Voltage Drop


Before we talk about voltage drop (Vd), lets refresh a bit on the three electrical values used when discussing electrical circuits; voltage, current, and resistance.

Voltage is electrical force or the pressure that pushes electrons (electricity) through a circuit. The higher the voltage in a circuit the more potential flow there is. Voltage is measured in volts.

Current is the amount of electrical flow in a circuit. Current is the actual amount of electricity flowing past a given point in the circuit. In electrical circuits; the higher the voltage in the circuit the higher potential flow will be. Current is measured in amperes.

Resistance is the opposition to electrical flow. In electrical circuits there is always resistance; most resistance encountered will be at the load. Resistance other than at the load is unwanted and affects the operation of the circuit. Resistance is measured in units of Ohm (Ω).

So, all things in a circuit working as designed will make for a proper working circuit. When any of the three values above become higher or lower than intended the circuit operation is affected.

Lower Voltage = Low Current = circuit performs less work

Higher Resistance = Low Current = circuit performs less work

Higher Voltage= Higher Current = circuit performs more work; which in most cases is unwanted as components will heat up and the fuse will blow.

Lower Resistance= Higher Current = circuit performs more work; which in most cases is unwanted as components will heat up and the fuse will blow.

As you can see current is what performs the work in a circuit. When current is changed the circuit will not operate as designed.

Circuit Faults:

Most circuit faults can be traced back to unwanted resistance. This resistance creates a voltage drop that reduces the voltage available to the load and the overall current flow in the circuit.

So let’s look at a basic circuit…….

Basic Circuit Operation:

circuit readings
Basic Circuit Operation

What Does a Voltmeter Measure?

All meters whether single or combination, measure the difference between the two meter leads. When measuring voltage the meter displays the difference in voltage between the two meter leads. The same applies when measuring resistance and current.

voltmeter, voltage drop, difference in potential
Voltmeter Readings

Voltage Drop:

So what is voltage drop? Voltage drop is voltage used. Voltage is used in a circuit anywhere resistance is encountered.

How is voltage drop measured? It is measure with your voltmeter, multimeter, or DMM.

Can I measure voltage drop in the positive or negative side of the circuit? Yes, absolutely. A voltage drop measurement is done with circuit energized and is ideal for finding loose connections or corroded wiring that increases in resistance when heated by current flow.


Max Drop Chart


Load Voltage Drop:

The circuit load is the component that uses the voltage in the circuit since it should be the only resistance in the circuit.

Load Voltage Drop
Load Voltage Drop

Measuring Voltage Drop Through a Connector:

Connectors often develop unwanted resistance due to corrosion, loss of terminal tension, or physical damage. This resistance creates a voltage drop that affects the operation of the load. in essence; a series circuit is created when unwanted resistance is part of a circuit.

Voltage drop through connectors, switches, fuses, and wiring should be minimal and should not exceed 0.2V on 12V circuits and 0.4 on a 24V circuit.

Connector Voltage Drop
Connector Voltage Drop

Powertrain and computers circuits are very susceptible to failing when unwanted resistance is present. These are low voltage circuits for the most part; follow specifications when troubleshooting these types of circuits.

Vd, connector, voltage drop
Connector Voltage Drop

Measuring Voltage Drop in the Whole Circuit:

Remember! voltage drop is voltage used. Most of the circuit voltage should be used (dropped) at the load. In the drawing below you can see that the load is only using about half of the voltage because unwanted resistance at a connection is using the rest. The 12V load is very dim since 5.5 volts is not enough to make it light normally.

bad connection, ground voltage drop
Voltage Drop
battery cable, Vd, unwanted resistance
Battery Cable Voltage Drop

In conclusion:

Measuring voltage drop is the most effective way of finding unwanted resistance. Why? with the circuit under load voltage will be divided between all resistance points in the circuit. If you were to just perform a resistance measurement; the reading will probably read within specifications, potentially leading you to a wrong diagnosis.


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