That's correct. The wasted 40W would have been used by the motor if the resistor wasn't there. The resistor reduces the voltage to the motor and therefore its current is also lower.

__________________
Uncle $crooge

 

I just pulled 0.1R out of the air to point out that 10R is impossible to use.

I was describing the use of a Diff-amp since it seems the next logical step in this is a current-controller.

If you are just after current-sense then yes a sense resistor will be by far the best


As to the hall effect. I am afraid you might have some problems with it. I have used a 100A version from them and their accuracy is good for 5A.

since you want to sense 20A max and 1A min (ish) you are going to have a sensing problem.

My usual rule is for amperage less then 20A use (correct sized) sense resistor, above correct hall effect

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details

 

Very instructive day, thanks everybody...

I didn't know they even manufactured resistors this low! I've ordered two or three from mouser just to make a few experiments.

I also ordered the 75A rated allegro IC. After I test the motor and realize what is the real power consumption, then I'll be able to better decide how I'll perform current sensing. Thanks again

 

As nigel said you will draw spikes around 100 amps depending on the wind of your motors.
I woulld start with 2 (.1ohm) in parallel. This will give you .05 ohms total.
If your average current draw is 20 amps, then two ten watt resistors should be fine. p = i^2*r = 20 watts. With this setup you will drop
1 volt at the resistor at 20 amps.

 

And you might not notice that your car might go only half as fast as it did without the current sensing resistors. :cry:

__________________
Uncle $crooge

 

Yes, I would suggest a far lower value resistor, and use an opamp to amplify the voltage drop across it!. You might even monitor the voltage drop across the wire feeding the motor in this way?, so you're introducing NO further restrictions at all to it's speed.

__________________
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Good point! :lol:
An MC33171 or MC34071 can measure voltages that are close to ground. A TL071 or TL081 can measure voltages that are close to its positive supply.

__________________
Uncle $crooge

 

yup, ACS750 has 0.00013 Ohm

 

I got my allegro acs750 yesterday... put it on a protoboard, run it and it works perfectly. Very simple.
Because it runs at 5V levels, I can wire its ouput pin directly to my pic's adc pin. One problem I see with that is the allegro outputs 0 amperes as VCC/2, and top amps (in my case 75A) as VCC. Therefore I'm gonna loose half of my ads's resolution, which I *think* is ok.


Now I have a more troubling problem: I want to measure the voltage on that line (the one I'm sensing the current). I know that I cannot do it directly for two reasons: first because the voltage of that line is greater than my circuit's (14 to 16V), and second because I don't want to contaminate my circuit with noise (not to say that such high amp would fry my circuitry I guess).


So, is there another magical IC to measure voltage with some internal programmable gain? (for example, 0-20V on the measured line outputs as 0-5V)

If you read from the start of this post, you will know that I'm electronically challenged, so if you will suggest something much more complicated, please give me a good explanation (or at least some pointers on how to leverage my knowledge)


thanks once more

 

?????

are you reading datasheet? allegro should be perfect for this.
it is isolated up to 3kV.
you can use it to measure current in AC circuit.
did you try measuring negative current?

 

I think I realize how dumb that question was... in my mind, the ACS750 would output current only....

of course it only outputs current, but if I read correctly, you're trying to say that I can multiply that by a certain resistance value to obtain voltage right?

The datasheet gives several types of resistance, which one do I use to calculate voltage?

 

ok, long day for me... why don't I just shut up?

of course it has nothing to do with any one of the resistances stated on allegro's datasheet. it has to do with resistance on the measured line

 

oh, sorry, i was multitasking and answering in a haste (like now :? ).
to measure higher voltage, all you need is voltage divider.
in your case you need 4:1 divider (20V input will be scaled down to 5V).
if you need gain, simple opamp circuit will do and gain can be adjusted by pot if needed.

 

Ok, I need to rest. In the last 10 minutes I had 10 theories of how to measure that voltage and re-invented ohms law in 8 different ways (I doubt any one of them would make sense for you).

I've heard about voltage dividers before, I'll take a look at them tomorrow

 

If you need to increase the resolution of the Allegro, add a couple of turns through the center (it is one with a cable-guide isn't it?).

This will increase the amp-turns seen by the current sensor, thus when you have 10A, it will output a voltage as if it was 20A.

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details