WEBVTT 00:00:01.296 --> 00:00:04.346 Now, let's talk about measuring resistance and voltage. 00:00:04.346 --> 00:00:09.243 This device right here is an ohmmeter and a voltmeter. 00:00:09.243 --> 00:00:14.134 So here's how I will measure resistance or I can measure DC voltage, 00:00:14.134 --> 00:00:16.593 or I can measure RMS, AC voltage. 00:00:16.593 --> 00:00:20.771 Today, I am interested in measuring ohms and measuring volts. 00:00:20.771 --> 00:00:26.024 So what I have built is a series circuit where I have four resistors and series. 00:00:26.024 --> 00:00:27.899 1k, 3k, 4k and 5k. 00:00:27.899 --> 00:00:31.050 They'll be connected to a three-volt source which you don't see yet. 00:00:31.050 --> 00:00:33.650 In this device right here, this is a protoboard and 00:00:33.650 --> 00:00:36.138 I've connected this circuit on my protoboard. 00:00:36.138 --> 00:00:40.813 The way a protoboard works is every row is an individual node. 00:00:40.813 --> 00:00:44.868 So this is a node, this is a node, this is a node and so on. 00:00:44.868 --> 00:00:48.361 When you cross this plastic barrier, here is a new node. 00:00:48.361 --> 00:00:49.656 Here is another node. 00:00:49.656 --> 00:00:51.794 Here is a third node and so on. 00:00:51.794 --> 00:00:55.526 So I have my 1k resistor connected through this node. 00:00:55.526 --> 00:00:58.839 This is actually the green node connecting to the the 3k. 00:00:58.839 --> 00:01:02.507 The 3k is connected to the 4k with the blue node and 00:01:02.507 --> 00:01:06.103 the 4k is connected to the 5k at the yellow code. 00:01:06.103 --> 00:01:08.791 The 5k is going to be connected to the negative side of 00:01:08.791 --> 00:01:11.678 my voltage through the black node, which is right here. 00:01:11.678 --> 00:01:14.013 That is this whole line and the red node or 00:01:14.013 --> 00:01:18.072 the positive three-volt source is going to be connected right here. 00:01:18.072 --> 00:01:23.112 Now if I want to measure resistance, I am not going to have my source connected, 00:01:23.112 --> 00:01:27.643 because that may end up being in series or parallel with my resistance. 00:01:27.643 --> 00:01:29.491 So I'm not going to put that there, but 00:01:29.491 --> 00:01:31.901 I want to just measure my resistors by themselves. 00:01:31.901 --> 00:01:36.719 So I'm going to touch one side of my lead to one side of the resistor. 00:01:36.719 --> 00:01:38.909 The other side will lead to the other side of the resistor. 00:01:38.909 --> 00:01:43.640 And look, this is 0.987 kiloohms which is just about 1k. 00:01:43.640 --> 00:01:46.896 These are five percent resistors, so that's good. 00:01:46.896 --> 00:01:48.805 Now, let's measure our 3k resistor. 00:01:48.805 --> 00:01:51.309 Touch one side, touch the other side. 00:01:51.309 --> 00:01:53.783 There's what my 3k resistor looks like. 00:01:53.783 --> 00:01:55.380 Here's the 4k. 00:01:55.380 --> 00:01:57.330 One side touch the other side. 00:01:57.330 --> 00:02:02.491 There's my 4k and here is my 5k resistor. 00:02:02.491 --> 00:02:06.040 Okay, now let's hookup our three-volt source. 00:02:06.040 --> 00:02:09.625 The three-volt source is going to be from two batteries in series. 00:02:09.625 --> 00:02:12.741 I'm going to connect the red side right there and 00:02:12.741 --> 00:02:15.401 the black side right there to my circuit. 00:02:15.401 --> 00:02:20.115 All right, now let's measure the voltage across this three-volt source. 00:02:20.115 --> 00:02:25.036 And actually, I have a couple little pins here that are just going to help 00:02:25.036 --> 00:02:27.545 me be able to hold my things in place. 00:02:27.545 --> 00:02:32.297 So here is a little pin that's going to allow me to connect my black loop there, 00:02:32.297 --> 00:02:36.549 I made it and here's the other pin that will let me connect my red lead. 00:02:36.549 --> 00:02:38.896 So I'm gonna connect those into the black and 00:02:38.896 --> 00:02:41.260 red rails on my circuit right here like this. 00:02:41.260 --> 00:02:43.950 Okay, so I don't want to measure resistance anymore now. 00:02:43.950 --> 00:02:45.064 Now, I want to measure voltage. 00:02:45.064 --> 00:02:46.423 So let's turn it over here. 00:02:46.423 --> 00:02:49.611 So I am measuring my three-volt source right there. 00:02:49.611 --> 00:02:51.736 That's 2.907 volts. 00:02:51.736 --> 00:02:53.841 Okay, let's probe my circuit. 00:02:53.841 --> 00:02:58.812 The first thing that I am going to do is measure the voltage at each of my nodes. 00:02:58.812 --> 00:03:02.642 So right now, I'm measuring the voltage at the red node, see? 00:03:02.642 --> 00:03:07.330 This is the part right there connected on to the red node, it's right there. 00:03:07.330 --> 00:03:09.080 That's measuring three volts. 00:03:09.080 --> 00:03:11.809 Now, I'm going to measure at the green node. 00:03:11.809 --> 00:03:14.786 Okay, at the green node, so actually let me bit. 00:03:14.786 --> 00:03:21.806 Let me write right here that this was 2.909 volts at the green node number. 00:03:21.806 --> 00:03:24.176 I said that was the green nailed right there. 00:03:24.176 --> 00:03:28.851 At the grade node, this is two point six eight seven 00:03:28.851 --> 00:03:32.982 volts at the blue node there between the 3 and 00:03:32.982 --> 00:03:38.526 4k is 2.010 volts and at the junction between the 4 and 00:03:38.526 --> 00:03:42.567 5k at the yellow node is 1.138 volts. 00:03:42.567 --> 00:03:46.522 Down here at the black node, we're back to zero. 00:03:46.522 --> 00:03:49.418 What I just did was that I measured the node voltages. 00:03:49.418 --> 00:03:53.304 I measured the red node, the green node, 00:03:53.304 --> 00:03:58.091 the blue node, the yellow node and the black node. 00:03:58.091 --> 00:03:59.645 Those are node voltages. 00:03:59.645 --> 00:04:04.552 The way you measure a node voltage is you plug the black lead of your volt meter 00:04:04.552 --> 00:04:05.779 into the ground and 00:04:05.779 --> 00:04:10.545 you move the red lead to touch each of the nodes that you are interested in. 00:04:10.545 --> 00:04:12.463 Now, let's measure voltage difference. 00:04:12.463 --> 00:04:16.991 The way you measure voltage difference is you put your plug on one side of your 00:04:16.991 --> 00:04:18.478 resistor right there and 00:04:18.478 --> 00:04:21.894 you put your black plug on the other side of the resistor. 00:04:21.894 --> 00:04:25.824 This is now going to measure the voltage that's across R1. 00:04:25.824 --> 00:04:31.799 From red here to black there, that voltage is 0.223. 00:04:31.799 --> 00:04:34.952 We also can say that this is the plus side or the red side and 00:04:34.952 --> 00:04:37.322 this is the minus side, or the black side. 00:04:37.322 --> 00:04:39.541 Let's do the same thing for this resistor. 00:04:39.541 --> 00:04:41.585 Let's measure across R2. 00:04:41.585 --> 00:04:46.130 So let's put the black lead on one side and the red lead on 00:04:46.130 --> 00:04:51.183 the other side just like that, and that is 0.678 volts. 00:04:51.183 --> 00:04:54.683 Put a plus where your red lead is and a minus where your black lead is. 00:04:54.683 --> 00:04:55.723 Let's get this one. 00:04:55.723 --> 00:04:58.266 Make sure you fill out the cross our 4k now. 00:04:58.266 --> 00:05:06.852 That is 0.873 volts measuring from here to here plus on this side minus on that side. 00:05:06.852 --> 00:05:09.260 And now, let's measure across the 5k. 00:05:09.260 --> 00:05:14.797 Right there, this is 1.140 volts plus a new side minus 00:05:14.797 --> 00:05:21.233 on that plus it's always the red lead minus is always the black lead. 00:05:21.233 --> 00:05:25.372 So what we measured there were the voltage differences. 00:05:25.372 --> 00:05:29.314 Now, let's take a look at what we expect to happen. 00:05:29.314 --> 00:05:32.716 So we went from our approximately 3 volts and 00:05:32.716 --> 00:05:38.040 we're going to have dropped 0.223 volts down to our next node. 00:05:38.040 --> 00:05:38.931 Is that what happened? 00:05:38.931 --> 00:05:41.442 Take 2.9 subtract off 0.2. 00:05:41.442 --> 00:05:44.176 Yep, we've gotten down to about this voltage, 00:05:44.176 --> 00:05:48.192 then we're going to have lost another 0.678 volts across our 2. 00:05:48.192 --> 00:05:49.983 In fact, subtract that off from here. 00:05:49.983 --> 00:05:52.340 Yep, you get down to the next node. 00:05:52.340 --> 00:05:55.500 Now subtract off the voltage difference here, 00:05:55.500 --> 00:06:00.248 0.87 and that is bringing you to the 1.138 volts at this node. 00:06:00.248 --> 00:06:03.582 Let's also see how those measurements could work this way. 00:06:03.582 --> 00:06:07.070 Let's measure the voltage across two of our resistors. 00:06:07.070 --> 00:06:10.435 So I'm measuring across R1 and across R2. 00:06:10.435 --> 00:06:12.478 What voltage do you expect there? 00:06:12.478 --> 00:06:17.360 You would expect to see the voltage drop across R1 and the voltage drop across R2. 00:06:17.360 --> 00:06:21.265 So this volt is difference right there is 0.90 and 00:06:21.265 --> 00:06:25.358 that's plus were the red is, minus where the black is. 00:06:25.358 --> 00:06:27.010 That's another voltage difference. 00:06:27.010 --> 00:06:30.007 What if I measured across three of my resistors? 00:06:30.007 --> 00:06:34.732 Add up the voltages that you originally had across the individually and 00:06:34.732 --> 00:06:39.141 that should give you the voltage that's across from R1 all the way 00:06:39.141 --> 00:06:44.121 over here to R3 plus on this side, minus on that side, it's 1.775. 00:06:44.121 --> 00:06:46.175 Add those up, so that you can get the right number. 00:06:46.175 --> 00:06:49.572 And then finally, when I measure across all of my resistors, sure thing. 00:06:49.572 --> 00:06:51.504 I'm back to my 2.9 volts. 00:06:51.504 --> 00:06:57.671 Now wait a minute, this says, 2.915 when I started out with 2.909. 00:06:57.671 --> 00:07:02.573 Your voltages are not exactly accurate and your volt meters is not exactly accurate. 00:07:02.573 --> 00:07:07.005 So you can already see that I wouldn't be trusting these last two digits in this 00:07:07.005 --> 00:07:11.080 particular measurement between this volt meter and these voltages. 00:07:11.080 --> 00:07:17.995 So let's just focus on the first digit, the 2.9 which means let's get rid of this. 00:07:17.995 --> 00:07:19.574 Let's get rid of this. 00:07:19.574 --> 00:07:21.176 Let's get rid of this and so on. 00:07:21.176 --> 00:07:22.446 Those are your significant digits. 00:07:22.446 --> 00:07:24.794 So today, we have measured resistance. 00:07:24.794 --> 00:07:28.224 We've measured node voltage and we've measured voltage difference.