# Help- how to use multimeter to determine current drawn by old locos



## musicwerks (Jan 4, 2012)

Hi,

How do I use a multimeter to determine the current drawn by locos in DC?

I want to dcc my old hornby tender drive steamers.

I had been advised to test out the current drawn by these old locos (stalling current?) so as to determine if they are worth converting to dcc incase the huge loads burn out decoders (bah, hornby decoders! I better stick to digitrax dh123d)

Thanks 

Kiong


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## Don Trinko (Oct 23, 2012)

For current the meter is hooked up in series; You interrupt the track power and use the two meter leads as a jumper to reconnect it. Make sure the meter is set to the proper scale; Amps and DC. Make sure your meter has a scale that will measure several amps.
You may not get a 100% accurate number because most DC controllers provide pulse to get better motor control . If your using a DCC controller your meter will not read correct. Don


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## DonR (Oct 18, 2012)

My Harbor Freight cheapo requires that you move
the red probe to a special amps jack. It is labled 10 ADC
indicating only DC reading. As Don T said you most
likely won't get a decent amp reading on DCC.

CAUTION:

If you do use it to read amps...BE SURE to reset
the selector and probe after that use. Else when
you go to read volts or resistance you'll have a
short circuit. I know this because....

Don


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## waltr (Aug 15, 2011)

Another method is to measure the Voltage drop across a low value resistor then use Ohm's Law to calculate the current draw. Example:
Use a 1 Ohm resistor is series with the power supply and the loco motor.
Clip the multimeter, set to DC Volts across the resistor.
Apply power and measure the Voltage.
The current is then the Voltage divided by the resistance. With a 1 Ohm resistor the calculation is trivial.

If the motor draws very little current that the Voltage drop is too low to measure then use a large resistor value (10 Ohm). 
Be sure to use a resistor with large enough power rating. I like a minimum of a 1 Watt resistor and may use a 5 or 10 Watt resistor. Do not apply power to long since the resistor can get hot especially for a smaller Wattage resistor.


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## musicwerks (Jan 4, 2012)

Hi,

I think I will try the resistor method suggested by waltr. I feel safer measuring voltage than current using the multimeter. Its not expensive, but I don't see the point of risking busting the meter fuse.


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## Hutch (Dec 19, 2012)

fuses are cheaper than high watt resistors and you have the meter. I don't see why you would want to go the the resistor route. Experiment with a power supply or a battery and any resistor to get used to the ammeter. Start with the highest setting until you have an idea of the current for that resistor and then move to a next lowest setting that is above your initial current reading and you'll have an accurate reading and you'll know how to use your meter.


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## musicwerks (Jan 4, 2012)

Okay...I will try that out


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## shaygetz (Sep 23, 2007)

musicwerks said:


> ...I don't see the point of risking busting the meter fuse.


Aye...you can't be a member of "The Brotherhood" until you smoked at least 1 fuse, 1 motor and one DCC chip...


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## musicwerks (Jan 4, 2012)

shaygetz said:


> Aye...you can't be a member of "The Brotherhood" until you smoked at least 1 fuse, 1 motor and one DCC chip...


:smilie_daumenpos: I am 2/3 there on the way. I have smoked 1 Hornby decoder and some old tyco motors. Almost there on the brotherhood, just missing the fuse :smilie_daumenpos:


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## DonR (Oct 18, 2012)

Oh oh...

I may have to turn in my HOG probe. So far, as clutzy as I am,
All my decoders, fuses and other gear are smoke free. 
Well, I did have a flash in the pan when I hooked a 1.5 v 
grain of rice bulb onto 9v...

But now that I've said that...well, I'll keep my fingers crossed. 

Don


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## gunrunnerjohn (Nov 10, 2010)

I've taken out a number of fuses, one PS/2 board when the locomotive threw a traction tire and I didn't stop it before it smoked the driver FET. I also got a TMCC board when a errant wire got loose and shorted the drivers. Smoked the driver triacs and some PCB traces. I was able to rescue that one by replacing all the drivers and jumpering the traces that smoked.

I've only smoked small motors, so I'll have to work on a large one.


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