 In this video, we're going to be talking about voltage dividers and what happens when you actually throw a load across a resistor in a voltage divider. Let's take a look at what I'm talking about here. Here we have a typical voltage divider circuit. We have three different resistors, the 100K ohm, 200K ohm, 300K ohm. I have a reference point here, ground reference point. I have some tap off points, A, B, and C. So what we need to do is, first off, with this being unloaded, we want to find out what our volt drop will be across each individual resistor. The best way we can do that is by using the voltage divider formula. So our voltage divider formula looks like this. The voltage across the resistor is equal to the voltage source times the ratio of the resistor to the total circuit resistance. So in this case here, if I have 100K ohms, I'm going to take 100 over 600 because I've got 300 plus 200 plus 100. The total circuit resistance right now is 600K ohms. So we can work out our volt drops across each individual one using this formula. In this case, I will just take, and let me just write up in my wonderful writing here, I can take the source voltage, which in this case is 120 volts, I can multiply it by 100K ohms, 100K divided by 600K. And if I do that, that's where we get the 20 volt drop from. In this one, it's a very similar idea. All I'm doing is taking the 200K ohms, dividing that by the 600K ohms, and multiplying it by 120, and I get a 40 volt drop across that resistor. And in this case here, same idea, same old, same old. I'm going to take 300K ohms divided by 600K ohms, multiply that by 120 volts, and I get a 60 volt drop across that resistor. Those are the voltage drops across all three resistors on a voltage divider when it is unloaded, meaning there's no loads connected to any of these tap off points. So let's go ahead and we're going to throw a load across one of our tap off points and see what happens to the circuit. So here what we've done is, on tap off C, we've added a 300K ohm resistor in parallel with the voltage divider resistance. So what we have to do is we have to remember that that is going to change our circuit resistance here, and in effect change the entire circuit resistance of the voltage divider circuit. So what we'll do here is, if we go 1 over 300K plus 1 over 300K equals 1 over RT, or because they're both equal, we can just cut this in half. We see that the total resistance of this parallel branch is 150K ohms. Next up, what we need to do is to determine what our total circuit resistance is before it was 600K ohms. Now we're going to go 100K ohms plus 200K ohms, which is 300K ohms, plus 150K ohms gives us 450K ohms. Previously with no load on the circuit, it was 600K ohms. That is extremely important. We're going to go ahead and we're going to use our formula again. The V source times that ratio equals the V of the resistor. So we're taking 100 divided by 450K ohms and multiplying it by 120, we get 26.7V drop. We do the same thing to the second, 200K ohms divided by 450K ohms times 120 gives us 53.3V. And the last one here is going to be 150K ohms divided by 450K ohms times 120 gives us 40V. So that's the volt drops across this circuit in a loaded situation. And if I take a look at that other resistor there, if I kind of take this branch and put it out to what it actually is, we know that voltage in parallel stays the same. So across that 300K ohms resistor is 40V and across that 300K ohms resistor is 40V. So that shows the effects of loading the circuit. Before when we had, without having this load here, when we had 600K ohms here, we actually had a 60V drop across here. Now by putting the load on it, it changes the whole circuit and we end up with a 40V drop across that. Loading your voltage dividers makes a significant difference and so it's very important to be aware of how to make those calculations.