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PICList Thread
'Current measurement [OT]'
2000\04\19@062604 by Russell Farnhill

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Hi all,

I want to measure current used by a load using an ADC.

The load is a R/C Servo (Hitech 605BB). The ADC already
exists on a micro controller which takes a voltage of
0-5V giving me a reading of 0-255. My idea was to put
a 1 Ohm resistor in series with the servo power supply
and try to measure the volt drop across it. I guess
that the volt drop would be quite small and would need
an op-amp of some sort to make the voltage change big
enough so the ADC could measure it. This is where I need
help as I don't know how to configure an op-amp in such
a way. I've tried bashing out a circuit in a demo version
of electronics workbench but without much luck.
Can anyone help in the way of a circuit diagram or
something. The op-amp would preferably only need one
positive supply rail and not a +/- supply if possible.


Any help would be greatly appreciated.

Thanks,

Russ.

2000\04\19@071023 by Thomas C. Sefranek

face picon face
Russell Farnhill wrote:

> Hi all,
>
> I want to measure current used by a load using an ADC.
>
> The load is a R/C Servo (Hitech 605BB). The ADC already
> exists on a micro controller which takes a voltage of
> 0-5V giving me a reading of 0-255. My idea was to put
> a 1 Ohm resistor in series with the servo power supply
> and try to measure the volt drop across it. I guess
> that the volt drop would be quite small and would need
> an op-amp of some sort to make the voltage change big
> enough so the ADC could measure it.

The voltage is in DIRECT proportion to the current.
1 amp is 1 volt, 1 milliamp is 1 millivolt.
I notice you have not said WHAT current level you are trying to read.

> This is where I need
> help as I don't know how to configure an op-amp in such
> a way. I've tried bashing out a circuit in a demo version
> of electronics workbench but without much luck.
> Can anyone help in the way of a circuit diagram or
> something. The op-amp would preferably only need one
> positive supply rail and not a +/- supply if possible.

If as you propose you put the current shunt in series with the supply,
you have the supply voltage as a common characteristic.
Perhaps you could put it in the return lead, and share ground as the
common?

The Op-Amps circuit is easy enough, but until you say what level of
current
you are expecting to read the values for the gain programming resistors
can not
be specified.

A single supply Op-Amp will probably have to be CMOS technology as
others do NOT do rail to rail voltages.  (They don't do 0 volts well!)

> Any help would be greatly appreciated.
>
> Thanks,
>
> Russ.

--
 *
 |  __O    Thomas C. Sefranek   spam_OUTtcsTakeThisOuTspamcmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

2000\04\19@072513 by Andrew Kunz

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face
Servos usually idle at pretty low currents, but when the motor is running and
there's a good load, the suck the electrons pretty bad.

1 ohm is going to make a lot of difference to the servo.

If you check the recent archives, look up the subject "resistance" which I
started for a similar thing.  THere's even a GREAT PDF on the links you'll find.

Andy









Russell Farnhill <.....r.farnhillKILLspamspam@spam@STOR-WAVE.CO.UK> on 04/19/2000 06:22:03 AM

Please respond to pic microcontroller discussion list <PICLISTspamKILLspamMITVMA.MIT.EDU>








To:      .....PICLISTKILLspamspam.....MITVMA.MIT.EDU

cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Current measurement [OT]








Hi all,

I want to measure current used by a load using an ADC.

The load is a R/C Servo (Hitech 605BB). The ADC already
exists on a micro controller which takes a voltage of
0-5V giving me a reading of 0-255. My idea was to put
a 1 Ohm resistor in series with the servo power supply
and try to measure the volt drop across it. I guess
that the volt drop would be quite small and would need
an op-amp of some sort to make the voltage change big
enough so the ADC could measure it. This is where I need
help as I don't know how to configure an op-amp in such
a way. I've tried bashing out a circuit in a demo version
of electronics workbench but without much luck.
Can anyone help in the way of a circuit diagram or
something. The op-amp would preferably only need one
positive supply rail and not a +/- supply if possible.


Any help would be greatly appreciated.

Thanks,

Russ.

2000\04\19@105215 by M. Adam Davis

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face
Another option you may have is that on some PICs the A/D converter has a Vref.
Once you find the expected voltage level (say, 0-2.7v) you can input 2.7v into
Vref+ and 2.7v will read full scale on the A/D.  This will also eliminate the
noise an opamp would introduce.

Right now I am assuming you are using a 7805 or similar regulator for your
circuit, which means your supply voltage (and therefore Vref right now) can vary
up to 1%(or more).  This is reducing your accuracy by about one bit.  If that is
acceptable, you can get 1% resisters and simply make a resister ladder for
Vref.  You could then read the Vref with a meter and use that value for your
calculations to keep close to 1%.

I hope this helps.

-Adam

Russell Farnhill wrote:
{Quote hidden}

2000\04\19@124524 by Thomas C. Sefranek

face picon face
"M. Adam Davis" wrote:

> Another option you may have is that on some PICs the A/D converter has a Vref.
> Once you find the expected voltage level (say, 0-2.7v) you can input 2.7v into
> Vref+ and 2.7v will read full scale on the A/D.  This will also eliminate the
> noise an opamp would introduce.

How low (Voltage) can you set Vref. to?

>
> -Adam

--
 *
 |  __O    Thomas C. Sefranek  EraseMEtcsspam_OUTspamTakeThisOuTcmcorp.com
 |_-\<,_   Amateur Radio Operator: WA1RHP
 (*)/ (*)  Bicycle mobile on 145.41, 448.625 MHz

ARRL Instructor, Technical Specialist, VE Contact.
hamradio.cmcorp.com/inventory/Inventory.html
http://www.harvardrepeater.org

2000\04\20@044821 by Russell Farnhill

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face
Hi,

Thanks for all the advice guys, I didn't
realise 1 ohm would be too high. I followed
the "Resistance" thread and got the link
to the PDF you talked about but the link
is broken for that file :
http://redrival.com/mcgahee/100_amp.pdf

could anyone mail this to me ?

Thanks again,

Russ.

2000\04\20@060306 by Arthur

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This a job for those zero ohm link i Think <grin>
Art
----- Original Message -----
From: Russell Farnhill <r.farnhillspamspam_OUTSTOR-WAVE.CO.UK>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, April 20, 2000 9:47 AM
Subject: Re: Current measurement [OT]


{Quote hidden}

2000\04\20@081756 by Andrew Kunz

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Just send a note to the site's owner.  He's a really nice guy and you'll be glad
you did.

Andy








Russell Farnhill <KILLspamr.farnhillKILLspamspamSTOR-WAVE.CO.UK> on 04/20/2000 04:47:35 AM

Please respond to pic microcontroller discussion list <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>








To:      spamBeGonePICLISTspamBeGonespamMITVMA.MIT.EDU

cc:      (bcc: Andrew Kunz/TDI_NOTES)



Subject: Re: Current measurement [OT]








Hi,

Thanks for all the advice guys, I didn't
realise 1 ohm would be too high. I followed
the "Resistance" thread and got the link
to the PDF you talked about but the link
is broken for that file :
http://redrival.com/mcgahee/100_amp.pdf

could anyone mail this to me ?

Thanks again,

Russ.

2000\04\20@174001 by Thomas McGahee

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face
Unfortunately redrival.com is down at the moment.
Try this secondary website of mine:

http://mcgahee.freeservers.com

Fr. Tom McGahee

{Original Message removed}

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