shunts

[M.J.Lawrence]

Hi all,

I have a multimeter. It will read 2.5-1000v ac/dc and also an amp range up to 10 amps max. I would like to measure amps up to 40 amps. Can i use car bulbs as a shunt if so how. What wattage bulbs and would they be in series or parralell

Thx,
Mike

[hoppy]

no
There are many threads on using a volt-ohm meter for amp measurements - try searching this forum as well as google.com

[Miami Mike]

No, that probably won't work with light bulbs because they change resistance with temperature. You might be able to get it to work with an appropriate length of ordinary wire in parallel with the meter, but you'd have to experiment and compare the results with an accurate reference in order to calibrate it.

[hoppy]

http://www.uoguelph.ca/~antoon/gadgets/shunts/shunts.html

[whanderson]

Hi all,

I have a multimeter. It will read 2.5-1000v ac/dc and also an amp range up to 10 amps max. I would like to measure amps up to 40 amps. Can i use car bulbs as a shunt if so how. What wattage bulbs and would they be in series or parralell

Thx,
Mike

M.J.

Car bulbs can and are used as shunts. However, their resistance changes with temperature. This leads to less than accurate measurements. The best thing to do is connect power resistors in parallel. Place this parallel resistor combimation in series with preferably the negative battery to motor wire. Set your multimetr to read DC volts and measure the voltage accross the power resistors. Use Ohm's Law to convert the measured voltage to current.

For example, 20 1 Ohm 10W power resistors (Radio Shack) in parallel is .05 Ohms. At a battery voltage of 20V and 40A the voltage accross the resistors is 2V (E=I*R, 2V = 40A * .05 Ohms). The power dissipated by the resistors is 80W (P=I*E, 80W = 40A * 2V). The maximum power that the resistors in this case can handle is 200W (20 resistors * 10W).

Good luck.

[Weisse Luft]

I have a $20 Sears multimeter that will measure down to 0.001 Volt so with the Kowalski shunt, I can measure current down to the 1/10th amp. Plenty precise for my work.

[vintage1]

The secrets of Shunts:

(i) The lowest V drop that will allow decent measurements to be made. With analog meters typically 100mv full scale.

(ii) made of something whose resistance is constant with tempereture. Constantan.

(ii) Man eniough for the job. That tends to go with the low Vdrop = 100mv at 10A is 1/10th watt for example. At 50A its 2.5W. So at high currents it need sto be substantial. You DON't want it heating up.

If you can but them at sensible prices rather than make them up, do so.

Its a lot of hassle getting the right materials, and getting the length 'just right'

[AndyOne]

I like to use Welwyn wire wound resistors as shunts, they have low temperature coeffecient and you don't have to experiment. If you need high power just make sure you use a big enough one or put enough in parallel.

Here's a selection...
http://www.welwyn-tt.co.uk/products/sql/res_class_search.asp?application=Current_Sense&technology=Wirewond&package=Axial&Submit=Submit

Andy.

[simingx]

If you can find them cheap, try using shunts made for industrial moving iron meters... They are usually rated like "60mV/50A" or something like that, which means, with a current of 50 amps through it, the voltage drop will be 60mV... These will usually be very accurate.

[Retlaw]

Shunts untill 100A
if you have a voltmeter with a 2 Volt selection f.e. you can use it together with a shunt to messaure untill 100A.
i'll have made a combination with a tachometer & voltmeter by simply switch over from tach to voltage i'l can read both parameters, you need at least 2 hands, one for holding the plane and the other for switching over.
Its simple : 2.5mm² wire with battery connectors on it. The calibration can be done with a battery loader where you can readout the Amperes.
see http://users.telenet.be/aaxfly/retlaw/electric.htm