Troubleshooting Parallel Circuits



Movie(s) created by Tim Fiegenbaum at North Seattle Community College.


Lecture Transcript


Point two, troubleshooting parallel circuits. Parallel circuits are a basic way of connecting components but may be difficult to troubleshoot. Logical, professional methods need to be used when troubleshooting complex circuits. Three rules that also apply to troubleshooting parallel circuits are, and we introduced this in section 5.1 on series circuits; make a measurement only if you know what a good reading should be; make as few measurements as possible and select the best tool for the task at hand. This is one of the tools that can be used with parallel circuits: a current probe. A current probe senses the magnetic field produced by a current carrying conductor and produces a corresponding voltage. Current probes are useful for circuits with discreet interconnecting wires. Some will only work with AC and must be clamped onto the circuit. Others will work with DC as well.

Troubleshooting with a current probe and here, we have an example. We have a circuit and we are showing what you would see with a current probe if you connect it across these components. Notice we have R1, R 2, and R3. R2 is open. With R1, our current probe will sense a current in this component. R3 it senses a component. R2 it doesn't sense any current and that would be our indication that this component is in fact open.

Troubleshooting with a temperature probe. A temperature probe is another attachment that can be used with a digital multimeter that is with some digital multimeter. Now all will support them. Power dissipation and a circuit produces heat and therefore relative values of temperatures can be measured and used to troubleshoot a circuit. Temperature probes can be used to compare dissipation to other components. You can use this to get a relative sense of power dissipation between other components on the same circuit board. Temperature probes can provide a general indication of current or no current based on heat. You can use this to go in and it can just give you a general indication, is there current or is there no current flowing through a given circuit?

Schematics don't usually address expected temperatures, so on a schematic diagram you don't typically have values that say you should have such and such a temperature on a component. You usually – these schematics would indicate voltage, current, resistance and such but not typically temperature.

Effects of a short circuit. As in a series circuit, if a branch in a parallel circuit develops a short extremely high currents may flow through the short circuit. The voltage across all branches of a parallel circuit will be zero with a short circuit. The offending component will usually be damaged by the high current unless protected by a fuse or circuit breaker. Here we have a parallel circuit and from our parallel resistance studies, we would find that the parallel resistance from these three would be would 333 ohms. If we were to calculate the current in this circuit if we took Ohms Law and we said E/R = I we would get about 30 milliamps of current would be expected in this particular circuit. If we had a short, let's pretend we had a short from here to here. Notice that short is directly across the power supply. This is a little bit different than we saw in series circuits.

This was going to put a tremendous load on this power supply. Where there was 333 ohms and about 30 milliamps of current being drawn from this supply suddenly we are going to go down to a value, if we could say, it shorts and it goes down to 0.1 ohms. If that was the value then suddenly our current that is going to be required from this was going to jump up 100 amps. That simply isn't going to happen. A couple of things might happen. For one, the power supply could just fail. For another likely the current through this component would be so severe that it would heat up and then it would simply open.

Probably the most likely scenario is that you would have a fuse in this circuit and when the current draw on this power supply became more than it could handle, the fuse would melt and the circuit would remain undamaged. However when you put another fuse back in you would just have the repeat fault and then you would need to go in and begin to measure components and determine, “Well what is the faulty component in this particular system?” That is the overview of symptoms you might see in a short circuit.

Effects of an open circuit. An open in a parallel circuit will have less impact on the overall circuit operations. Voltages and currents across other components remain unchanged, total current and power across the complete circuit goes down. Let's take a look at this. If we have an open here, and we talked about this earlier in this particular lesson. Let's just talk about this a little bit. If we have an open here, notice this ten volts, it is really not impacted by that because it still provides ten volts across R2, still ten volts across R1 and we may not even notice that R1 is open. When troubleshooting this, one of the things we could consider. Remember we looked at our parallel resistance. It would have been 333 ohms if we had turned power off and we measured resistance across, and this was in fact the case. Our parallel resistance would be up to 500 ohms because then it would be the parallel of just these two, that would be an indicator that one of these components is in fact open.

Commonly with a situation like this when you have an open component it is usually quite obvious is you just do a visual inspection because usually the reason it's open is because it's overheated and it's open. Typically, the component will be burned or it might even be warm to touch if the circuit's been on. Those are some of the indicators that you might see in troubleshooting a circuit like this.

Okay, so we've been looking at troubleshooting a parallel circuit. We looked at opens. We looked at the effects of a short, and remember that a short and a parallel circuit can be quite devastating. We looked at troubleshooting using a temperature probe as well as a current probe and that concludes 5.2, troubleshooting parallel circuits.