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### Topic: Measuring h-bridge current? (Read 1 time)previous topic - next topic

#### Kariky

##### Jun 13, 2011, 02:24 pm
Hi,

I have two DC-motors driven by single h-bridge L293 as described in http://itp.nyu.edu/physcomp/Labs/DCMotorControl.
H-bridge is driven by Arduino Uno. My problem is how can i measure the current dc-motor is takeing from L293.
Stall current of motors are about 0,5 A and i have some 0,5ohm resistors to use but how can i connect them to
be able to measure the voltage drop ? Or do i have to use one L293 for each motor and connect all ground pins through resistor to ground  ?

Are all ground pins 4,5,12 and 13 seems to be internally connected together ?
My software has no bugs, only random features.

#### Grumpy_Mike

#1
##### Jun 13, 2011, 06:17 pm
Problem with that is the sensitivity it is only 66 to 185 mV/A
Your stall current is 0.5A so the running current is likely to be closer to 100mA.
That only gives you a range of 6.6 to 18.5 mV.
With only 4.8mV per step on the A/D you are not going to get a lot of precision out of it.

#### Kariky

#2
##### Jun 15, 2011, 02:19 pm

So my possibilities seems to be to use some separate measuring circuits or two separate L293's
with one for both motor and little bigger resistors like 1 ohm ?

I re-measured the stall current of the motor more exactly but it's difficult because axle seems still be slipping in wheels... XD
Current is very near 1A and current on "normal drive" <150 mA so with 12VDC and 1 ohm resistor by normal
drive voltage drops 1 ohm * 0.15A =0.15V , and when stalled 1 ohm * 1A = 1.0V if i did understand this right way ?
But that makes one watt so can my 1/4 watt resistor handle it for a very short period ? (only those i have now)

What about the VCC voltage of the L293?  If the "earth" is +1V there will still be 4 Volts difference in circuit inputs
so i think i don't have to worry about that ?
My software has no bugs, only random features.

#### Grumpy_Mike

#3
##### Jun 15, 2011, 07:08 pm
Quote
But that makes one watt so can my 1/4 watt resistor handle it for a very short period

Yes the rating of your resistor is 1/4 watt continuous, the peak rating will be much higher, you should have no trouble with the power rating.

Quote
What about the VCC voltage of the L293?  If the "earth" is +1V there will still be 4 Volts difference in circuit inputs
so i think i don't have to worry about that ?

That is slightly more worrying, not because of the lack of Vcc volts but the fact that you will be applying signals to the chip that are lower than the supply voltage to the chip. It is only the stall condition you have to worry about as a chip can take 0.6V lower than its ground. You might be wise to put a clamping diode to ground (just one), see this link:-
http://www.thebox.myzen.co.uk/Tutorial/Protection.html

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