By Kevin Roberts
As we showed last time, sometimes there is no electrical difference between an intended load and an unintended load in a series circuit. We used balanced loads to make that clear.
Today we will look at the principles behind unbalanced loads and how we can use what we learn to train our brain to think in a certain way. It is in the development of thinking skills that we can learn to recognize that odd problem that may seem as though the vehicle is playing tricks on us.
If you have two loads, one intended and one unintended, odds are that they will not be balanced. Even using a relatively coarse 1-volt increment across a 12-volt range makes it 12 times more likely that the loads will be unbalanced. No surprise there.
But you may be surprised at one bit of consistency even in unbalanced loads in series. You see Kirchhoff did not stop at a single law. His Voltage Law that states that all Voltage Drops in a series circuit will add up to source voltage, is not only eminently useful, it is also intuitive. Many times in class, I have shown Source Voltage with the intended load’s Available Voltage labeled. When I add an additional corrosion-based Voltage Drop unlabeled, the students always seem to figure out that the Source Voltage minus the Available Voltage equals the unintended Voltage Drop.
Kirchhoff’s other law is not so intuitive. This law refers to amperage in a series circuit. “In a series circuit, any amperage that flows into a connection must have an identical amount of amperage flowing out of it”. This may seem intuitive when expressed this way. But in my experience, just looking at a wiring diagram doesn’t bring this idea to the surface as easily as Kirchhoff’s Voltage Law. Settle in, this might take a little time.
Consider Figure 1 below.
Figure 1
Even though the loads show a different Voltage Drop, according to Kirchhoff, the amperage into each connection must equal the amperage out.
If you ask, “Who cares?” or “Is this just theory?” or “Is this something useful?” you are the reader that this article is intended for.
It takes time to diagnose electrical problems. As anyone who has had to find wire corrosion on a 40-foot apparatus or an intermittent short on a 60-foot equipment trailer has found, the time spent can be frustrating. It is in these moments that a clear understanding of the theory can be your friend.
Let’s say you have an admin vehicle with one slow power window: the driver’s front door. It’s the chief’s vehicle and he wants it back in service ASAP. If you have seen this, the fork in the road is an important one. Do we take the door apart or not? Depending on the year make and model, this may be relatively painless or a total nightmare. How can we get the most information in the least amount of time?
First, what could cause a window motor to be slow? Could it be the dreaded Voltage Drop? Maybe. Could it be a faulty window regulator overloading the motor? Maybe. Can we know the answer to that question before we perform surgery on a door panel that is made roughly the same strength as your average potato chip? Let’s try.
Second, let’s use the “known good” principle. If you have ever wished you had an identical rig in the parking lot to perform a simple test on a “known good” candidate, you know where I’m going. If only one of the power windows is slow, you have a known good one on the other front door. (And no, we don’t want to take that door apart either).
Third, do we find a fuse/relay? We could but that might take time. Rather, let’s just use the battery cable. This is where Kirchhoff’s Amperage Law shines.
See Figure 2 below.
Figure 2
Notice there are no identifying labels on anything in the figure. You probably don’t need them to figure out what the battery is and what the fuse box is. But the point is, you don’t need the labels to test the vehicle. When the engine is not running, the Source Voltage is from the battery. The battery feeds the fuse/relay box. The fuse box feeds the window motor(s). We really only need the battery. Once you find the battery positive cable, you open the front doors and put the vehicle into KOEO (Key On Engine Off) mode, with either the key or the push-to-start button. This will begin to draw amperage from the battery. Grab your amp clamp and place it around the battery positive. If you like math, that’s good enough. If you don’t like math, zero the amp clamp. Then, without doing anything else to the vehicle, first have a helper operate the “known good” window. Note the amperage. Then operate the faulty (slow) window. Note the amperage. The readings will likely be different. If the working window draws more amperage, you are starving the slow window for amperage (using the Current Limiter function of a Voltage Drop). If the slow window draws more amperage, you have a problem inside the door (probably). What we just accomplished is we gained information in a very short time that told us which fork in the road we should take (do or do not take the door apart) without guessing or hoping.
The amperage that any motor circuit draws is based on what the load needs, and the ability of the harness to flow that amperage. Any unintended resistance limits the amperage (current) to the intended load. Since we don’t want to check Available Voltage inside the door, we can compare the amperage draw to a known good. If the amperage draw on an improperly functioning motor can be measured and matches the amperage draw on a properly functioning and identical motor, we have no reason to suspect a Voltage Drop. If the amperage is lower, that is sign of a Voltage Drop. If the amperage is higher that is a sign of a heavier load on the motor.
By using Kirchhoff’s Amperage Law, we can infer Voltage Drops where we cannot access the component to measure Available Voltage.
Thanks again, Mr. Kirchhoff.
