How does an amp meter work?

(Almost) all digital multimeters, on a 10A DC (or AC) range, will pass the current through a low-value shunt, with very low additional resistance in the connections inside the meter. Most will drop

0.01V/amp (0.01 ohms), plus some extra for the internal connections, jacks and plugs, and leads, and possibly a fuse, for this sort of range. Note that the meter may apparently be capable of reading up to 20 amps, but may be limited lower than that, because if you drop 0.2V at 20 amps, you end up with 4 watts power dissipation, which is quite a lot for a precision shunt in a small package.

If that's the case for your meter, you might try putting about 0.02 ohms of resistance in the circuit, or more if your meter leads are long. A foot of 20AWG copper wire will get you 0.01 ohms, and if the wire is in open air, cooling it should be no problem, at least for a trial. Another thing to try, if you have a second meter, is measure the actual voltage drop where the meter leads are connected into your circuit. That, along with the current, will tell you the net meter+leads+banana jacks&plugs+fuse resistance, and it just may be significantly more than 0.01 ohms. The meter and its leads will add a bit to the inductance, but I'd expect that to be low compared with your

100uH.

Cheers, Tom

Why not just use a small resistor in series with circuit and measure the voltage drop with a voltmeter, or scope ? Then convert to current with calculator. If the resistor makes the circuit work, you can leave it installed.

-Bill

On 23 Mar 2006 13:18:34 -0800, "Andrew" Gave us:

A high precision "shunt resistor" with a known value is across the "ammeter" terminals provided by the meter (internally). The voltage is measured across this precision resistor, and that voltage is expressed on the display as a directly converted amperage reading. Any resistor passing current through it has a voltage expressed across it that is directly proportional to the current in the element. That voltage can be used by the meter's internal electronics to create an exact current reading.

The resistor is kept VERY low in value so that it appears as not to affect the circuit under test. At least not enough to make a "significant" difference.

Typically 0.1Ohm, 0.01 Ohm, or 0.001 Ohm. This allows the voltage to be directly converted to a current reading by simply moving the decimal point over from the read voltage numbers to "obtain" the current. It works perfectly.The only things that affect it are typically temperature, and moisture causing a change in the base value of the shunt resistor OR affecting the volt meters accuracy in reading the voltage impressed across the resistor.

On 23 Mar 2006 13:54:43 -0800, snipped-for-privacy@att.net Gave us:

"Small resistor". You likely meant LOW value resistor.

Just for my education: What sort of speed controller? PWM type (like in RC model stuff)? If so - why not ramp up the rpm control more slowly to avoid rush in peaks? The inductive element of the motor armature limits the current if the PWM freq is sufficiently high. Incidentally, there *are* RC motor controllers that have exactly this current limiting function. (9A is considered "whimpy" in RC circles :-)

If it is a "bang on" controller (not actually a controller IMHO), why not just place a series resistor (car lamp!), that is shorted after a short time period. That is how ancient electric train traction systems did it.

--
 - René

I would love to redesign to ramp up, but unforunately that isn't possible.

I'll have to try some different wire combos, but just as I suspected, a wire should be the same thing as the fluke.

Argh!!

On 24 Mar 2006 05:40:26 -0800, "Andrew" Gave us:

Not at all. A wire will change as it heats up, and it WILL heat up. It will have a huge value swing. It will throw off your readings. At the very least, buy a precision high stability resistor from mouser or digi-key. Then a voltmeter can provide more exact readings.

Depending on the precision of the current reading you need (range), you need a 0.1 ohm or 0.01 ohm or 0.001 ohm resistor to use the voltmeter with no conversion required (just a decimal move) for amperage readout. It depends on whether you wish to measure microamps, milliamps, or 100s of milliamps or amps or tens of amps. The current you expect to pass will determine both the resistor value you need as well as the wattage rating it needs to be at to remain accurate through the range of utilization.

There are other odd values you can get, but then you have to calculate each time you take a reading, OR construct a spreadsheet printout that provides a table of meter readings cross referenced to actual current flow in the circuit.

A simple wire will not work. First off, you would have to have a precision instrument just to determine the resistance value of the wire. Not cheap. Then, it will drift badly with temperature.

I can understand what you are saying about the change in resistance vs constant resistance, but what I'm confused about is why my circuit works with the meter, and does not when it is replaced with a wire? I wouldn't expect a (relatively) small change in resistance to make the difference between working and not working. How is the meter working with the DC choke to "work" vs. "not work" ? Well, at least, what are some possibilities? I'm completely stumped here, I'm trying to think about this logically, but I suspect I don't know enough about inductors. From what I would have figured, a fluctuation of a few tenths of an ohm (MAX) shouldn't affect it like this.

It is very surprising. I can understand a circuit being right on the edge, and a slight change having a disproportionate effect, but it is surprising that this small change makes a repeatable and stable difference.

Even when the circuit works (with the ammeter) it can't be far from malfunction. But I would like to look at the internal signals with a scope to see what is changing to understand why the circuit is so touchy. That is, if connecting the scope doesn't change the operation again.