Troubleshooting Series Circuits



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

Related Links


Lecture Transcript


We are continuing section 5.1 on troubleshooting and in our previous section, we had looked at troubleshooting open circuits and troubleshooting shorted circuits. We are picking up now on effects of component tolerances. Because of aging or overheating, component, values may change an electrical circuit. If a component value has increased in resistance then the effects would be similar to an open circuit. Notice that it is similar to. Remember when you have a circuit and there is a complete open, then the resistance over the cross of the component will be infinite. In this case, the resistance has increased so the resistance has gone up but not to the point where it is infinite.

If a component has decreased in resistance, then the effects would similar to a short and again, the same idea with a short there is going to be zero resistance. In this case, the component value has decreased then we just have less ohms than it should be, but it's not to the point of being a short.

This is a summary for troubleshooting series circuits and this picks up on the previous lesson that we looked at and also items that we just mentioned just now.

First of all the type of defect and remember we talked about an open circuit and this is typically because a resistor or a component is just physically deteriorating. Either it's over current and burned up but you have an open circuit. The whole thing with respect to ground, what should you read? You put a bolt meter into the circuit; you are measuring a cross that component with respect to ground. Near or equal to the supply terminal on the same side of the defect. In this case, measuring across that open you would typically see the supply voltage because there's no current drops in the circuit. The voltage across, in affect full supply. The voltage across other components, zero, because remember with an open there is no current and if there's no current there will be no voltage.

Now, the increased resistance, which is similar to an open. It's not the same thing as an open. An open is more severe. It is infinite in resistance were increased resistance is going to be larger resistance but not infinite. Near or equal to the supply terminal on the same side of the effect. Your text just points to the same voltage with respect to ground that you would see with an open. I would expect it to be a little bit less. In fact, the voltage across the defect here it says, well it has increased. The voltage across the defect will increase but you wouldn't see the full supply voltage and the voltage across other components notice it has decreased and the reason that it is decreased is because the total R has gone up, and so if total resistance has gone up, the current has gone down and the voltage measured across these other components will be lower because there's a lower current.

Shorting and this is the one we talked about in previous lesson. The voltage with respect to ground near or equal to the supply terminal on the opposite side of the defect and we talked about this, that the voltage would not equal the supply voltage but it should be higher across every component because every component now is going to have increased current, which will result in increased voltage because the shortage component resistance is now out of the circuit.

The voltage across the defect with the short should be zero and the voltage across other components should go up and it is for the opposite reason that the voltage went down here because now the total resistance has down, the current is going up, the other resistances remain the same. With the currents going up, the resistance remains the same, the voltage across other components will increase.

Then decreased resistances. Now again, this is similar to this but it is not quite as severe and we will see the same fault here. The voltage across the defect. Now notice with the decreased resistance the voltage has decreased but it isn't zero because there is still some resistance there. The voltage across other components, it will increase but it won't increase as much because there is still some resistance in that component.

This is a summary for trouble shouting of series circuits.

Circuit connectivity problems. In practice, circuit boards can develop defects and these can take the form of opens, shorts, higher resistance, lower resistances and these are the faults that we've previously discussed. When circuit board trace is open or short they are very difficult to repair since they are often within the layers of the circuit board. If we have components just out in the open, they are relatively easy to identify and replace but within a circuit board often times you'll have traces and sometimes the failure will often be inside of a layer within the circuit board. That makes it very difficult to repair.

In this case, there are options and one of the common options is that the entire board is simply discarded and often times the cost of a new circuit board is less than the cost of repairing it, so the board is just discarded. If this is a very expensive board, you might call upon the services of miniature component repair technicians and their function is to repair these kinds of things. They would come into play and the effect is this is very high-end, very expensive equipment. They would go in and actually repair the circuit board internally. Sometimes these guys will come into play with obsolete equipment. Maybe you have some equipment and I'm using the term obsolete because it's not manufactured any more. You may have a very critical piece of equipment that you simply can't buy components for them or buy the replacement pieces because they simply aren't for sale. In this case, sometimes you would spend the money to repair it because there just simply aren't any to purchase.

Effects of power supply defects. The power supply can also be a source of defects that must be identified and corrected. A power supply that produces no output voltage is easy to identify. The system simply won't work because the power supply isn't producing any voltage and that is relatively simple to diagnose. A power supply can also produce voltages that are incorrect. They can be higher or they can be lower or there can be no output. Not just power supplies, but electronic systems in general typically will have tolerances. In this case, let's pretend we have a power supply that's supposed to put out positive 15 volts, well typically there are going to be acceptable tolerances. If there's a 10 percent tolerance this could be plus or minus 1.5 volts and it still would be providing an acceptable voltage for use. Just because the voltage is a little bit lower or a little bit higher then it doesn't necessarily mean that it is bad but if it goes outside the tolerances then of course something would need to be done.

In some cases, the power supply trying to deliver more current than it is capable of will deliver a lower voltage. In this case, we have potentially a power supply. Maybe the circuit is supplying, something has gone wrong and it's pulling more current than it should or maybe poor design and the power supply is just simply being asked to deliver more current than it is capable of. This can also be a potential failure point.

In the section we looked at effects of power supply defects, we looked at the problems that can occur in circuit boards. We also considered a summary for trouble shooting series circuits and then we looked at the effects of component tolerances and this has to do with values of components changing in resistance either going up or going down. This concludes section 5.1.