By Kevin Roberts

Today we will go a little deeper into the third voltage test from last time, that of Available Voltage.

To review, Available Voltage is the voltage that is delivered by the Source Voltage (battery, for our purposes today) through the harness to the load. Clearly, it is a problem with Available Voltage that captures the attention of the driver. Either the engine cranks slowly, the bulb is dim, or the wipers barely move.

If you have a poorly functioning load, you likely have some sort of Voltage Drop in the harness that is feeding that load.

As we alluded to last time, an understanding of Kirchhoff’s Voltage Law applies here. If you find that the Available Voltage to a load is less than Source Voltage, you know two things. You know that you don’t have an Open Circuit. If you did, the Available Voltage would be zero. This may seem obvious but sometimes the obvious gets missed. You also know something is causing the Available Voltage to be low.

This is where Mr. Kirchhoff and his law come into play. The difference between Source Voltage at the battery, and Available Voltage to the load is what we refer to as Voltage Drop. Plus, the numbers add up. If your Source Voltage is 12.2 volts and your Available Voltage at the load is 10.2 volts, according to our friend Kirchhoff, you have a 2.0 volt drop. A Source Voltage of 12 volts with an Available Voltage of 4 volts shows an 8 volt drop, and so on. And finally, a Source Voltage of 12 volts with an Available Voltage of 8 volts shows a 4-volt drop.

See Figure 1 below.

Figure 1

We know we have a Voltage Drop, and we know it (or they) will be 4 volts, but we don’t yet know where it is hiding. In order to find it we need to do a Voltage Drop test. See Figure 2 below.

 Figure 2

Now for the fun part. A Source Voltage of 12 volts, and an Available Voltage of 8 volts shows a 4 volt drop as in Figure 2 above. But the reverse is true as well. A Source Voltage of 12 volts, and an Available Voltage of 4 volts shows an 8 volt drop. Figure 3 below.

Figure 3

Do you see the pattern? If all you knew was there were two voltage drops in series, you could not tell the difference, just from the meter, between which one was intended and which one was unintended. Electrically there is no difference. Remember that, it’s important.

So, let’s equalize the Voltage Drop and see where that leads us. There would still be no difference electrically. See Figure 4 below.

 Figure 4

Our goal today is to gain an understanding of the similarity between Available Voltage and a Voltage Drop. In our first article we stated as much. Now we want to drill it into our consciousness.

If you have seen an engine cooling fan circuit similar to a late model Silverado or Suburban, you are familiar with the operation of the two fans in series for low speed, and in parallel for high speed. When two loads are operated in series (intentionally) each load functions as a Voltage Drop for the other load.

If you remember our first article in this series, we stated that a component “is a Load, that functions as a Current Limiter, because it has Resistance and that can be measured using Voltage Drop Testing with a meter.”

In the case of our Suburban, we have two intended Voltage Drops in series. Each fan is a Load, that offers resistance, that drops voltage, and limits current to the other fan.

Notice how Figure 5 below is almost identical to Figure 4. This is because they are electrically identical. The only difference is that we have identified the second load in Figure 4 as unintended, but both are intended in Figure 5.

Figure 5

The point here is that this explains why you need to have a circuit live in order to find an unintended Voltage Drop. No Tech that I have ever spoken with has ever forgotten that in order to have a component function, you need to close the circuit. But many have forgotten that to find a Voltage Drop you need to close the circuit. Once you have trained your brain to understand the two (intended Load and unintended Voltage Drop) as the same thing electrically, you can’t make that error. You will remember this more consistently when the basis is understanding and not merely memorization.

To summarize: The Available Voltage to an intended load is electrically identical to a Voltage Drop across an unintended load. We know of the existence of the unintended load by comparing Available Voltage to Source Voltage. We know the amount of unintended Voltage Drop by using the math provided by Kirchhoff.

As I say in class, Kirchhoff’s Law is the Law you will use. With all due respect to Georg Simon Ohm and his law, I really don’t use Ohm’s Law as often as I use Kirchhoff’s Voltage Law. Hats off to Kirchhoff.

Next time Voltage Drops and Amperage Flow.