:

  • Connect the DMM and fused jumper wire in series between the power supply and the load

  • This allows the current to flow through the DMM

Step 8

If applicable, turn on the DMM using the ON/OFF button on the meter.

Step 9 

Finally:

  • The amperage measurement should display on the DMM

  • Use a clamp or current probe if the amperage exceeds the DMM rating