Measuring 20AMP

[Nigel Goodwin]

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

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.

 

[audioguru]

[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.

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.

 

[panic mode]

yup, ACS750 has 0.00013 Ohm

 

[padu]

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

 

[panic mode]

?????

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?

 

[padu]

?????

are you reading datasheet?
allegro is perfect for this. it is isolated and good up to 3kV difference.

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?

 

[padu]

?????

are you reading datasheet?
allegro is perfect for this. it is isolated and good up to 3kV difference.

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

 

[panic mode]

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.

 

[padu]

oh, sorry, i was ,ultitasking and answering in a haste.
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).

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

 

[Styx]

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.

 

[pike]

If these are R/C model motors (as used in Tamiya R/C car models?), then the stall current is likely to be far greater than 20 amps - a LOT depends on your power source, which 'may' be the limiting factor?. NiCd batteries have very low internal resistances, and can provide enough current to destroy themselves - which is what makes them fairly dangerous.

Listen to this guy. 20 amps is too small for a stall current from something like a traxxas emaxx. If its got a mabuchi rs-550 (a common choice in a monster truck of that model) your looking at 70-80 amps!!

here: **broken link removed**

If it has the red dot on the back of the motor, you know what you have...a mabuchi 550.

 

[padu]

Listen to this guy. 20 amps is too small for a stall current from something like a traxxas emaxx. If its got a mabuchi rs-550 (a common choice in a monster truck of that model) your looking at 70-80 amps!!

here: **broken link removed**

If it has the red dot on the back of the motor, you know what you have...a mabuchi 550.

No, they are not mabuchis, the e-maxx is powered by two titans 540 in parallel.

From the e-maxx newsgroup, I got the idea that amp spikes can go as high as 70A. If you're using brushless, it can go as high as 120A.