Today I want to talk about voltage and voltage drops and some of the common confusions that relate to Ohm's law. Don't worry, we're not gon na do a whole bunch of Ohm's law math, it's probably one of the most boring and one of the more useless things that are taught in HVAC school, not that it isn't useful to some people, but to us it doesn't apply very Much but I'm gon na show you why I think it matters to at least understand how this all works, so first off we're gon na be using a voltmeter. In order to demonstrate this, you really one of the first tools, one of the first tools that a technician should be really comfortable with using is a voltmeter and I'm going to show you it kind of just touching it to the board as an example. But then I'll also show you what I'm talking about with a couple with a contactor and a relay alright.

So let's start with the boring thing, which is Ohm's law, and that is e equals I times R or if you would. Rather, we can just say: volts equals amps times. Resistance and resistance is measured in ohms voltages. We're gon na call V and amps we're gon na call a so V equals a times the ohm symbol, if you like, whatever whatever you prefer there, but this is what we're gon na straight is how Ohm's law works in real life.

So, let's use for case of simplicity, this is going to apply to an HVAC technician, mostly when something like a compressor. Compressor has a couple windings, and so that can confuse you a little bit same principle applies, but let's do it with a with a light bulb to start with, so we're gon na say all right. We got a circuit, it's 120 volts ac and it connects in between these two points. We've got a switch here and we're gon na make a little light bulb here.

So that is our circuit, so we have 120 volts. So if I were to measure, if I had you know lugs here and I were to measure in between these two points with a voltmeter, this is how I would do it take my voltmeter set it to volts, and I would measure between these two points and I would get 120 volts, so there it's the same as what we do when you connect to a contactor. So it's the same thing with a contactor. If you were going to measure the input voltage to a compressor, you might go here and you would expect to see 240 volts whether the contactor was open or closed.

You would expect to see 240 volts applied to that contactor, which is what we call input voltage right. So this is often how we use volt meter's will measure between two points of potential difference or areas that we expect to have potential difference or will measure across a load, for example. So in this light bulb here, let's draw a circle around it, so you can tell it's a light bulb. We wouldn't measure across the light bulb and if this was 120 volt light bulb, we would expect to see 120 volts on the display, which means that we're providing 120 volts of potential difference to that light bulb and then we're gon na have some some current moving Through the circuit, so I'm assuming that you know generally what volts amps and ohms are.

But if you don't we'll just do a quick, quick overview. Volts is electrical pressure. Amps is the amount of current or the amount of electrons moving through the circuit, and then resistance is the electrical resistance that the current has to overcome. So what does it have to push through in order to do its work? So we'll focus on the basics here? Equals I times R, the biggest thing you need to know is that, as the resistance increases, the amps decrease, so as the resistance increases, the amps decrease.

If you have more resistance to overcome, then you're going to do less work. So, if there's more resistance to overcome this you're gon na do less work, a good way to think about it is: are you going to be able to more effectively drive your truck over a nice flat pavement, or are you going to be able to more effectively Drive your truck through mud, it's gon na take more work to drive your truck through thick mud. Then it's going to be over flat pavement. You can think of the same terms with with a ball.

So if you kick a ball across a path, that's a nice hard dirt path versus. If you kick a ball through really high grass, the high grass is going to act as more resistance. So it's going to take more work to kick the ball as far through the high grass makes sense at all. It all works for us, but we run into a quick problem here again, this class isn't about Ohm's law.

This is about practical application. The quick problem we run into is when we think of a motor, for example, you take you, take a ceiling, fan and ceiling fan is running and you run up and you grab it. You know your mom taught you not to do that when you pretty young grab the ceiling fan you stop it. What do you think's going on inside that motor? Well, the motor is drawing higher current, so the amperage of that motor is going higher because it's pushing against your hand and what happens is there's this thing called inductive reactance we're not going to go into that.

But what happens? Is the resistance inside that motor actually decreases and so there's an inverse relationship in magnetic loads? What are called inductive loads? Primarily, we see it in motors and electromagnets the more force you apply against a motor, the lower the electrical resistance is going to go inside the motor and that's why you see this result of a locked compressor. For example, when you make it real practical, you had a compressor, that's locked. It draws really high amperage when it's trying to start. The reason that that is is because the physical resistance inside that motor has it locked up and now that motor, instead of creating this magnetic motion that we want to have in a motor.

Instead, it becomes a big heater, the windings just become heaters and it goes off on internal overload. You see this really high current, but that's because the physical resistance against that motor results in a decrease in electrical resistance and with that decrease in electrical resistance. We see higher amperage or more circuit work. That makes sense so when you have a physical resistance against a motor, the electrical resistance decreases when the electrical resistance decreases.

The amperage goes up. So that's this reverse inverse relationship between amperage and resistance. Now, in the electrical circuit, the thing that primarily stays constant, at least in our minds - it doesn't really stay constant, we're gon na get into that. But the thing that we think of as staying constant is the voltage.

If you have an outlet, you say well, that is 120 volts, 115 volts. If you have a typical residential air, conditioner, you're gon na say it's 240 volts, and so you expect this to stay fixed. I'm gon na show you how, in reality that doesn't always stay fixed, because what we do with a voltmeter is. We measure voltage drops we often think of a voltmeter, and this is one of the more difficult things to overcome when somebody sees it is they'll.

Ask where do I put my meter right so they mean. Where do I put the probes on my meter, but they often think in terms of a single point, so they want to be able to say I want to see if I have 120 volts right here and then, where they put. This second probe becomes sort of an afterthought. They not really thinking about where to put this second probe, so they just kind of pick a spot they'll put it on ground.

They'll put it. You know wherever that, isn't the best way to think about this, because, when you're measuring you're measuring a voltage drop between these two points, that's really what you're measuring so between here and here. If we measure 120 volts on the meter, we're measuring on a hundred and 20 volt voltage drop, so your total voltage drops across the circuit. This is a principal - are equal to the total resistance of the circuit.

So the voltage drop that we measure is equal to the total circuit resistance. So we're gon na assume in this initial circuit that we've drawn here that all of the resistance is in the light bulb. So if all those resistances in the light bulb, then we would have a voltage drop of 120 volts across the light bulb because - and this is where Meg start getting a little confusing for you - so slow this one down I'll try to talk slow here, 120 volts Here: 120 volts across the load. That means that, because this is equal to this, that the total circuit resistance is in the light bulb, that's an ideal circumstance that is not ever going to represent reality because wires have resistance.

Witches have resistance. Now they should be minimal. We should always have a minimal resistance and all of our connectors and our wires and our contact points switches all those sorts of things we want as low of resistance as possible in all of those points, and if there's only one thing you take away from this Video, that's what I want you to take away. You want all of your circuit resistance to be in your load as much as physically possible and the load is the part of the circuit that does the work.

In this case. It's a light bulb. It's your compressors! Your condenser fan motors or heat strips it's whatever does. The work actually accomplishes something in your system.

That is the load, and you want the fully applied voltage to go to that load. That's the goal, but when we measure 120 volts here and 120 volts here, that means that there is no resistance in the rest of the circuit. Now, that's not realistic, it'd be more likely. We would see 120 volts here and maybe 118 volts or 119 119 point.

Five and whatever is left over that tells us how much resistance is in the rest of the circuit so measuring applied and then at the load. That's a really nice measurement to do now. It's important that you recognize that that is only valid to do when you are under load, meaning when you are actually doing electrical work on the circuit. So you can't do this if this switch is open like it's shown right here.

If this switch is open, then that measurement doesn't become valuable because it's not under load. So at that point of course, I'm gon na see 120 volts here, I'm gon na see 120 volts here. I'm gon na see nothing. If I measure here if this switch is open, because now you have an open path on the other side, there's not going to be any potential difference between these points.

It's as soon as this switch closes, should go ahead and draw it closed just so. You're learning diagrams as you go, it says this switch closes that now we're gon na be able to measure 120 volts here and hopefully, as close to 120 volts here as possible. Now, let's say that we're reading less, let's say we're reading a hundred and fifteen volts here. This is where it becomes really interesting and you can use your volt meter as a voltage drop measuring device.

Let's see you're say you're measuring 115 volts here now that may or may not be a problem. It's probably not a big problem to have 115 volts there all right. So let's say that I'm measuring 120 volts here and I'm measuring 115 volts here. That tells me that I've got five volts of voltage drops somewhere else again.

That may not be a problem, may just be due to the cumulative effect of all the wires and the length of wire and whatever again under load with the switch closed with the light bulb on that's what I'm measuring now. Let's say I take my my voltage drop measurement device, my voltmeter here now the first thing quickly - and I say this all the time - but for those of you who are newer before you use ammeter, always put it on ohm scale, an ohm between your leads to Make sure that you have a path through your leads. I hate it when guys like. I don't have any voltage and it's just that one of these jacks is a little bit out and but when you do the ohm test, you can see that that's the problem.

Okay, first tip so now we're using this voltmeter and now it is a voltage, drop, detecting device, 120 volts of voltage drop between here and here now only 115 volts a voltage drop here. Where else do we have olders drop? Now I go to this switch. I measure across it and I measure 4 volts across a switch now. That would be definitely a problem, but let's just say that that's what I measure now.

This tells me that I've got 120 volts voltage drop between these points 115 volts here, which means that I've got five other volts somewhere else. This switches, four volts. That means the rest of the circuit is equal to one volt, and this is where Ohm's law becomes interesting, because what that's telling me is that this makes up a percentage of that total circuit. Ohms 115 volts makes up a percentage of the total, and now we have 1 volt left, which is what's left over so 1 volt now between all of the rest of the circuit, and that is the rest of the voltage drop on the circuit.

And that explains where all of our voltage drops are remember. Our goal is to reduce our voltage drops in everywhere, but the loads to essentially zero wires switches contacts. They should all be power, passing devices, meaning they should just pass the power through with a very little resistance and that's how they stay nice and cool and work. Well, you know intuitively if you've worked on anything electrical that when you have a poor connection.

Those connection points get hot. That means, when you add resistance in at a point that point is going to increase in temperature. Okay, so this switch here. If this is drawing 4 volt sorry this switch here.

If it has 4 volts across it, I can actually calculate what the I can actually calculate the amperage of this circuit. I can actually look at the Ohm's of this circuit and then I can. I can calculate the Ohm's district and then I can calculate the amperage. Not valuable to do now, we know that there is a voltage drop across this switch and there should not be a voltage drop across the switch.

That's that's what we want to eliminate, so I can calculate what the ohms are of this based on what the ohms of the total circuit is, so I could de-energize it and I can measure those resistances and I could calculate that from a practical standpoint, though it's Much easier to do this wall, you have a piece of equipment running. So how would you do it? Let's look at the contactor here on the on the bench. If I measure the applied voltage across the contactor, that's going to show me what the total input is and now, if I measure across a switch whatever voltage, I'm displaying is the voltage drop across that switch. When it's closed now, when it's open, I'm gon na measure the full applied voltage in most cases again, it depends on the the type of circuit, but this switch is closed.

So if it's pulled in like this, so if this switch is closed, meaning the electromagnetic coil has it pulled in, and I measure a voltage on this display - that's going to show me what the voltage drop is across that switch and they're normal circumstances. It should be very low. You should read essentially almost nothing to the extent that you do read something. That's telling you that you have resistance in that switch and when you have resistance there, there's going to be localized current draw meaning actual conversion from electrical energy to heat energy and that's: what's that's essentially what happens when you add resistance into a contact point? It gets hot you can see.

This set of contacts here has been hot, see all that carbon buildup around the edges, that's generally due to either an overcurrent condition on the entire circuit, or these contacts weren't, making good connection. This happens in the field, a lot when bugs get into it. If there's something got stuck in there or just over time with where, as they open and closed many cycles over the life. As these contacts make poorer and poorer connection, the resistance is increased, which means that there's current that's drawn here now.

Here's an interesting fact as you increase resistance in the circuit as you increase resistance current decreases, so your overall circuit current will decrease. Now a lot of people will say it depends on the motor depends on the load. There is some variance there because, as a motor locks up as it starts to run more slowly, you will start to see the resistance of the motor itself decrease and so it sort of acts as a balance or to that that's a little bit more advanced. But the point is, is that we don't want to reduce the applied voltage to our load and so from a practical standpoint.

One of the best things you can do is to measure the voltage applied to your load. So the voltage going into your compressor, for example, across the compressor with it running and then measure the applied voltage with everything off so a practical example would be. You know, let's say that this is a let's. Let's draw this as a contactor circuit, as you can see, I have lovely artwork, I'm an artist at heart, as you can tell here, but this is a typical compressor.

We don't have anything else connected in we've. Just got our contacts which are represented these. These two points here are breaking both legs because it's a two pole contactor. So these are our contact points here and when I measure here with the contacts open, I'm going to read 240 volts coming in now, I'm gon na go ahead and close the contacts.

Now my compressor should run. But now I can measure past the contacts which would generally be the top of the contactor or the load side of the contactor. I can measure between here and here and if I'm still measuring 240 volts are very close to it. That means I have very minimal, voltage, drop, minimal, voltage, drop on the circuit under load and also minimum voltage drop through the contacts.

That's one good way. We can do that, but another thing I can do if I do have a voltage drop that seems abnormally high. Then I can measure across each contact point and see what I measure on my voltmeter, because whatever I show on my voltmeter as I measure across these contact points, is the voltage drop that that contact is adding. So I can measure here in here here and here and now that gives me the full picture so by measuring coming in to the compressor here before you've energized it and then at each contact.

Now I know not only what is my total applied voltage to my compressor? Am I giving it the voltage it needs I'm seeing what are the effects of having it under load, meaning running vs., not under load, meaning, not running, and then I can also see. Are my switches, my contact points, adding resistance to the circuit, which, of course, they shouldn't be adding any significant resistance, because all because I'm using my voltmeter as a voltage drop device? What you've noticed here - and this is the point of this whole video - is that at no point did I switch my meter to the ohm scale other than just to touch my leads. I didn't go through an ohm out the individual components in this circuit. I just used voltage in order to measure voltage drops, which then tells me, where are my points of resistance, because points of resistance are points of heat in the circuit and it results in lots of unintended consequences which make your compressors run inefficiently.

All that sort of thing, but the other side here that I always like to mention, because a lot of people get confused, is that physical resistance is inverse to electrical resistance and an electric in a motor, which is why, when you lock a lock, a motor or Grab a ceiling fan, the amperage goes up because the amperage goes up because the resistance is coming down because resistance and Brij are also inverse so long as the voltage stays the same. So that's four, maybe a little bit more nerdy viewer there there's some things that, if you don't understand them correctly, you're actually gon na come to the opposite. Conclusion of what you should take. Aways are provide motors and loads with the proper voltage and you do that by reducing voltage, drops meaning making better connections using proper wire size on the circuits that feed the electrical devices and when you want to test for a voltage drop use.

Your Volt use your volt meter as a voltage drop testing device. In fact, that's the way that I will talk about the volt meter. I did a podcast about this think about your volt meter as a voltage drop measurement device, and it's gon na make a lot more sense to you, because now you're gon na know what to do with both leads versus saying. What is the voltage at a particular point, which is never a valid thing? A meter is always a measurement, a voltage drop between two points.

That's why you have two leads all right, i'm brian with hvac school. If you have not checked out the HVC school podcast, i would encourage you to do that. You can find it in any podcast app. Also, we have an app that you can listen to the podcasts and see our tech tips just go to the Apple App Store on your phone or the Android.

Google Play Store and type in HVAC school, and you will find our app right there. Thanks for watching. You you.