Simple current measurement with cheap multimeter

[StaalBurger]

Hi All,

A quick and simple way to measure current fairly accurate is as follows:
Get hold of some sort of high wattage resistor (15W or more), preferably a wire wound type,
a value of 1 Ohm or less would do the trick, bigger values isn`t good.
Put the resistor in series with the + line of the battery, measure the voltage drop accross the resistor. It will be a direct indication of the current being draw.
eg.

Ohms LAW (V=IR)
A voltage drop of 1v over 1 Ohm = 1A Beware 1Watt dissipated by resistor
10V 0ver 1Ohm = 10A etc. (10Watt)

Higher current can be measure quickly, don`t run the motor for more than a couple of seconds, the wire wound resistor is fairly forgiving for very high peak currents.

Hope this helps someone out there.

Ben

[Andy W]

1 Ohm is VERY high for our purposes.
A .1 Ohm or less shunt is better - discussed here many times.
..a

[zagisrule!]

For power dissipation:

W = IxIxR

Therefore at 10A, you are dissipating 100W into the resistor, not 10W.

Like mentioned earlier it is best to use a .1 or less, .01 would be even better so long as the meter as sufficient resolution.



-Matt

[FredericG]

I have recently done that with a very thin wire. I calibrated it with a 2A current source. About 3 cm gave 20mV voltage drop. However, it did not seem very linear: 5A produced around 60 mV.

Frederic

[Andy W]

.. because your wire was thin, had too much resistance, and warmed up. As it warms up, resistance increases..
..a

[adamdb]

So is there some common, everyday material you could use as a shunt? I noticed when building a multi-meter from a kit that the shunt was just a piece of wire about 2" long and 3/32" or so in diameter. Guess I will start testing various bits around the house.

Adam

[vintage1]

Try a fattish bit of piano wire.

But ideally you want 'constantan' which has a low temperature coefficient.

Fpr e.g a 10A shunty on a 50mV type movement you only need 5mOhm. That's a short fat bit of wire that is!

[Andy W]

A length of 10 or 12ga household wire will work - go to your local home improvement store, see if they have offcuts, or ask if you can get just a few inches. Ground wire will work..
..a

[FredericG]

because your wire was thin, had too much resistance As it warms up, resistance increases

Yes, probably. This is why I took 3 wires and take +/- 3cm instead of +/- 1cm but it did not improve things a lot...

Frederic

[Andy W]

1cm of 1mm² wire has the same resistance as three, 3cm lengths of 1mm² wire!
..a

[FredericG]

1cm of 1mm² wire has the same resistance as three, 3cm lengths of 1mm² wire!
Exactly, so the same power dissipation, BUT over 3 cm instead of 1 cm, so less temperature increase... Am I mistaken?

I was already at 0.01 Ohm, lowering it further for measuring 1->5 A would be too low, I thought.

Frederic

[Leftyretro]

Hi guys;

At work we use commercial current shunts for measuring larger currents over ten amps, while using our Fluke DVMs for currents up to 10 amps. The standard shunts we use are .001 ohms (really!) and seem to use a special metal to deal with the heating. One standard shunt I have is a 50 amp job that reads 1 amp = 1 millvolt, 50amps = 50 millivolts. One could probably find such shunts on e-bay such as this:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=4678&item=3856672174&rd=1&ssPageName=WDVW

A homebrewed shunt would probably work just fine. While not as accurate as a commerical shunt it would at least give relative readings as one makes test changes to their motor/prop setups...

Lefty

[vintage1]

Yes.

Accurate shunts need to take heating into account, and be made of special 'zero temp coefficient' metals alloys. Not perfect, but better than most..

But if y don';t need it that accurate anything will do. Even a thick lump of copper busbar or whatever. Your local electrical stores will net you large lumps of thick wire, busbars and so on.

With a busbar, you can fne tune the meter reading by picking off thr meter signal at different places on the bar, like a crude potentiometer.

[StaalBurger]

Matt

You are right in the calculation, I used the wrong formula,
I calculated the voltage drop and not watts as such (P=IxV) where V equates to (V=IxR)
Thus 10v drop over the 1 Ohm resistor
and then 10A x 10v = 100W !

My mistake,

Ben

[hwhall]

One approach to this that I've seen in commercial gear: Make the shunt out of metal strap. It has high area to mass ratio for better cooling when high current is passed through it (helps to keep the resistance constant). You can also fine-tune the resistance by filing one or more notches into the cross-section (easier sometimes than adjusting the length by tiny amounts). --Wayne