 You've been warned many times not to use an extension cord in the manner. You're about to see I plug in one item The extension cord is Alright plug in another one Extension cord is alright now. These are two small items Plug in another one. Why are starting to get a little bit warm? Plug in another The wire is really warm now. What's what happens when I plug in this last? It's obvious that the results could be very very dangerous. Of course, I suggest that you don't try something like this It's really a mess Now the wire burn Because I overloaded the circuit remember overload simply means that the demand for power is Greater than the conductors can handle in this case We exceeded the power rating of the wire and of course it went up in smoke. I have a demo that will show why this happened Now be using a DC source, but the principles are the same as it is with AC This battery will supply the power This represents the extension cord with the multiple sockets This meter will monitor the current we're demanding and we'll use bulbs to represent the various items demanding power Okay, I'll plug in the first one Notice the current it requires from the source Now watch current as I plug in another one current increases and also notice what we've done Both bulbs are across the power source. So we're adding components in parallel When I add another bulb Current increases again, and it's obvious that if enough components are added in parallel We could demand more current from the source than even these wires could handle Well, that's exactly what happened before Although it's quite a mess This is a parallel arrangement So all of the items that we plugged in are in parallel and as each was turned on more and more power was required We finally exceeded the rating of the extension cord wires. It became hot enough to burn the insulation and eventually short out the wires Now from this we should realize that the number of components in a parallel circuit Determines the power demanded from a source Let's go into this further and find out what determines power in Each branch of the parallel circuit, and I have another trainer that will show this Okay, I'll make the voltage connections first and close the switch Now we know that power is a measure of how much work is accomplished in a given time Here the work being accomplished is a production of light by the filament in each bulb Then we can say that the amount of light is an indication of how much power is used With this in mind, then let's see the factors that determine power Notice that each bulb is producing a certain amount of light If I decrease current by changing this rheostat Less light is produced The filaments are dissipating less heat then less power is being used If I increase current More work is being produced. So power increases Less current less power More current more power then current and power are directly related in a parallel circuit Another factor that determines power is voltage Now to show this I'll use these bulbs which are wired in a parallel arrangement and two different batteries Now if I connect the bulbs to this 12 volt battery Notice the amount of light being produced We can say then that the filaments are dissipating a specific power But when I connect the same bulbs to this six volt battery Well, you can just barely see that they're glowing. So I'll disconnect and there they're not glowing There they're glowing. It's pretty obvious that there's very little light here Then the bulbs are dissipating much less power Less voltage less power More voltage more power Then power is directly related to the amount of voltage Okay, resistance also determines how much power is dissipated by a branch Now I have a different trainer to show this. So let me change out the trainers. Of course first I'll make the voltage connections and Close the switch Now in this circuit These two bulbs are dissipating the same amount of power We know this is true because the current in each branch is the same and The voltage across each branch is the same Now I'll replace this bulb with one that has a smaller resistance Notice that the smaller resistance caused the current in this branch to increase Notice also that this bulb is brighter. It's dissipating more power The smaller the resistance the larger the power dissipated remember then The power dissipated by a parallel branch is determined by the applied voltage branch current and branch resistance Also, the total power dissipated by a parallel circuit is equal to the sum of the power dissipated by each branch We saw this in the beginning of the lesson Now also you must be careful not to add too many components or you could exceed the power rating of the circuit wiring And of course burn it up the way we did in the open next let's place some troubles in the parallel circuit and Develop the symptoms caused by an open and a short As I make the voltage connections, let's notice that the circuit will be troubleshooting contains three parallel resistors With a meter to monitor total current Okay, I'll close the switch Let's start with an open in one of the branches Now I'll open this middle branch and the first time I'll do it here so that we can visually see the indication You see that there is no current in this branch the bulb isn't burning Let's try it again now this time notice that when I open it It will have no effect on the remaining branches. I Open it current is still flowing in them then one symptom for an open is no current in the branch. That's open Now I'll reconnect it and we'll see what indications we get from the meters Let's try the ammeter first Watch total current as I open the branch and this time I'll open it by removing the ball Total current decreases. Well, let's see that again. There's total current with all three branches in When I open one branch total current decreases then with an open there's no current in the branch that's open and The total current decreases Next let's try the voltmeter Since the applied voltage is felt across each branch We'll have to use a range that will measure the applied voltage and in this case. We'll use the 50 volt range Of course the function switch is on DCV Okay, I'll connect the meter across The center branch and as always when using a voltmeter you must observe polarity So I'll go directly across the bulb socket We see with all three branches in there's about 12 volts across this branch Remember the middle set of numbers on the DC scale Watch as I open the branch The meter is still reading About 12 volts There's 12 volts with the branch in 12 volts with a branch open Then it's pretty obvious that a voltmeter can't be used to locate an open in a parallel circuit Then let's use the o-meter I'll change the function switch to ohms and we'll use a range of ohms times one and as always we must check meter zero it's a little off so I will zero it and Also anytime we're using an o-meter. We must remove power from the circuit. So I'll open the switch Now I'll connect directly across the bulb socket. We get a reading of about Let's call it 34 ohms. We're only ohms times one Range now. Let's think about this reading for a moment if we're checking across an open Which should be an infinite resistance. Why do we get a resistance reading? Well, let's check the meter connections This side of the meter is connected through the wiring to each side of the good balls This one is connected through the wiring to the other side of the balls. So actually we're reading the resistance of the good branches Okay, now let's try this again so that we can pinpoint the o-meter indication for an open This time I'll put the ball back in and Notice the resistance reading with all three bulbs in it's a little over 20 ohms Now watch when I open the branch total resistance increases Then the o-meter symptom for an open is an increase in total resistance Now if you can't visually check and find the open you must Isolate and check each component with the o-meter. Let me show you what I mean For example, if I physically disconnect one end of this branch I'll isolate it from the rest of the circuit now you watch the meter reading when I do that I isolate the Suspected component and the meter indicates an infinite resistance. It indicates that that is an open branch Now remember this about an open in a parallel circuit a Voltmeter can't be used because the applied voltage is always across the branch and Ammeter shows a decrease in total current Remember this is because there's no current in the open branch and an o-meter shows an increase in total resistance Okay, let's look at a shorted branch Okay, I'll remove the o-meter and Return the circuit back to normal put this bulb back in and reapply power Now first let's notice that if a short were to occur in any of the branches We're providing a very low resistance path for current of course this pencil is simulating the short We're effectively placing a wire across the battery terminals Of course, this means a very large increase in current But if this circuit is designed properly what should happen when the short occurs? Well, if this fuse is the proper size it should blow and protect the circuit components from the excess current Let's see if it does Now I'm going to place a short across this bulb and first let's watch the fuse when I Place the short across it see that flash that indicates that the fuse blew Well, let me put in another fuse and we'll try it again And we'll try it again now watch for the flash when I when the short is completed Okay, there it was now let's do it one more time, but This time let's watch the ammeter and the fuse now watch carefully You should watch for a sharp rise in the current when I make the connection Did you see the needle peg and then fall to zero when the fuse blew Well, let's try it one more time so that you can see it Remove the bad fuse and put in a good one now watch the meter closely When I make the final connection Okay It rose sharply then fell back to zero so the fuse is doing its job. It's protecting the components now Where do we go from here? There's no current so the ammeter doesn't tell us which component is shorted and With no current there's no voltage across any of these components So a voltmeter won't tell us anything Then that only leaves the ometer to troubleshoot the short. So let's try it Before I do of course, I must check the meter zero and it's still all right Now to make sure that there's no power over here. I'm going to be doubly safe and open this switch Now I'll check directly across the short one lead there one here Well, you can see that we get an indication of zero resistance The short has reduced the total resistance in the circuit to zero Now you can see why the fuse kept blowing with zero resistance The current tries to exceed the rating of the fuse, but it blows and protects the circuit components To locate the component that's shorted if you can't visually see which one is shorted You must isolate and check each component, but this time of course you're looking for one with zero resistance Let's go over some of the points that we should remember about Power and troubleshooting in a parallel resistive circuit Remember the power dissipated by a branch is dependent upon current and voltage The larger the current our voltage the more power dissipated by the branch Also remember that the largest resistance dissipates the least power in Troubleshooting the circuit. Here's what we found when we opened a branch resistance This happened There's zero current in the branch that's open Total current decreases because of an increase in total resistance when we placed a short in the circuit These are the symptoms that we found Total resistance is zero and Current tries to go to an excessive value But remember also that if the circuit is fused properly it should blow and protect the circuit components Now remember also that you can't haphazardly add components in parallel the way we did here If you do you could demand more power for the components than the circuit can handle even though it is fused properly And you could end up with results like this Remember the more components they are the greater the power dissipated in our next lesson will look at bridge circuits a specific use for a parallel resistive circuit