0:00:01.296,0:00:04.346 Now, let's talk about measuring[br]resistance and voltage. 0:00:04.346,0:00:09.243 This device right here is an ohmmeter and[br]a voltmeter. 0:00:09.243,0:00:14.134 So here's how I will measure resistance or[br]I can measure DC voltage, 0:00:14.134,0:00:16.593 or I can measure RMS, AC voltage. 0:00:16.593,0:00:20.771 Today, I am interested in measuring[br]ohms and measuring volts. 0:00:20.771,0:00:26.024 So what I have built is a series circuit[br]where I have four resistors and series. 0:00:26.024,0:00:27.899 1k, 3k, 4k and 5k. 0:00:27.899,0:00:31.050 They'll be connected to a three-volt[br]source which you don't see yet. 0:00:31.050,0:00:33.650 In this device right here,[br]this is a protoboard and 0:00:33.650,0:00:36.138 I've connected this[br]circuit on my protoboard. 0:00:36.138,0:00:40.813 The way a protoboard works is[br]every row is an individual node. 0:00:40.813,0:00:44.868 So this is a node, this is a node,[br]this is a node and so on. 0:00:44.868,0:00:48.361 When you cross this plastic barrier,[br]here is a new node. 0:00:48.361,0:00:49.656 Here is another node. 0:00:49.656,0:00:51.794 Here is a third node and so on. 0:00:51.794,0:00:55.526 So I have my 1k resistor[br]connected through this node. 0:00:55.526,0:00:58.839 This is actually the green[br]node connecting to the the 3k. 0:00:58.839,0:01:02.507 The 3k is connected to the 4k[br]with the blue node and 0:01:02.507,0:01:06.103 the 4k is connected to[br]the 5k at the yellow code. 0:01:06.103,0:01:08.791 The 5k is going to be connected[br]to the negative side of 0:01:08.791,0:01:11.678 my voltage through the black node,[br]which is right here. 0:01:11.678,0:01:14.013 That is this whole line and[br]the red node or 0:01:14.013,0:01:18.072 the positive three-volt source is[br]going to be connected right here. 0:01:18.072,0:01:23.112 Now if I want to measure resistance, I am[br]not going to have my source connected, 0:01:23.112,0:01:27.643 because that may end up being in series or[br]parallel with my resistance. 0:01:27.643,0:01:29.491 So I'm not going to put that there, but 0:01:29.491,0:01:31.901 I want to just measure my[br]resistors by themselves. 0:01:31.901,0:01:36.719 So I'm going to touch one side of my[br]lead to one side of the resistor. 0:01:36.719,0:01:38.909 The other side will lead to[br]the other side of the resistor. 0:01:38.909,0:01:43.640 And look, this is 0.987 kiloohms[br]which is just about 1k. 0:01:43.640,0:01:46.896 These are five percent resistors,[br]so that's good. 0:01:46.896,0:01:48.805 Now, let's measure our 3k resistor. 0:01:48.805,0:01:51.309 Touch one side, touch the other side. 0:01:51.309,0:01:53.783 There's what my 3k resistor looks like. 0:01:53.783,0:01:55.380 Here's the 4k. 0:01:55.380,0:01:57.330 One side touch the other side. 0:01:57.330,0:02:02.491 There's my 4k and here is my 5k resistor. 0:02:02.491,0:02:06.040 Okay, now let's hookup[br]our three-volt source. 0:02:06.040,0:02:09.625 The three-volt source is going to[br]be from two batteries in series. 0:02:09.625,0:02:12.741 I'm going to connect the red[br]side right there and 0:02:12.741,0:02:15.401 the black side right there to my circuit. 0:02:15.401,0:02:20.115 All right, now let's measure the voltage[br]across this three-volt source. 0:02:20.115,0:02:25.036 And actually, I have a couple little[br]pins here that are just going to help 0:02:25.036,0:02:27.545 me be able to hold my things in place. 0:02:27.545,0:02:32.297 So here is a little pin that's going to[br]allow me to connect my black loop there, 0:02:32.297,0:02:36.549 I made it and here's the other pin[br]that will let me connect my red lead. 0:02:36.549,0:02:38.896 So I'm gonna connect[br]those into the black and 0:02:38.896,0:02:41.260 red rails on my circuit[br]right here like this. 0:02:41.260,0:02:43.950 Okay, so I don't want to[br]measure resistance anymore now. 0:02:43.950,0:02:45.064 Now, I want to measure voltage. 0:02:45.064,0:02:46.423 So let's turn it over here. 0:02:46.423,0:02:49.611 So I am measuring my[br]three-volt source right there. 0:02:49.611,0:02:51.736 That's 2.907 volts. 0:02:51.736,0:02:53.841 Okay, let's probe my circuit. 0:02:53.841,0:02:58.812 The first thing that I am going to do is[br]measure the voltage at each of my nodes. 0:02:58.812,0:03:02.642 So right now, I'm measuring[br]the voltage at the red node, see? 0:03:02.642,0:03:07.330 This is the part right there connected[br]on to the red node, it's right there. 0:03:07.330,0:03:09.080 That's measuring three volts. 0:03:09.080,0:03:11.809 Now, I'm going to measure[br]at the green node. 0:03:11.809,0:03:14.786 Okay, at the green node,[br]so actually let me bit. 0:03:14.786,0:03:21.806 Let me write right here that this was[br]2.909 volts at the green node number. 0:03:21.806,0:03:24.176 I said that was the green[br]nailed right there. 0:03:24.176,0:03:28.851 At the grade node,[br]this is two point six eight seven 0:03:28.851,0:03:32.982 volts at the blue node[br]there between the 3 and 0:03:32.982,0:03:38.526 4k is 2.010 volts and[br]at the junction between the 4 and 0:03:38.526,0:03:42.567 5k at the yellow node is 1.138 volts. 0:03:42.567,0:03:46.522 Down here at the black node,[br]we're back to zero. 0:03:46.522,0:03:49.418 What I just did was that I[br]measured the node voltages. 0:03:49.418,0:03:53.304 I measured the red node, the green node, 0:03:53.304,0:03:58.091 the blue node, the yellow node and[br]the black node. 0:03:58.091,0:03:59.645 Those are node voltages. 0:03:59.645,0:04:04.552 The way you measure a node voltage is you[br]plug the black lead of your volt meter 0:04:04.552,0:04:05.779 into the ground and 0:04:05.779,0:04:10.545 you move the red lead to touch each of[br]the nodes that you are interested in. 0:04:10.545,0:04:12.463 Now, let's measure voltage difference. 0:04:12.463,0:04:16.991 The way you measure voltage difference[br]is you put your plug on one side of your 0:04:16.991,0:04:18.478 resistor right there and 0:04:18.478,0:04:21.894 you put your black plug on[br]the other side of the resistor. 0:04:21.894,0:04:25.824 This is now going to measure[br]the voltage that's across R1. 0:04:25.824,0:04:31.799 From red here to black there,[br]that voltage is 0.223. 0:04:31.799,0:04:34.952 We also can say that this is[br]the plus side or the red side and 0:04:34.952,0:04:37.322 this is the minus side, or the black side. 0:04:37.322,0:04:39.541 Let's do the same thing for this resistor. 0:04:39.541,0:04:41.585 Let's measure across R2. 0:04:41.585,0:04:46.130 So let's put the black lead on[br]one side and the red lead on 0:04:46.130,0:04:51.183 the other side just like that,[br]and that is 0.678 volts. 0:04:51.183,0:04:54.683 Put a plus where your red lead is and[br]a minus where your black lead is. 0:04:54.683,0:04:55.723 Let's get this one. 0:04:55.723,0:04:58.266 Make sure you fill out[br]the cross our 4k now. 0:04:58.266,0:05:06.852 That is 0.873 volts measuring from here to[br]here plus on this side minus on that side. 0:05:06.852,0:05:09.260 And now, let's measure across the 5k. 0:05:09.260,0:05:14.797 Right there,[br]this is 1.140 volts plus a new side minus 0:05:14.797,0:05:21.233 on that plus it's always the red[br]lead minus is always the black lead. 0:05:21.233,0:05:25.372 So what we measured there[br]were the voltage differences. 0:05:25.372,0:05:29.314 Now, let's take a look at[br]what we expect to happen. 0:05:29.314,0:05:32.716 So we went from our[br]approximately 3 volts and 0:05:32.716,0:05:38.040 we're going to have dropped 0.223[br]volts down to our next node. 0:05:38.040,0:05:38.931 Is that what happened? 0:05:38.931,0:05:41.442 Take 2.9 subtract off 0.2. 0:05:41.442,0:05:44.176 Yep, we've gotten down[br]to about this voltage, 0:05:44.176,0:05:48.192 then we're going to have lost[br]another 0.678 volts across our 2. 0:05:48.192,0:05:49.983 In fact, subtract that off from here. 0:05:49.983,0:05:52.340 Yep, you get down to the next node. 0:05:52.340,0:05:55.500 Now subtract off the voltage[br]difference here, 0:05:55.500,0:06:00.248 0.87 and that is bringing you[br]to the 1.138 volts at this node. 0:06:00.248,0:06:03.582 Let's also see how those[br]measurements could work this way. 0:06:03.582,0:06:07.070 Let's measure the voltage[br]across two of our resistors. 0:06:07.070,0:06:10.435 So I'm measuring across R1 and across R2. 0:06:10.435,0:06:12.478 What voltage do you expect there? 0:06:12.478,0:06:17.360 You would expect to see the voltage drop[br]across R1 and the voltage drop across R2. 0:06:17.360,0:06:21.265 So this volt is difference[br]right there is 0.90 and 0:06:21.265,0:06:25.358 that's plus were the red is,[br]minus where the black is. 0:06:25.358,0:06:27.010 That's another voltage difference. 0:06:27.010,0:06:30.007 What if I measured across[br]three of my resistors? 0:06:30.007,0:06:34.732 Add up the voltages that you originally[br]had across the individually and 0:06:34.732,0:06:39.141 that should give you the voltage[br]that's across from R1 all the way 0:06:39.141,0:06:44.121 over here to R3 plus on this side,[br]minus on that side, it's 1.775. 0:06:44.121,0:06:46.175 Add those up, so[br]that you can get the right number. 0:06:46.175,0:06:49.572 And then finally, when I measure across[br]all of my resistors, sure thing. 0:06:49.572,0:06:51.504 I'm back to my 2.9 volts. 0:06:51.504,0:06:57.671 Now wait a minute, this says,[br]2.915 when I started out with 2.909. 0:06:57.671,0:07:02.573 Your voltages are not exactly accurate and[br]your volt meters is not exactly accurate. 0:07:02.573,0:07:07.005 So you can already see that I wouldn't be[br]trusting these last two digits in this 0:07:07.005,0:07:11.080 particular measurement between this[br]volt meter and these voltages. 0:07:11.080,0:07:17.995 So let's just focus on the first digit,[br]the 2.9 which means let's get rid of this. 0:07:17.995,0:07:19.574 Let's get rid of this. 0:07:19.574,0:07:21.176 Let's get rid of this and so on. 0:07:21.176,0:07:22.446 Those are your significant digits. 0:07:22.446,0:07:24.794 So today, we have measured resistance. 0:07:24.794,0:07:28.224 We've measured node voltage and[br]we've measured voltage difference.