This is the 14th installment of these short episodes, where we just talk about a single topic, single technical topic, and today we're talking about the voltage drop measurement tool and before we do that, I want to mention, are excellent sponsors carrier carrier, comm, Mitsubishi, Electric cooling and Heating, the UE I hubs market with their high quality induct thermal hygrometers, as well as temperature, clamps and refrigerant probes, all in one nice kit, if you look at the hub 6, the wrs connected scales from uei. Those are the scales that I prefer refrigeration technologies at refridge tech comm, some of the best made American chemicals for the HVAC industry, check out refrigeration technologies that were French tech, comm air, Oasis makers of the bipolar and nano air purifiers. The air purification products that we use at Kalos services and then finally busy pal org if you're looking for a technician, then talk to Patrick over at visited org and see if he can help. You find your next technician for your company all right.

Here we go so a voltage drop tool, just a voltmeter people, it's just a voltmeter. The way that I'm teaching volt meters now and how they're used is to use it to check for a voltage drop, and that seems really counterintuitive, because what most people do with a voltmeter they're checking across the line. So they check l1 to l2. Well, there's no voltage drop, they think to themselves, but in fact, every time that you are measuring voltage, you are measuring a voltage drop.

You are measuring a potential difference, true, but you are measuring a voltage drop between those two points, and so you have to have one higher and one lower, and so a way to think about that is a voltage drop. What do we say? Why is that important? What does that matter? Well, it helps clear up some confusion, so you give a common example. This will be sort of the prime example we use here in this episode, at least, although there are many, if you have a contactor, where the contact points are beginning to add some resistance to the circuit, so they're not making good connection, you could disconnect power energize. The contactor coil an ohm across it, but that's not really under real conditions.

That's not under the real temperature conditions at which normally operating and so to really see if that contactor is potentially causing a problem, if that pitting or that carbon buildup is potentially causing a problem, you can measure across those contact points. So imagine if you're looking at a contactor, so you just pick one side so say the right side and you've got a contact points. We would normally read from bottom to bottom and top to top. But a really good test to do is to just measure across the contacts with voltage with your voltmeter and if you see anything on your meter that equals your voltage drop.

So whatever you measure on your meter equals your voltage drop between those two points when you're doing something like measuring voltage drops on series circuits where you have loads in series. That comes in really handy because there are different voltages applied to each load depending on the number of loads in the ohms of each load. That's sort of school stuff, stuff that you do that math in school and then you forget about it. But in real life we don't deal much with series circuits in air-conditioning, at least in electronics.

You see it, but in air-conditioning we don't have much in the way of series circuits, at least not intentionally, but we do have unintentional series circuits quite often, and an example of an unintentional series circuit is switch gear or wiring. That's adding more resistance than it should and when it does add more resistance, then you're going to see a voltage drop across it. So using your meter on an energized load and measuring across switches on the Volt scale. Looking for a whole tidge drop is a very effective way finding resistance in a circuit, but then also to find an open circuit.

So you may or may not know this, but a really good way to use a voltmeter is to measure through a circuit, so you're gon na be measuring from one side of the circuit to the other and kind of walking your way through and then all of A sudden you'll see where you no longer have voltage that's a typical way. People will use it. I see where the voltage disappears. You keep your 1 lead pegged to ground or to neutral, should normally be neutral of the other side of power, and then you kind of walk your way through the circuit, but another really good thing to do with a switch that you know has potential applied to It is to measure across it and if you measure across it - and you don't have any voltage, that means that you have no voltage drop, which means the switch is closed and in good connection.

Now again, this only works. If you know that there is potential applied to the switch, so it doesn't work obviously on that has no potential in the first place, but once you've established that the circuit has potential voltage on it, then, when you measure across a switch that is closed, you will Read nothing and when you read across a switch that is open, then you will read generally. There are some additional caveat to that, but that's sort of the general idea - and why is that? Well because an open circuit has voltage drop right. You have a voltage drop across that air gap in the switch when a switch is open, you're going to have voltage drop across that open switch, and so there's some other examples too, like if you've seen the crank case heaters that actually feedback feed through the compressor Windings that's another example of where we actually see a series circuit, a true series circuit where that crank case heater is wired in across the open contacts, so from top to bottom on one set of contacts, and what happens is that when the switch is open, the Voltage is allowed to feed through that crank case heater and then through the compressor winding back to the other side.

You've never seen this before it's going to be hard for you to imagine this, but what you'll notice is that the current on that circuit is very low. It's just like a trickle charge. Why is that? Well, it's because you have cumulative voltage, drops you have a voltage drop across the crank case heater and then you have a voltage drop across the compressor windings. So there's a law for this and it's probably not something you're gon na remember, but it is considered to be a pretty fundamental law of circuits and there's a couple of them, but there's called Kirchhoff's voltage, law or Kirchhoff's second laws.

Guy named Gustav Kirchhoff made this law, and this is sort of technical, lingo and I'll break it down a little more simply, but the law states that for a closed loop series path so for a complete loop, a complete circuit, the algebraic sum of all the voltages Around any closed loop in a circuit is equal to zero. So another way of saying that is is that across the entire circuit you're going to read the entire applied voltage, because you have voltage drop across that entire circuit. A lot of guys, I think, get confused. Sometimes because you'll notice, with your voltmeter, that when you measure from one side of a load to the other side of a load like a contactor coil, for example, you'll read the voltage so let's say 27 volts whatever so you'll read 27 volts across that coil.

But then you'll also read 27 volts across an open switch and you'll. Also read it right at the transformer, and why is that? And I think a lot of people think why is the load the same as an open, well they're, similar in that almost all of the resistance and a properly constructed y circuit? So if you're thinking of the low-voltage Y circuit that powers your contactor coil, almost all of the resistance should be in that contactor coil and so because almost all of the resistances in that contactor coil you're, going to measure almost all of the voltage drop across that Contactor coil now the wires have a little bit of resistance and you'll have a little tiny bit of resistance in the contact points inside the thermostat, but in general the resistance in that circuit is in that contactor coil, which is why you read the full applied voltage Right now imagine for a second that you had a wire that was feeding that contactor that had some additional resistance in it. It was who knows what something was wrong with it, and so you had some additional resistance in that wire had a bad connection. Whatever now you wouldn't read the full applied voltage at that contact or under load, you would read less than the applied voltage, because that additional resistance is going to take up some of that voltage drop.

So if you could kind of imagine if you're thinking of a series circuit, so you're going from one side to another in a 24 volt circuit is just for imagination sake and if a hundred percent of all of the resistance is in that contactor coil. On this 24 volt circuit, we're going to say it's exactly 24 volts if 100 % of the resistances in that contactor coil. Then, when you measure across that contractor coil you're going to read 24 volts but say, for instance, that the wire feeding it has a voltage drop of 2 percent of the overall circuit resistance. Well, then, 2 percent of the voltage drop is going to be in the wire and then you're gon na read 98 percent of the voltage drop across that contactor coil.

So remember, the point of this podcast is to get you to start thinking of a volt meter. Think of it as a voltage drop tool that you're using your volt meter. You have those two leads in your hands and what you're doing with those two leads is you're. Looking for voltage drops and so you're gon na see a whole tidge drop across open circuits.

If there's a potential difference between those two points, you're gon na see a voltage drop across a load naturally, but you're. Also going to find other voltage drops that maybe design voltage drops now keep in mind. A voltmeter is most effective in this purpose. When it's used under load, because when you have an open circuit, where there's no electrons moving well, then you're always going to read the full applied voltage.

I use the example of imagine that you wired up a compressor, but instead of wiring it up properly, you wired it up with thermostat wire, so instead of using proper number 10 wire and a brake wire to wire up the compressor you're using thermostat wire. Alright. So when you have this thermostat wire, what will happen when the circuit is open? Think of running thermostat wire to the disconnect for the condenser? For example, when you have the disconnect pulled - and you measure the voltage at that, disconnect you're gon na measure, the full apply voltage, you're gon na measure, the full voltage drop of, say, 240 volts, for example, feeding this condenser disconnect. So in our imagination.

Right now. Let's just set our imagination out, you literally wired the wires to a disconnect for a condenser with thermostat wire, and you have the disconnect pulled and you're measuring the voltage between those two, the l1 and l2 little number 18 wire or whatever number 20 wire and you're Gon na measure 240 volts there, because you have that full voltage drop across the line there and it's not under any load now as soon as you apply a load to it. As soon as you push that disconnect in your volt is just gon na drop down to nearly nothing why it's gon na drop down to nearly nothing because of the voltage drop across that wire. You're gon na have a huge voltage drop at amperage, and so, as you put that thing under load, there's gon na be an enormous voltage drop on those conductors and you're gon na measure that across those two lines there, because when it's not under lead, you can't Really measure and see what your voltage drop is going to be so anyway, voltage drop in application works best when it is under load, but you also will see a voltage drop when it isn't open because, of course, an open circuit, there's no path whatsoever.

So, of course, you're going to read the full voltage drop. Alright, that's it! So what you can take away from that is. If you want to sound fancy, Kirchhoff's second law, you can quote Kirchhoff's second law, if you'd like and for a closed loop series circuit. The algebraic sum of all the voltages around the closed loop in a circuit is equal to zero.

In other words, the voltage drop across the entire circuit is equal to the resistances in the circuit, so the additive effect of all of those resistances, and so, if you want to calculate the voltage drop, you can do that mathematically, but we don't care about that. Generally. We're not usually sitting there with her calculator doing math with voltage drops out in the field unless we're calculating allowable voltage drops and I'll. Give you another little quick, takeaway something you can take home with you.

You generally only want to see about a 5 % voltage drop in total, so when you have a 247 volts applied at the panel mains coming in, you only want to see a maximum of 5 % voltage drop under load, not during start during start, you're holders Drop will be a little higher because you have that really high amperage during the start of a compressor but running appliance. You only want to see about a 5 % voltage drop from start to finish so from where it comes in, on the main into the panel to where it hits your appliance. So there you go think of your volt meter as a voltage drop tool and see how that works. For you, I'm sure some of you are gon na, be like that's way too confusing, and maybe it is, but I've been thinking about it that way lately and it's made a lot more sense.

All right, we'll talk to you next time on the HVAC school podcast.