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I need measuring the current through a dc motor with an analog multimeter, but I don't know how to select the correct function to that.

The meter is a FT-360TR. I know it can measure currents of miliamps and it also has a function to measure currents <= 10A, but I don't know how to select this function.

FT-360TR http://www.aeletricaararas.com.br/loja/media/catalog/product/cache/1/image/5e06319eda06f020e43594a9c230972d/f/i/file_11_3.jpg

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    \$\begingroup\$ Check your manual! The high-current setting often uses a dedicated bus, maybe the top-left one, I can't read the legend. \$\endgroup\$
    – Wouter van Ooijen
    Mar 27, 2015 at 17:25
  • \$\begingroup\$ I don't have the manual. \$\endgroup\$
    – user49894
    Mar 27, 2015 at 17:27
  • \$\begingroup\$ Really? What's that in the right bottom corner of your picture under the probes then? Even if you don't have it, go download it from someplace. There is no excuse. Besides, this doesn't look any different to use than any other meter. \$\endgroup\$
    – Olin Lathrop
    Mar 27, 2015 at 17:29
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    \$\begingroup\$ You can always put a small resistor in series with the motor and measure the voltage across it. \$\endgroup\$
    – George Herold
    Mar 27, 2015 at 17:34
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    \$\begingroup\$ From the circuit diagram (esily found on the web) I don't think this meter has a 10A range. What you can select with the big knob is what you get. \$\endgroup\$
    – Wouter van Ooijen
    Mar 27, 2015 at 17:43

2 Answers 2

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Here's a link to a manual

Unfortunately it is in a foreign language (Portuguese based on the website suffix of .br?) but there are a lot of tables with English units so it might be some help.

And here is a schematic:

enter image description here

from this link.

The extra terminal in the upper left corner appears to be some sort of AC output.

The current measurement does appear to be limited to 250 mA.

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  • \$\begingroup\$ Thanks, tcrosley. I read the manual and it tells me the current measurement is limited to 250 mA. \$\endgroup\$
    – user49894
    Mar 27, 2015 at 22:06
  • \$\begingroup\$ Capacitor C1 was often found on old analog multimeters because the AC ranges are, in fact, DC coupled. Trying to measure AC voltage on the plate of a tube-type amplifier would be impossible. Hence C1 was added. It is often labeled "Output" - I don't recall exactly why but that's the way it was back then. I do remember making use of that input terminal for exactly that reason: measuring the AC voltage on the plates of vacuum tubes. \$\endgroup\$
    – Dwayne Reid
    Mar 27, 2015 at 23:53
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The website that photo came from says that the Smart FT-360TR has a 10A range, but it is wrong.

To measure current over 250mA you need a shunt, which is a low value resistor connected in parallel with the meter. The shunt passes most of the current, while the meter measures the voltage across it. If you choose the right shunt value then one of the meter scales should match the current you are measuring.

Most shunts are designed to give a full scale reading at 50mV. The lowest voltage range on your meter is 100mV, so a '5A, 50mV' shunt would give 5A at half scale and 10A full scale.

Precision shunts are expensive. If you want to save money and aren't too concerned about accuracy then you could just use a resistor of the correct value. The required value can be calculated using Ohm's Law. For 10A you need 0.1V/10A = 0.01 Ohms. At 10A it will dissipate 0.1V x 10A = 1W, so to keep the resistor cool it should be rated for 2W or higher (I would use a 5W wire-wound resistor).

To avoid excessive voltage drop you should wire the resistor directly in line with the motor, then have separate 'sense' wires going from the resistor to the meter. These wires only have to carry 50uA, so they can be quite thin.

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