High-side current measurement

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tblough
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Posted by tblough: Wed. Nov 5, 2008 - 12:11 PM
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I have a 10R0 resistor connected to +9V from a battery on one side and the other side is connected to a load that draws 0-100ma.

Since the voltage drop across this resistor would be 0-1V, I originally thought it would be no problem to measure it using the differential ADC on my Tiny84. Now I realize even though the differential is only 1V, I still need to drop the actual voltages below the 5V my Tiny is running at.

So, it would appear that I need a resistor divider network to drop the 9V on the high side of the sense resistor as well as a resistor divider on the low side 9V-8V.

Do these two divider networks need to be matched? Or is there another way to get this 1V differential into the ADC without the two divider networks?

Cheers,

Tom

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Learning and Information, General Electronics
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bobgardner
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Posted by bobgardner: Wed. Nov 5, 2008 - 12:38 PM
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Every chip maker has hi side current monitors. Why build when you can buy?

Imagecraft compiler user

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valusoft
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Posted by valusoft: Wed. Nov 5, 2008 - 12:51 PM
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Tom,

I agree with Bob that there are specific devices available that handle high side current measurements, eg TI's INA19X. But you could also place your 10 ohm resistor on the low side and use the A/D providing you can tolerate the floating ground on the load.

Cheers,

Ross McKenzie ValuSoft Melbourne Australia

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oddbudman
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Posted by oddbudman: Wed. Nov 5, 2008 - 12:53 PM
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Have a look at:

AVR450: Battery Charger for SLA, NiCd, NiMH
and Li-Ion Batteries

Page 39.

oddbudman

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DocJC
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Posted by DocJC: Wed. Nov 5, 2008 - 02:25 PM
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If you use a fixed resistive divider on one side you could use a fixed resistive divider on the other side if you use very high precision parts, or want to do a software calibration.

If one leg includes a pot, then you can adjust the offset from the fixed leg. The down side is you need to calibrate it manually.

JC

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tblough
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Posted by tblough: Thu. Nov 6, 2008 - 01:43 AM
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Ross & Bob,

TI's chip looks great, but with the lowest gain at 20X, I would still need a voltage divider to handle the output of the chip since my input voltage range is 0-1V.

Oddbudman & JC,

Using resistor dividers on both legs is what I though I would need to do - thanks for the confirmation. JC - I'll probably do the calibration in software. I had hoped that would be an alternative to matching the divider pairs.

Thanks, all. I'll try to breadboard it this weekend and let you know how it works out.

Cheers,

Tom

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JoystickTX
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Posted by JoystickTX: Sun. Jan 25, 2015 - 11:24 PM
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Tom,

 

I know it has been a while since your post, but did you ever solve your problem?

 

I'm trying to do a similar thing, but I want to measure current in both directions. I thought about putting my series resistor in the battery negative wire and connecting the ADC analog gnd and the IC gnd to one side of that resistor. I should get a positive and negative voltage drop without the ADC seeing the high positive battery voltage on the inputs.

 

Thanks

Steve

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Kartman
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Posted by Kartman: Sun. Jan 25, 2015 - 11:40 PM
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Something like a ACS712 might suit. Being hall effect, you can run the current carrying wire lengthwise across the chip and measure the current. No need to cut the wire. It will also measure both ways and interface easily with a microcontroller.

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JoystickTX
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Posted by JoystickTX: Mon. Jan 26, 2015 - 12:51 AM (Reply to #8)
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Kartman,

 

That chip looks great, the 5A version would work fine for what I'm working on. 

 

I may need to use an external IC, but I have cost, space, and current consumption issues to deal with.  I can put Tiny to sleep; can't do that with the other IC's and that drain adds up.  

 

I was hoping there was a simple solution, like the series resistor in the return line, or maybe putting the ADC ground on the VCC line to keep the Differential inputs from being at the VCC level.

 

Thanks,

Steve

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Kartman
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Posted by Kartman: Mon. Jan 26, 2015 - 01:58 AM
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Zetex, ti and maxim have specific devices to address your problem - not sure about power consumption though.
Putting agnd at anything different to the voltage on gnd will cause smoke on the AVR.

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