Ohm’s law plays a pivotal role in helping commercial trucking technicians diagnose problems as part of regular truck maintenance. According to Purkeys Sales and Service Engineer Rylar Masco, Ohm’s law is the relationship between voltage, current and resistance. When understood properly, this relationship can really help in the troubleshooting process and in understanding a truck’s electrical system as a whole.

Ohm’s law is most commonly written in the form:

V = I x R

The V stands for voltage, I stands for current, which is measured by amps – A, and R stands for resistance, which is measured in ohms – Ω. As Purkeys Chief Engineer Dale Henningson points out, the formula seems simple but has many applications.

In the video Dale uses a relay with a coil that has a resistance of 100 Ω. When using this relay in a 12 V system solve for the current passing through the relay by rearranging Ohm’s law to:

I = V / R

Therefore, the current would be:

12 V / 100 Ω = 0.12 A

Now, if you tested the coil and found the resistance to be 50 Ω instead of 100 Ω, it would mean that the relay is drawing twice the amount of current than it should. This could result in the relay overheating and possibly melting. Avoiding issues like this is just one of the reasons why understanding Ohm’s law is so important.

Purkeys recognizes the difficulty in remembering all the different variations of the Ohm’s law equation and has developed a “cheat sheet” listing all the various symbols and formulas to further assist in troubleshooting and diagnosing electrical problems.

Are you dealing with battery issues? Are you looking for advice? We welcome your comments and questions below.

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