We are in section 5.3, troubleshooting series-parallel circuits. Because the complexity of series-parallel circuits, troubleshooting them may be quite challenging. The same rules apply as before with regard to measurements and tools. I won't review them again. Those techniques and troubleshooting charts used for series and parallel circuits individually should be combined for series-parallel circuits. Here we have a circuit that is a combination of series and parallel. What I want to do is just to walk through some of the things that I might do if I was going to be troubleshooting this circuit.
The first thing that I'll note is that there are five test points in this circuit. What we had mentioned is previous lessons is that doing lots of measurements doesn't mean a whole lot unless I have some idea of what I should be measuring. Now if we look at this circuit, we can make a couple of quick deductions. Test point one, if I'm measuring in respect to ground this should be 20 volts. If I'm looking at test point five with respect to ground that should be zero volts and those are kind of givens. What we'll see at two, three, and four we are going to have to do a little bit of thinking and we can actually quite quickly get an idea of what those values should be. In a parallel circuit, remember we had our rules that said, in a parallel circuit the parallel resistance will be slightly smaller than the smallest component.
I'm going to look at this and I notice that 500 over here. I'm going to make guestimation and this is just going to kind of guess. I'm going to say about 400 ohms and often times when you are troubleshooting, your calculations don't have to be exact. Sometimes they'll need to be exact but they don't always have to be exact. Then down here in R5 and R6, I know that if you had 200 and 200 the parallel resistance is going to be about 100, not above, but it'll be about 100 ohms. In looking at these circuits I can, I've got 100 ohms here, 400 here and then the parallel here does me about 400 and this would be about 100. I can see that I have about 500 here and about 500 here. Given that since I have 20 volts supplied and I've got half my resistances here, my guestimation is that this would… test point three should be about 10 volts because it is half-and-half.
Down on this side I'm going to note that, let's see, 100 to 400. That's about one-fifth of this ten volt so my guess is that test point four should be about two volts. Up here the other extreme, let's see, this is about four-fifths, about 500 ohms here. About fourth-fifths of 500 would be… given ten volts I would guess about 18 volts. These are some guestimates about what I would expect to see here.
I want to do a quick look at a few faults that could occur here. First of all, let's look if R1 shorted. If we had a short, went from R1, across R1 one of the things we would notice, a number of things that we could – there would be increased current in the circuits. All the drops across every component would slightly increase a little bit just because there's less resistance in the overall circuit now. As a troubleshooter, if I looked at test point one I would see 20 volts and since there's a short directly across here, now at test point two I would read the same voltage. There should be a two volt difference. Now there is zero volts difference. That tells me something that this component is shorted.
R4, if R4 was shorted. Let's pretend that this part of the circuit's okay. We'll open that short. Now we are going to short R4. In this case, again, the test point three and test point four would be the same voltage whereas before they should have been eight volts. That would be one indicator. Another indicator would be that if we went in and measured test point three we would find that it would be quite a bit less than it used to be because now the total resistance of this circuit is, let's see, there's 500 ohms up here and there's only 100 ohms down here. One sixth of this 20 volts, I should now see a test point three vice one-half of it. That would be an indicator. R4, the TP3 and TP5 having the same value would be the biggest indicator. We'll put this value. We'll say this is not short anymore.
So R6, open. R6 is down here. If R6 were to open, then the resistance across from between test point four and five was 100, it would jump up to 200. Now we would see an increase in the voltage at TP4. We would also see an increase in the voltage at TP3 because there was just 500 ohms of resistance between these two and now there are 600. There's 500 here. We expect this voltage to go up. Those would be some of the indicators. I guess I would probably be concerned about this test point four having to increase because that would be a substantial increase at that point and then I would need to go in and measure resistance and probably observe that this, usually when they open this because they overheated and they might be burned. They might even smell. So anyway, R6.
Then R4 being open now. In this case, if R4 were open. If we had a volt meter and we went in here and usually you are measuring voltage in respect to ground and if I measured the voltage at this point. I would see the supply voltage and that would be the giveaway that I have an open circuit is because I am measuring the supply voltage at a midpoint in the circuit. If I was to measure the voltage across any of these components, I would find that they would be zero because there's no current flowing in this circuit at this point. Also, I might mention that if I measured this side of the component I would see the supply voltage but let's say I measured from here to here, now I am measuring from ground to the bottom, say, I would measure zero volts.
This is some of the thinking that a technician might go through in troubleshooting a parallel or a series-parallel circuit. These are a lot of fun. It's kind of an exercise in applied physics to see how our circuit parameters are changing when given components either open and/or short. This has been a quick look at troubleshooting series-parallel circuits.