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How to (safely) use a multimeter?


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Been doing this railway thing for 4 years or so, never used one yet.

 

Lets say I want to test how many volts are running through my track - how would I do this?

 

And what about if I wanted to test how many volts are going through a particular cable?

 

It worries me that I've seen people plug in and turn on some sort of power cable, then just touch the bare ends with the multimeter sticks and somehow get a reading.

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First of all volts don't run through anything, current runs through cables, etc.

 

If you wanted to test the track voltage you turn on the track power and place one of the multimeter probes on each rail of the track.

 

If track power is 8V then the multimeter will read either +8V or -8V depending on which way round the leads are. 

 

When you measure a voltage you are always measuring the DIFFERENCE between two points on an electrical circuit.

 

Regards,

 

John P

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When measuring voltage you first set the range switch to an appropriate high range. Eg DC volts 200. Then place the probes across the circuit, ie one probe to each 'rail'. Then reduce the range until you have a sensible reading. Obviously most of the time you guess it's going to be say 12V so you set it to the next highest range, perhaps 20V or 25V. Much of this will be irrelevant if you have a self ranging meter. Measuring current is a whole different ball game. The meter has to be in series with the circuit. So disconnect a wire, connect one meter probe to the wire you just took off, and the other probe to the terminal you just took it off !! A good  trick is to try all these moves out on a working circuit before you have a fault, so you become familiar with using the meter, and familiar with likely readings.

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What type of meter are you using Sir Topham?
 

Is it digital or analogue (numbers or needle) and if digital is it autoranging or are there lots of ranges you can select?

 

 

Autoranging digital is by far the easiest as you select DC Voltage, AC voltage, resistance, etc and the gubbins inside deals with everything else if you know where to put the probes.

 

If it’s a meter with lots of ranges and you don’t know what you might be dealing with then start at the highest and work down. I.E. track voltage should be about 12v at full whack but you could start with 200v then down to 20v in case your controller has gone mad

 

 

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2 hours ago, cliff park said:

A good  trick is to try all these moves out on a working circuit before you have a fault, so you become familiar with using the meter, and familiar with likely readings.

 

I felt that was worth emphasising.

It is much easier to find something wrong when you know what should be correct in the first place.

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Analogue meters with a moving pointer are easier to use for fault finding  than digital meters. You can hear the needle clicking back and forth. Digitals often scroll rapidly through a series of numbers with minor variations in voltage which is annoying.

When working in the motor trade I made a tester from a door bell, and several old bulbs which I soldered wires to so the crocodile clips on the bell wires could be attached.  It just meant I didn't need a second person to assist.  The same 12 volt bell has since seen use with crocodile clips for track power testing. It gives a yes/no binary power on or off indication which is really all you need.

Finding one digital meter shows 15 volts and another shows 19 volts on the same circuit can be disconcerting.

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All digital meters will show the same voltage on the same circuit when using DC, it is when you use DCC that the variation *may* appear - and that same variation would be seen on an analogue meter.

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Posted (edited)
22 hours ago, corax67 said:

What type of meter are you using Sir Topham?

 

Digital but has a dial for me to choose the range / what I'm measuring.

 

I guess it just worries me when I see a video of someone that seems to put one probe on the live wire of a power supply (goodness knows where they put the other one?) and get a reading.

 

If a power supply has just two cables - how will I know which is positive 12v (coming from a 12v PSU) and which is negative (I guess this would show 0v?)?

 

 

Never needed to use one before as the one time I had a short just after soldering, I just re-traced where the cables were plugged into the bus and swapped it round.

Edited by Sir TophamHatt
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1 hour ago, Sir TophamHatt said:

 

If a power supply has just two cables - how will I know which is positive 12v (coming from a 12v PSU) and which is negative (I guess this would show 0v?)?

 

Hi,

 

You really need to get your head around how voltage measurements work.

When you measure a voltage, you are measuring the voltage drop between two points, using the two leads on the multimeter.

 

If a power supply has just two cables it doesn't matter which is positive or negative when you are measuring the voltage as the result on the multimeter screen will read either +12V or -12V. You can't measure the positive or negative outputs in isolation, you must always measure them with reference to one another, that's why the multimeter has two leads and not just one.

 

Generally speaking if you measure a positive voltage on the power supply cables, then the lead which is connected to the common on the multimeter is the one which is connected to the 0V or GND of the power supply. This is true when you are talking about a simple DC power supply like that use for a laptop computer.

 

On a model railway DC supply, you can switch the polarity of the output voltage to make your trains run forwards or backwards, so if you connect up a multimeter to a DC track power supply the meter will give either a +ve or -ve reading depending which way the trains are running.

 

Regards,

 

John P

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If you are using a proprietary PSU, then you need have no concerns about safety. Short of cutting it open you won't be able to get anywhere near the mains voltage, and 12 V (or even twice that) won't cause you any harm. However, if you have a homemade PSU it might be better not to try anything at all until you know what you are doing.

 

There is also very little that you could possibly break, except possibly for the meter itself, and I suggest you don't try using the 2000 mV or 200 mV DC settings. 20 V DC, or something like it, will be fine. Even 200 V DC or 600 V DC will work well enough for most things I imagine you want to do. Do make sure you plug the probes in correctly (unless they are connected permenently). The red lead must go to the red terminal on the meter or, if there is more than one, the one marked V. The black lead must be plugged into the black terminal, which may be marked COM. You can leave the probes plugged in the meter permanently, although you may need to move the red lead if ever you do current testng.

 

When using the meter on a voltage setting, the probes are unpowered and you can touch them together and treat them like ordinary metal rods. Don't try sticking them into a mains socket or your toaster or anything like that. The only damage I can imagine you doing with them is if your PSU does not have short circuit protection. If this is the case, then you must take care not to allow a single probe to bridge across both positive and negative outputs. Touching one probe to the positive output and the other probe to the negative output is fine - it is what the meter is designed to do. Note that you have to touch both probes to the outputs (one to  positive, the other to negative); touching just one will not give any reading.

 

You asked about knowing which is the positive and which is the negative output. If, after turning on the PSU and touching one probe to one output and the other probe to the other output, you get a positive voltage reading on the meter, then the output connected to the red probe is positive. If the reading on the meter is negative, then the output connected to the red probe is negative (or zero, of you prefer).

 

Apart from checking polarity, another common reason for using a voltmeter is to check for open circuits. Open circuits can be on either the positive or negative side, and one common approach, which might be what you saw in the video, is to measure the voltage close to the PSU or controller, where it should be fine. You then leave one probe connected at the PSU or controller, and then move the other probe progressively further away, ideally testing after each soldered joint or switch. If you get a reading on one side of a soldered joint but not on the other, then that is where the open circuit is. If you have tested all of the positive (or negative) circuit up to the track or whatever it is that does not work and not found a problem, then do the same with the other probe on the other circuit.

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You will should have 3 or 4 main settings on your meter: DC Volts, Amps, Ohms, possibly AC Volts. 

Volts tells you how much push the electricity is getting between one point and another. Tells you if there is power available between the rails or the terminals on the controller. Also use to see if a battery is dead.

Amps is the current flowing. I think it is some millions of electrons per second. This is related to the load and the amount of heat generated. It is measured by putting the meter in place of one of the wires. Older OO/HO locomotives could draw 1 amp on a bad day. Old O gauge locos might pull more. It is interesting to watch the meter as you run a train around the layout and see how the draw varies. Some people check a loco by holding the coupler and turning the throttle up to see what it consumes when the wheels spin.

Ohms is resistance. 0 ohms is a short or you've soldered the wires together properly (depends). Infinite is an open circuit, again either something isn't connected or you haven't dripped solder across a printed citcuit board. You can test track by measuring resistance from rail to rail. I suggest that you disconnect the throttle or you will measure its resistance.

 

Watts is Volts times Amps. A 100 Watt bulb at 200 Volts consumes 1/2 amp. At 12 Volts it would consume over 8 amps.

 

 

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4 hours ago, BR60103 said:

Watts is Volts times Amps. A 100 Watt bulb at 200 Volts consumes 1/2 amp. At 12 Volts it would consume over 8 amps.

This isn't true. Power does not usually remain constant regardless of voltage, and power ratings are usually given as the maximum power expected at the usual voltage.

 

With tungsten filament incandescent bulbs, there is a sudden current draw when they are turned on which quickly heats up the filament and increaes the resistance, which in your example ends up as about 400 ohms. When cold, the resistance is a lot less, perhaps 30 ohms. from which we can work out that the initial current at 12 volts is 0.4 A. This is enough to heat up the filament, increase the resistance and so reduce the current, perhaps to 0.1 A, making the power of the 1.2 watts at 12 volts. The bulb will barely glow. These numbers might not be accurate but they should be in the right ball park.

 

Don't use a multimeter with mains voltages unless you know what you are doing. You also need to take a little more care with current readings, and sometimes a lot more care with the meter set to measure resistance, than you do with voltage readings.

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Sir Topham

 

Seeing this and your other "electrical" questions, you might want to start with a very basic intro to electricity.

 

This one is pretty good, and it does quickly get on to using a multimeter https://www.explainthatstuff.com/electricity.html#:~:text=Electricity is a type of energy that can,one place to another is called current electricity.

 

BBC Bitesize also has quite good material, although their resources are so many that I find using them involves looking for the most suitable needle in the haystack first.

 

Apologies if these are patronisingly junior introductions, but the danger of RMWeb for something like this is that it has members who are experts, but tend to take off at the deep end, and members who are actually  a tad confused themselves.

 

As an aside, a multimeter is definitely in the "luxury" class of tools when it comes to railway modelling - you can well get by without one. I'm an electrical engineer by profession, have been playing trains for a lifetime, and own a multimeter which comes out of the drawer probably no more than once a year!

 

Kevin

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Useful thread, I have a multimeter but while I can measure volts and resistance, that’s as far as my knowledge goes. So this is helping my understanding.

 

Earlier someone said the red probe must go in the red socket, the black in the black socket.  Just wondering what happens if you put them in the wrong one?

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Posted (edited)
34 minutes ago, ColinK said:

Earlier someone said the red probe must go in the red socket, the black in the black socket.  Just wondering what happens if you put them in the wrong one?

 

You'll get a reverse reading on DC Range  i.e   -10V   instaed of   +10V

Edited by Pannier Tank
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34 minutes ago, ColinK said:

Useful thread, I have a multimeter but while I can measure volts and resistance, that’s as far as my knowledge goes. So this is helping my understanding.

 

Earlier someone said the red probe must go in the red socket, the black in the black socket.  Just wondering what happens if you put them in the wrong one?

Nothing, both the probes are identical. They are colour coded so that you don't have to keep looking at the meter connections to see which one is connected to common.

 

Regards,

 

John P

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4 hours ago, ColinK said:

Earlier someone said the red probe must go in the red socket, the black in the black socket.  Just wondering what happens if you put them in the wrong one?

If you are doing DC voltage testing (which the OP is) then having the wires the wrong way round will show -12 V instead of 12 V, for example. If you aren't interested in polarity then this does not matter, but the OP specifically asked about testing the polarity of his PSU outputs.

 

You will get a similar polarity error with DC current testing.

 

If you are doing resistance, continuity or diode testing, where the meter applies a voltage across the two probes, if the leads are plugged in the wrong way round then the black probe will be positive and the red probe negative. This won't make any difference unless you are testing electronics (diodes and transistors), when you will get unexpected and possibly confusing results (till you work out the leads are swapped over).

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9 hours ago, Nearholmer said:

Apologies if these are patronisingly junior introductions, but the danger of RMWeb for something like this is that it has members who are experts, but tend to take off at the deep end, and members who are actually  a tad confused themselves.

 

Don't apologise!

 

To be fair, some sort of pre-GCSE / junior school type of approach may be the way for me to learn.

 

I've made a few similar but fairly different threads about power as I'm sort of thinking out loud with planning and options but I think I'll stick to the Hornby controller and a laptop PSU for other things.

 

Cheers! 

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Many digital multimeters on the lowest resistance setting emit a warning sound when no resistance is found; quite useful in tracking down that annoying short circuit as you do not need to keep on looking at the display.

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One use for the Resistance meter is checking rail continuity if something stops running.  Put one probe on a rail where it runs and the other on the same side rail a bit away. You should get a reading of almost 0.  Move the second probe down the track. At some point you may see the reading jump to very high. This is where there is a break of some sort. You will often get this at points where contacts are dirty or you forgot to add frog wiring.

I have an extra set of probes with alligator clips on them. And a long wire with clips. 

 

(My watts explanation may have been a bit crude, to get an example.)

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On 15/07/2020 at 10:19, Nearholmer said:

Sir Topham

 

Seeing this and your other "electrical" questions, you might want to start with a very basic intro to electricity.

 

This one is pretty good, and it does quickly get on to using a multimeter https://www.explainthatstuff.com/electricity.html#:~:text=Electricity is a type of energy that can,one place to another is called current electricity.

 

BBC Bitesize also has quite good material, although their resources are so many that I find using them involves looking for the most suitable needle in the haystack first.

 

Apologies if these are patronisingly junior introductions, but the danger of RMWeb for something like this is that it has members who are experts, but tend to take off at the deep end, and members who are actually  a tad confused themselves.

 

As an aside, a multimeter is definitely in the "luxury" class of tools when it comes to railway modelling - you can well get by without one. I'm an electrical engineer by profession, have been playing trains for a lifetime, and own a multimeter which comes out of the drawer probably no more than once a year!

 

Kevin

To suggest a £10-£20 DMM is a luxury for an hobby powered by low voltage electricity is rather far fetched 

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If you are a beginner in the hobby, and build your tool kit in order of necessity, or frequency of use, you can happily put the multimeter a very long way down the list, is what I’m saying.
 

 

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On 16/07/2020 at 02:56, BR60103 said:

One use for the Resistance meter is checking rail continuity if something stops running.  Put one probe on a rail where it runs and the other on the same side rail a bit away. You should get a reading of almost 0.  Move the second probe down the track. At some point you may see the reading jump to very high. This is where there is a break of some sort. You will often get this at points where contacts are dirty or you forgot to add frog wiring.

I have an extra set of probes with alligator clips on them. And a long wire with clips. 

 

(My watts explanation may have been a bit crude, to get an example.)

 

There is an easier way to use a resistance meter than that.

Remove the controller & replace it with a wire.

Then simply test the resistance across the running rails. Move further away from where the controller is & if you see a sudden increase in resistance, you have found your fault. You don't need long leads to do it that way.

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