A VOM tutorial
#1
Jim Currie suggested that I do a tutorial on the use of a VOM meter some time ago. This is essentially copy of that tutorial that was posted elsewhere over two years ago. His thinking is that every modeler needs this tool, and should know how to use it. A VOM meter is indispensable for use during wiring and troubleshooting layouts and accessories. This meter is an electrical device used to measure Volts, Ohms and Milliamps, thus the name, VOM. There are two types of these meters, one is analog where the value of what you’re measuring is shown by the position of a needle on a dial face, and the other has a digital readout. I have an analog meter that is probably a good 50 years old and still works, but finding one is difficult nowadays, so we’ll talk about the digital type.

Let’s spend a little time discussing what is it that we’re actually measuring. Volts are the amount of electrical potential between two points, kind of like the amount of water behind a dam, the higher the water, the more potential there is. There is AC volts and DC volts. AC is what you get out of the wall socket in your house and can only be created by a generator. Current flows back and forth 50 or 60 times a second. DC volts are what you get out of a battery or a power pack that has change AC into DC. Current flows only in one direction in this case. There is another tutorial on power packs here that you might want to read. Voltage by itself can do nothing. You can frequently measure voltage between yourself and a water pipe, but it is normally harmless. Notice I said “normally”. If you measure a few volts between you and the pipe, that’s what’s know as “pick-up”, if you measure a lot higher, say 110, don’t touch the pipe, that’s not a good thing. What is required is current, measured in amps, to get any work done. Think of this as the water pressure that flows through the pipes leaving the dam. Resistance, measured in ohms, is the ability to oppose the forces of electrical current. This would be the same as when the water pressure hits a restriction or narrowing in a pipe, lowering the water pressure. Or in our case, lowering the amount of current that flows. There is a relationship between these and can be expressed in a formula: Voltage equals Current times Resistance, or, E = I x R. If you know any two values, you can calculate the other one using this basic electrical formula. “E” is used to denote voltage and stands for “Electromotive force”, “I” stands for current and “R” for resistance. Power is the Voltage multiplied by the Current, or P = E x I, and cannot be measured using a VOM.
A basic meter looks like these:
   
You can purchase an inexpensive meter for under $10US, and up to a couple of hundred dollars for a very sophisticated instrument. The one on the left I paid about $3 for on sale at Harbor Freight, the other two were about $40-50 purchased at an electronic supply house. There is no need to spend more than that. Some meters can also measure things like capacitance or even temperature. Simple meters now days have the ability to check transistors. Features and accuracy are depended on the individual model. Some are designed for more rugged use than others. For instance, if I were to drop my $3 meter, I’d just leave it and sweep it up with the trash, but my $50 meter has a rubber cradle to keep dropping damage to a minimum. Analog meters are also very sensitive to abuse as well. All meters are set up about the same. There’s an on/off switch somewhere, holes at the bottom for probes, a digital readout and a large switch used to set the meter to select what you are measuring. Make sure the meter is set for the measurement you’re about to take and at least the highest value you expect to see. Remember, the numbers on the meter selection switch represents the highest value that can be read in that switch position. Also, make sure the two probes are in the correct holes for what it is you are measuring. You can damage the meter, or blow an internal fuse, if you have it set for 20 mA and you are measuring 2 amps or if you’re measuring voltage and the meter is set to measure resistance. Each meter is different in some respect. The meter on the right is self-seeking in that you can just set it for what it is you’re measuring and it will seek the proper scale for the value. To me these types of meters are confusing. The smaller meter does not measure AC current and has limited range positions.

Measuring Volts:
When wiring up anything, it’s the wires that bring the voltage to wherever it’s needed. In our case, the wires go from the power pack to the rails (maybe through a control or switch panel), then to the loco’s motor. If you don’t have voltage, you don’t have anything. When testing, I always start at the farthest point. If I don’t have voltage there then I check the power pack. If I have voltage at the source, then I can work back in either direction to locate the point where I have voltage on one side, but not the other. In the case of rail wiring, it’s necessary to move both probes as you go since there is no common connection (or grounds) in DC rail systems. When measuring DC voltage, there is polarity, the red probe goes to plus, the black to minus. If they are reversed, the meter will read a minus (-) in front of the value. Remember, if you don’t have a load, you will read the same voltage all along the line, that can change when you add a loco or a light somewhere.
   
Measuring Current:
There is no current on an open circuit. Once you add a load, or some resistance in the circuit, you will generate current. The lower in resistance the load is, the higher the current. The higher the voltage, the higher the current will be for the same load. That’s why when you increase the voltage from the power pack; the current increases and the train will run faster. Current is always measured in series with the load, that is, you must connect the meter in line with one of the power wires. Remove one wire from the power pack; connect one probe to the power pack, the other to the wire you just disconnected. Set the meter to the highest current setting. On some meters that will require moving the positive probe to a different probe hole. There may be a fuse in the lower settings while a setting of 10A or more usually aren’t fused. If for instance you want to find out how much current your new loco draws at slow speed, adjust the power pack to the setting you want and put the loco on the track. Turn on the power pack and read the meter, if it’s too small a number, you may have to lower the meter setting. If the meter value keeps changing as the loco goes from one section of track to another, that’s a good indication that you have some rewiring to do. It could mean that a rail joiner is not making good electrical connection or that you need to add a new voltage drop to the rail section that is affected. Undersized wire will cause these problems as well. As you increase the speed setting on your power pack, the current will increase as well. If you have a lamp connected to your power pack, as you increase the voltage, or speed setting, the lamp will get brighter and the current will increase. The total current at the power pack is equal to the sum of all the currents required by all the loads you have. Let’s say you have two locos and at the 50% speed setting, they each draw .250 amps, and you add four lights, each drawing 50 mA, or .05 amps. Your total current draw will be 700 mA or .7 amps. You have to check the label on the power pack to see what the maximum output is. Some are rated in amps, others in watts, or power. If you exceed the rating, one of two things will happen. The unit will shut down until the load is removed and or the power pack it reset, or on some cheaper units, the voltage will drop to compensated for the increased current and it will heat up. Some have thermal overload protection, but the ones that don’t can be damaged this way, so whatever you power pack is like, it’s best to know how much current it can safely deliver and how much your equipment is loading it.
   
Measuring continuity:
Continuity is basically when you have an electrical connection from one point to another, or a “short”. For instance, of you have an open or loose rail joiner, you most likely won’t have continuity along your rails. Continuity is essential to complete a circuit and for current to flow. Frequently, when troubleshooting wiring, it is easier to use the resistance setting on the meter to test continuity, which should appear as a short. One word of caution, do not measure anything on the ohms scales with power on. It is best to just disconnect the wiring to the power pack. Many meters have a continuity setting that is denoted by a musical note next to the switch position. With the meter set for that, the meter will buzz whenever the two probes are touched together. If your meter doesn’t have that, then use the lowest setting for ohms. A short will then show as “0” and an open circuit as a “1” on the left.
To test for a short between the rails, connect one probe to one rail and the other probe to the other rail. If the power pack is disconnected, you should see an open circuit. Note that if you have some lamps in the circuit or have the power pack still connected, you will see some resistance, but you shouldn’t see a short circuit.
To test for an open circuit to your rails, connect one probe to the end of the wire that went to the power pack, and touch the other probe to the closest rail where the other end of the wire is connected. You should see a short. Run the probe down the rail and you should see a short all along the rail. If at any point the meter shows an open or high resistance, you have located a problem area. If you have a switch or control panel in the circuit, you’ll have to be sure the switches are in the right position. You should repeat the process with the other wire and rail.
   
Measuring resistance:
Everything that uses electricity has resistance. For the most part, you shouldn’t care. If you know the voltage and current ratings of things, you can calculate the resistance. On the other hand, if you know the resistance and the voltage, you can calculate the current. If for instance you have a 5 volt lamp that you want to use in a 12 volt circuit, you need to know either it’s current rating or it’s resistance so you can put a voltage-dropping (or current-limiting) resistor is series with it so as not to damage the lamp. Set the meter to one of the ohms scales, in this case a lower one. Touch each probe to one of the lamp contacts or wires and read the meter. Let’s say it reads 50 ohms. Using the formula above, you can calculate that it takes .100 amps, or 100mA to light. You’ve got to use a series resistor with a value that will drop 7 volts across the resistor and the remaining 5 volts across the lamp. Using that same formula, you can determine that you need a 70-ohm resistor, or a standard 75-ohm resistor. In this case, a one-watt resistor would be necessary. You can use the resistance scales to check continuity as described above. This is a good way to do it since it can show when you have a connection, but a poor or high-resistance one. If you’re checking continuity on your rails and you’re getting 0, or a short, then your OK, if you see a higher number like 100 or 1000 ohms, you can have either a poor solder joint, a corroded rail joiner or even a dirty track.
   
Don (ezdays) Day
Board administrator and
founder of the CANYON STATE RAILROAD
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#2
This a great tutorial for anyone who has little or no experience using meters. For those buying meters I personnally recommend the manufacture Fluke. They have proved to be the best meters I have used both in school and now in electronics design. They are usually fairly pricey but seem to work very well and are very accurate. Also I have had good luck with the Wavetek Meterman brand. That is what is on my desk at work right now (model 15xp). However for personal and hobby use I have a Mastercraft meter that Canadian tire puts on sale for 9-15$ every couple of months. The one thing you want to be sure of when picking out a meter is the Current rating (Amps) Be sure that your meter can handle the currents you want to measure. If you try and measure the current of a big club layout with a small meter that is only designed to handle a couple amps then it may make a nice popping noise and begin to burn up. Smile Just like a z scale decoder in a G scale train.
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#3
Great tutorial..!! Having used one of these VOMs for as long as I've been involved in this hobby, it's proved an invaluable tool to check wiring, locos, circuits, etc....
It should be noted that a "standard" VOM will NOT measure current (Amps) on a DCC circuit. For that you will need a special DCC "enabled" Ammeter.
Gus (LC&P).
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#4
Beamish Wrote:This a great tutorial for anyone who has little or no experience using meters. For those buying meters I personnally recommend the manufacture Fluke. They have proved to be the best meters I have used both in school and now in electronics design. They are usually fairly pricey but seem to work very well and are very accurate. Also I have had good luck with the Wavetek Meterman brand. That is what is on my desk at work right now (model 15xp). However for personal and hobby use I have a Mastercraft meter that Canadian tire puts on sale for 9-15$ every couple of months. The one thing you want to be sure of when picking out a meter is the Current rating (Amps) Be sure that your meter can handle the currents you want to measure. If you try and measure the current of a big club layout with a small meter that is only designed to handle a couple amps then it may make a nice popping noise and begin to burn up. Smile Just like a z scale decoder in a G scale train.
Yep, like I said, most meters have a separate 10 amp input that is not fused, so be sure your range setting is higher than what you're measuring.

And fluke for sure is the top of the line. For those that are not using meters everyday on a professional level, it might be more than they want to invest. I would opt for a cheap one rather than none at all, and a Fluke if I could afford it.
Don (ezdays) Day
Board administrator and
founder of the CANYON STATE RAILROAD
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#5
ezdays Wrote:I would opt for a cheap one rather than none at all, and a Fluke if I could afford it.


Very true. This is exactly why I only don't have a Fluke at home. Big Grin
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#6
Hey thanks Don! For some one like me who is electrically challenged that was a great help!
Ralph
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#7
Thanks, always a valuable refresher. Cheers
Lynn

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Great White North
Ontario,Canada
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#8
I have a digital VOM that I picked up at Home Depot a few years ago for something like 20 bucks. Never have sat down and figured out how to use it and lost the instructions to boot. I always get confused as to which lead gets plugged into which socket. If I post a picture of it do you think you help me with it? :oops:

I should also print this off and give it another go.
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#9
tetters Wrote:I have a digital VOM that I picked up at Home Depot a few years ago for something like 20 bucks. Never have sat down and figured out how to use it and lost the instructions to boot. I always get confused as to which lead gets plugged into which socket. If I post a picture of it do you think you help me with it? :oops:

I should also print this off and give it another go.
Be glad to help, most VOMs are about the same, but some have different sockets for different functions which usually saves on switch positions. Post a pic and we'll figure it out for you.
Don (ezdays) Day
Board administrator and
founder of the CANYON STATE RAILROAD
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#10
Thanks Don. You're the best! Thumbsup
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#11
Better late then never. Hey Don, mind telling me how to use this contraption? :?

[Image: DSC02861.jpg]
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#12
tetters Wrote:Better late then never. Hey Don, mind telling me how to use this contraption?

OK, easy enough, just refer to my tutorial for how to use it, I'll tell you here how to set it up.

  1. To measure DC volts, put the black lead in "COM" and the red lead in "V". Turn the meter dial to any of the DC--- positions, always at a higher scale than you are trying to measure. Like, set it at "20" if you are measuring 12 volts. Always start at the highest setting if you're not sure of the voltage, or you could burn out the meter.
  2. To measure AC volts, do the same but set the dial to V~ positions.
  3. To measure DC current, put the black lead in "COM" and the red one in "A". Turn the meter dial to "200 m" position and start there, you can switch to a lower setting when you are sure the current is much lower. The maximum you can read with your meter is 200 milliamps, or .2 amps. If you try to measure anything higher you will either burn the meter out or you will blow an internal fuse if it has one.
  4. To measure resistance, use the "COM" and the "Ω" inputs. You can read anything from a few ohms to 20 megohms. It's best to start at a lower scale and work your way up.
  5. To measure continuity, you should be able to use the position below the 200 ohms setting. When you touch the probes together,the meter should buzz. That way you can use the meter to check for shorts.
Don (ezdays) Day
Board administrator and
founder of the CANYON STATE RAILROAD
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#13
Awesome! That's all I needed to know, I'm printing this off and taking it home with me to play with it tonight. Thanks Don. Thumbsup
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#14
tetters Wrote:Awesome! That's all I needed to know, I'm printing this off and taking it home with me to play with it tonight. Thanks Don. Thumbsup
No problem, just one other thing, be sure to turn the dial to "off" when you're finished. Some meters have an auto-off feature, but yours does not.
Don (ezdays) Day
Board administrator and
founder of the CANYON STATE RAILROAD
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#15
Great tutorial, Don. I would add one last bit of advice. If you don't use your meter very often, remove the batteries after use. When old batteries break down and leak, it pretty much destroys the meter.
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