Using Your Voltmeter As a Voltage Drop Detector

Ohm’s law describes the relationship between volts (electrical pressure), amps (electrons flowing through the circuit), and resistance (ohms or resistance that the circuit must overcome). Ohm’s law states that voltage is equal to the product of amps and resistance (E = I x R). As resistance decreases, amperage increases and vice versa. Motors are inductive loads, meaning that applying force against a motor increases the amp draw and reduces the electrical resistance.

Before using a voltmeter, set it on the ohm scale and ohm between the leads to make sure there is a path through them.

We can take a voltmeter and measure the electrical potential (voltage) between any two points. On a simple 120v circuit with a lightbulb and a switch between two legs of power, we would measure 120v between both legs of power. The voltmeter has two leads so that you can measure the potential (or voltage drop) between two points. The total voltage drop is equal to the total resistance of the circuit. In the case of a 120v simple circuit with all the resistance in one lightbulb, the total resistance would be in the lightbulb, and we’d read 120v across it. (However, that’s not realistic because wires and switches have a little bit of resistance.) These sorts of tests between points on a circuit are only valuable when the circuit is under load, and a case like an open switch would make an open path and make there be no potential between the two points.

If we read significantly less than 120v across a 120v load, we can determine that we have a voltage drop somewhere in the circuit. We can use our voltmeter as a voltage drop detector to isolate areas of voltage drop (such as 4v across a switch). Our goal is to reduce voltage drops to the best of our abilities, so we need to isolate them within the circuit and address them. Switches and wires should have very little resistance, as those are power-passing devices; a significant voltage drop across a wire or switch is usually a cause for concern, as those components can increase in temperature.

Measuring the applied voltage across a contactor (or open switch) can let us know what the total input is. Measuring the voltage across a closed switch will tell us the voltage drop across the switch. Situations that create resistance, such as insects on the contactor, will also cause current to be drawn. The overall circuit current will decrease, but the localized current will increase and generate heat.

On a compressor in a 240v circuit with the contacts open, we would measure 240v between each leg of power. When the contacts are pulled in, we can begin searching for a voltage drop; if the voltage stays pretty close to 240v at different points across the load and through the contacts, then there is a minimal voltage drop. If the voltage drop is significant, then we can start measuring the voltage drop across each contact point. We don’t need to ohm out the individual components; the voltage drop alone can tell us where our points of significant resistance are.

To prevent unwanted voltage drops, you can provide motors and loads with the proper voltage, ensuring that you have tight connections, and using wires of the proper size. When you suspect an area of high resistance, you can use your voltmeter to detect and isolate a voltage drop without ever switching to the ohm scale.