Voltage Drop Test

To understand how a voltage drop test works, understanding how a volt meter works, understanding Kirchoff's Voltage Law (KLV), and how corrosion affects circuits are necessary. I'm going to give a very high level explanation of these two concepts to allow us to move forward. For a deeper understanding of these topics, please utilize the Resources section of this web site to access more technical information.

How a Volt Meter Works

A volt meter determines the potential difference between two voltage points in a circuit. The following table will illustrate what a volt meter will register given the following conditions:

DVM

A web site named All About Circuits did a pretty good job at explaining how voltage sensing works and how voltage sensing could affect a circuit. <...Learn More...>

Red Lead Black Lead DVM Read Out
12V 0V 12V
0V 12V -12V
12V 5V 7V

Kirchoff's Voltage Law (KLV)

In a nutshell, we are going to leverage Kirchoff's Voltage Law in that all voltage is consumed by the expected load in a circuit. Well, after watching Jim Pytel's video below, you'll see that I've over simplified it. But, thinking of it this way will allow you to understand how a voltage drop test leverages this law.


The expected load of the following circuit is the starter.

Example Starter Circuit

In my humble opinion, Jim Pytel's YouTube video did an awesome job explaining Kirchhoff's Voltage Law, KLV. YouTube<...Learn More...>

 

Simple Circuit with No Corrosion

Single Load
Before using the JAVA Circuit Simulator, let's discuss what we expect the voltage values to be around a circuit. Let us look at a simple series circuit:

This healthy circuit allows the load to receive the complete 12V from the power source.
Here's a table of potential voltage differences in this circuit:
Red Lead Black Lead DVM Read Out
A B 0V
A C 12V
C D 0V
B D 12V
In the above table, I'm assuming the connecting wire does not have any resistance. Of course, in reality, wire does produce a little resistance.

How Corrosion Affects Circuits

Two Loads
Now, let's compare what happens to the primary load when additional resistance is added. The following diagram depicts a possible set up:

The mystery resistance steals some voltage from the primary load. This mystery load is corrosion, damaged wire, damaged connector, and/or corroded connector.

Due to Kirchoff's Voltage Law, we know that all the voltage will be consumed by all the loads in a circuit. So, we know that both loads will share the voltage. But, due to this, the primary load is not receiving all the voltage it needs to perform its job efficiently. Remember, corrosion adds resistance to a circuit. So as the corrosion in a circuit builds up more and more, the resistance increases and the voltage it robs becomes more parasitic. After a while, the primary load will not receive enough voltage.

Voltage Drop Test

Now that we have described how corrosion can add hidden resistance to a circuit. We leverge the voltage drop test to find it. Now, let's leverage the JAVA Circuit Simulator to visualize Voltage Drop and how the Voltage Drop Test functions.