panic mode said:use known low resistance in series with motors, measure voltage drop and apply Ohms Law.
that's exactly how your meter does it.
you can also ask for samples or buy chips from Allegro.
I got few ACS750 but there are ones that cover lower current range...
padu said:panic mode said:use known low resistance in series with motors, measure voltage drop and apply Ohms Law.
that's exactly how your meter does it.
you can also ask for samples or buy chips from Allegro.
I got few ACS750 but there are ones that cover lower current range...
Humnn, I'm not sure how to measure it though. I know Ohms law, but how to apply it in this case? (I told you I were a newbie)
Let's say I connect a 10ohms resistor in series with the motors (I know that somewhere I'll need to calculate power ratings in order to choose the resistor, but seems pretty obvious a 1/4W won't do it).
Then I measure current between going through the resistor on a certain scenario (let's say motor has no load, minimum speed) and current is 1AMP. Am I correct in assuming that at this point that 1AMP is going through the motor too? What if the resistor wasn't there, what would be the current going through the motor?
Now scenario 2: same resistor but now motor has a big load on its output shaft and I'm driving it to full speed. How do I calculate the current flow now?
Thanks and sorry if my questions are too basic.
Then I measure current between going through the resistor on a certain scenario (let's say motor has no load, minimum speed) and current is 1AMP. Am I correct in assuming that at this point that 1AMP is going through the motor too? What if the resistor wasn't there, what would be the current going through the motor?
Resistor Motor Total
E 'X' ? 14
I ? ? ?
R 10 ? ?
Resistor Motor Total
E 4 10 14
I 0.4 ? ?
R 10 ? ?
Resistor Motor Total
E 4 10 14
I 0.4 0.4 0.4
R 10 ? ?
you would NOT/CANNOT measure 20A with a 10R resistor
why?
1) the power dissipated would be 20*20*10 4kW !!!!!!!
2) the voltage developed would be 20*10 = 200V !!!!!
you would use a resistor with an ohmage of about 0.1R or less
.You would put the resistor in the return path of the machine windings and use a differential amplifier configured OPAMP to measure the voltage across that resistor. That voltage is proportional to the current flowing through it (the constant of proportionality is the resistor value).
simple as that.
..snip..
I think you need to take a step back and go over the basics. It is one thing jumping into PIC electroncis with next to no infomation, but when dealing with power the basics need to be such you dont even think abt them any more
Mr Kirchoff and MR ohm should be 2nd nature to you
stevez said:I quickly scanned the responses and didn't see "shunt" - which is more or less a low value resistor that can be used to extend the range of a meter. Most of the current flows thru the shunt - some thru the meter. I've made them from copper wire. What you need is wire big enough that it won't get red hot and melt under the current draw but yet be small enough so that a modest (a foot or two) length provides the value that you need.
Ron H said:Yeah, but he really shouldn't need a diff amp for DC measurements. All 2 wire, ungrounded voltmeters are differential.
For 20 amps and 14V, 0.1R is probably too high. 0.01R would cause much less degradation in current, torque, and RPM.
Optikon said:I agree, the OP said he was using a DMM, why the extra amp stages? Even 2 wire grounded voltmeters are differential. The fact that it is DC doesnt remove the requirement to reject the common mode voltage, so isnt a diffamp needed?
The dmm will average and the shape of the current waveform may cause inaccuracies (I'm not so sure its a clean DC amps) Also would it be better to pick a sense R such that the full scale current reading will match the full scale DMM reading? If the DMM has a 2V, 200mV scale Rsense = .1 or .01 respectively is good. If it has 1V, 100mV range then maybe R = .005 on the 100mV range is best?
Optikon said:Ron H said:Yeah, but he really shouldn't need a diff amp for DC measurements. All 2 wire, ungrounded voltmeters are differential.
For 20 amps and 14V, 0.1R is probably too high. 0.01R would cause much less degradation in current, torque, and RPM.
I agree, the OP said he was using a DMM, why the extra amp stages? Even 2 wire grounded voltmeters are differential. The fact that it is DC doesnt remove the requirement to reject the common mode voltage, so isnt a diffamp needed?
The dmm will average and the shape of the current waveform may cause inaccuracies (I'm not so sure its a clean DC amps) Also would it be better to pick a sense R such that the full scale current reading will match the full scale DMM reading? If the DMM has a 2V, 200mV scale Rsense = .1 or .01 respectively is good. If it has 1V, 100mV range then maybe R = .005 on the 100mV range is best?
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.padu said:I figured that .1 is still too high, because power dissipation at 20Amps will still be 20A*2V = 40W correct?
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