DC Loco Power [Using a meter to measure it]

[Chrissaf]

Jimbopuff,

Type into Google "how do I measure current with a multimeter". You can also measure the stall current [amperes] by briefly [very briefly] putting your hand in front of a loco to stop it moving. Power is some formula of volts x current, again Google will explain.

[Chrissaf]

Multi-meters measure three basic electrical conditions ... voltage, current and resistance. For voltage and current, there are two sub-categories ... AC and DC.

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In principle, you measure voltage by placing the meter probes across (in parallel) the component being measured.

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In the case of DC Analogue track voltage, then the meter probes would be placed across the two rails with the meter on the 0 - 20 volt DC range.

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In principle, you measure current, by breaking the continuity of the circuit being tested and inserting the meter probes in series with the circuit being tested.

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In the case of DC Analogue track current, then one of the connections from the controller to the track is disconnected. The positive meter probe would connect to the positive end of the break, and the negative probe would connect to the other side of the track feed supply break. I would suggest that the meter should be set to measure 0 - 2 Amps DC range.

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So if you wanted to measure the current drawn by the DC Analogue loco motor. Then you would connect the meter in series with one of the power feeds to the track as described above. Then operate the loco on the track using your controller monitoring the current drawn. Note that the current will go up and down proportionally to the speed. So if you want to know the nominal maximum current drawn, then you would set the controller to 'max' speed. If you want to know the absolute maximum current drawn, then you need to briefly (very briefly) hold the loco down to prevent the wheels turning and quickly take your current reading. This is termed the 'stall test'.

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Notes:

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Meters [subject to their price and capability] will have an upper limit of how much current they can measure. Very cheap meters may be limited to a mere 0.5 Amps. Some might go up to 10 Amps, but in the main, the majority of budget meters should be able to measure up to 2 Amps. You need to consult the manual that came with your meter to ascertain how to connect the probes to the meter when measuring these higher current ranges as they will usually use different sockets on the meter.

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With DC voltage and DC current, if the meter probes are connected the wrong way round, the measured value will normally still display but prefixed with the ' - ' [negative] symbol.

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For resistance .. rather than reinvent the wheel and duplicate info in this reply. Read my replies in yesterdays thread:

https://www.hornby.com/uk-en/forum/post/view/topic_id/32848/?p=2

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DCC Addendum.

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For DCC tracks. The meter should be set for Voltage 0 - 20 volts AC and current also for 0 - 2 Amps AC but note this. DCC track voltage is not pure sinusoidal AC. Thus normal meters will not read accurate values. They can only give indicative 'working' / 'not working' type indications.

[2e0dtoeric]

p.s. to Chrissaf's comments - then you need to briefly (very briefly) hold the loco down to prevent the wheels turning and quickly take your current reading. This is termed the 'stall test'.

DO NOT do this with a DCC loco with the decoder installed! You will let the magic smoke out!

Remove the decoder and fit a blanking plug, then do the stall test!

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And can someone change the title of the thread to something more meaningful?

[Chrissaf]

I have just bought a basic multimeter and want to measure the power drawn by my older locos. I am a complete novice at using multimeters so could someone please advise me how to do this ? 

I would also be very useful if I could have a few tips at other common uses of the multimeter on the model railway - mainly DC.  Thanks for some help.

[Chrissaf]

And can someone change the title of the thread to something more meaningful?

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Done...

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PS - I should have added Eric's caveat about 'stall tests' in my DCC Addendum paragraph. Eric is quite correct, 'stall tests' should only be peformed in DC Analogue test conditions (as I described in the main body of my reply) and never via a fitted DCC decoder.

[GWR 14xx Tank]

Once you have found out the readings for the voltage on the track and the current drawn by the locomotive (information provided by the other memebers above) you will need to do this calculation:

 

Power (in watts) = Voltage (in volts) x Current (in amps or milliamps)

 

So, for an example,

 

If the voltage was 12v DC, and the current drawn was 0.75 amps, then you would times 12 x 0.75, to get an answer of 9 watts of power used by the loco.

 

Because DC regulates voltage, the power drawn will be different on different positions of the dial on the controller.

 

Another, this time in milliamps,

 

If the voltage was 15v AC, and the current drawn was 697 milliamps, then work out what 697 milliamps is in amps (1 amp = 1000 milliamps, so to find 697 milliamps in amps, divide 697 by 1000, and that is 0.697 amps.)

 

Then, 15 x 0.697 = 10.455 watts of power would be the power that that particular locomotive would consume.

 

Although DCC doesn't change the voltage to change the speed, I would still imagine that different levels of current are drawn depending on the speed of the locomotive. So, similar to DC, the amount of power drawn will be dependent on the locomotive's speed.

 

To use a mutimeter to measure DC voltage, plug the black probe into the "COM" hole on the multimeter, and then plug the red into the hole marked "V" and "mA", something like the image below. Set the dial to 20 in the "V with the straight and dotted line" zone. Then, follow the steps to measure voltage given out by @Chrissaff.

 

To use a multimeter to measure AC voltage, do the same as DC, but switch the dial to the 200 in the "V with the wavy line" zone.  Normally, cheap meters only have 200 or 750 in this zone, but the 200 setting will be more accurate for the 15v AC voltage on DCC.

/media/tinymce_upload/028def79ef92baa433624493e6391557.jpg

To measure current, keep the black probe plugged into "COM", but switch the red probe to the 10A port. (Note: settings and numbers on the meter may be different on other meters.) Don't keep the red probe in the "V" and "mA" hole, as some currents may be more than the rated milliamp reading for the "mA" hole. It tells you underneath the hole with "V" and "mA" that the maximum rating for that hole is 500ma (this may be different on other meters, but it should say below the hole.) Like mentioned before, it is best to put it in the 10A hole. Also, set the dial to the "10" setting in the zone with the "A and the straight and dotted line" zone.

 

Have probably repeated a few things from other members, but I hope this information is helpful. Hopefully my descriptions of where to turn the knob and attach the probes make sense.

14xx Tank

[Jimbo1707820979]

Thanks for your help, Gentlemen. I will mainly test the Current drawn by the loco(I or A) by disconnecting the Positive power feed from the controller, touching the Red Probe to the disconnected wire, inserting a temporary wire (say, Black) into the

 track connection, then touching the Black wire with the Black Probe. Then running the loco at various speeds and noting the results.

The multimeter will be set at 0-20 Ohms DC Higher Reading -- or Lower Reading if possible to get a more accurate reading.   I assume that this is the correct method ?  And there is no danger of my ruining the multimeter ?  I haven't yet received the gadget so I can't comment on the instructions included. Meanwhile I have been swotting up on Ohms Law 🤔

[96RAF]

Not Ohms Jimbo - for a stall check you are measuring current which is amps.

 

As stated some cheaper meters can only measure 200mA on DC. This one does 10As on DC and AC.

Note the international symbols for AC (wavy line) and DC (solid line above a dashed line) rather than words.

The Greek letter capital Omega is used to signify Ohms.

 

/media/tinymce_upload/7e67d641610e957849e52fe4ff83670b.jpg

 

[Chrissaf]

The multimeter will be set at 0-20 Ohms

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As Rob says ... NOT OHMS

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If you select Ohms and try to measure live voltage current, then you may damage the meter.

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You need to select DC AMPs not Ohms. Preferably a 2 AMP or 10 AMP range, certainly higher than 1 AMP which should be considered the absolute minimum for loco current testing.

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This video covers the 'How to measure voltage, resistance, current basics' I have found one that has 'subtitles' as I believe you are deaf. Note that the 'subtitles' had to be enabled when I viewed this vid on my Windows 10 PC [Firefox].

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[GWR 14xx Tank]

I have a reply waiting for moderation as it contains an image (@Chrissaf knows, but I thought I might as well inform everyone) so I apologise if, in a few hours / days, a reply from me pops up that was written before the others, so may well contain duplicated information, similar to posts above.

 

14xx Tank

[ColinB]

You can do the stall test, but if you want an easier way this will give you a rough idea of current. Measure the resistance of the motor, easiest to put the multimeter between the two wheels on same axle of your loco. Make sure they they are the ones that pickup the power. Use the multimeter on the ohms range. Assuming your multimeter goes down to about 10 ohms, you should measure a value between 10 and 120 ohms, dependant on the type of motor. So now all you have to do is use ohm's law V= I * R . Where V is voltage, I is current, R is the resistance of the motor. So the I = V / R, so take the maximum voltage say 14 volts divide by the resistance of the motor and it gives the stall current. It might be a bit lower than the actual value, because of the restance of the multimeter leads, but it will be roughly there. Stall currents are not good for DCC decoders, I have found even though you do your calculations quite often the decoder cannot dissipate the heat when the motor stalls and subsequently dies. With modern locos not an issue as they consume less power, more an issue with old ringfield type motors and I assume X04s.

[Jimbo1707820979]

Thanks for all the detail and hard work you have put into your replies. I am now much clearer about things. I have photoed the multimeter  -- which is different from Rob's picture -- and will post it now just as a double-check that I am doing the right things. Cheers, Jimbo

[Jimbo1707820979]

As promised here is my new Multimeter. The  1 and 2 shown are the pointer settings I will try, in that order, to measure the AMPS ! that various locos will draw.

Please correct this duffer if he is wrong (again). 😛/media/tinymce_upload/5607fed426a437584d013d21ce26c1f5.jpg

[GWR 14xx Tank]

Absolutely correct, although I think it is best to use the 10A function, as locomotives often draw more than 200mA of current (1/5 of an amp). Just make sure that your black probe is connected to "COM" and that the red probe is connected to the "10A" slot.

 

Maybe an image is better - just checking!!

 

https://learnassets.getfpv.com/learn/wp-content/uploads/2018/08/22123941/current1.png

 

Let us know how it goes - no violent explosions I hope!!  😛

 

14xx Tank

[96RAF]

If you try a stall test on the 200mA setting you will blow the internal fuse. It probably says that in the instructions.

[Jimbo1707820979]

Unfortunately the "Instructions" are pretty useless, as usual. But thanks for the warning.

[Jimbo1707820979]

Thanks, 14xxTank. Even an electrical ignoramus like me could not misinterpret that excellent picture ! 😆

[GWR 14xx Tank]

Unfortunately the "Instructions" are pretty useless, as usual. But thanks for the warning.

 

Yes, the instructions are pretty vague - your multimeter is a different make than mine, which I also recently bought, but judging by the reply which I have quoted, the instructions were similar to mine. No safety warnings or messages about how to measure volts, current or resistance, or where to plug the probes.

 

I know that someone who knows nothing about electronics shouldn't really be measuring super high voltages, but at least for the people who use them for the first time on 15v AC or 12v DC like here on model railways, you'd have though they would have provided a little bit of info - like how to measure current without shorting the power supply!

 

14xx Tank

[GWR 14xx Tank]

Chris, regarding your post on my post, in the OO gauge outside thread, the image in the clickable link above is not the image I am waiting to be moderated.

 

It would be nice if it did eventually appear on here, but I think @Jimbopuff is well informed about using his multimeter.

 

If he needs more info, then I'm sure that our replies will be more tailored to his specific problem - on my original post, it basically contained duplicated information, as other members beat me to it before the post was moderated. I was just describing it in a different way. 

 

I have tried before to send images on the forum, like this example, and also an image on a new thread, but they haven't shown up. Maybe it is just beginners misfortune.

 

Thanks for keeping an eye out.

 

14xx Tank

[Chrissaf]

Miraculously, your belated image post has just popped up.

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The timestamp shows the original posting date/time, then your edited version where you added the image and the post disappeared. Then finally the approved date/time.

[GWR 14xx Tank]

Thanks Chris, did you have to do some prodding around to get this to apppear?

 

EDIT: Seen your following post, thanks for keeping your eye out for it. Most helpful.

[Chrissaf]

No ... it just happened.