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'[EE] Source for current-sense resistors?'
2005\07\17@135928 by PicDude

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I need to measure DC current up to 100A (at 12V) at multiple points and
planning to use the ZXCT1010 current monitor and a sense resistor for this.  
A drop of 100mV or even a bit more is acceptable; few percent error/accuracy
preferred.

(a) Calculated ideal is a 0.001-ohm, 10W, 1% sense resistor, or .0005-ohm 5W,
but can't find either at the usual sources (Digikey, Mouser, etc).

(b) I can use 4 sense resistors in parallel -- 0.005-ohm, 3W resistors, but
these add up to ~$6.  Replicating this a few more times adds up fast.

(c) Shunts are and option, but are really expensive ($19 ea. at All
Electronics).

(d) The other option is to use a couple inches of say 10-ga wire, but I'm not
sure how consistent this wire/resistance is.  Google hasn't found me any info
on this accuracy.  I do have the option of calibrating these individually,
but without any specific calibration equipment, I'd like to avoid doing this.  
I'm also not sure if I should be running 100A through a piece if 10-ga wire.

(e) Finally, I thought of using a PCB trace for this, which is an elegant
solution, but some high-level calcs of 1oz or 2oz copper yield some large
trace sizes -- ~2 inches wide and 5+" long and really high temps.

Any ideas on this?  Or perhaps a source for the ideal sense resistor in (a)?

Cheers,
-Neil.


2005\07\17@142653 by Charles Linquist

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Really low-ohm resistors are difficult to do, because even the solder
connections to the part change the resistance. Individual calibration of
each sensor becomes mandatory if you want any decent accuracy.

Have you thought of using the Allegro hall-effect sensors (like the
ACS750SCA-100) instead? I use them all the time with great results.
I use one op-amp to move the normal Vcc/2 offset to near gnd, and
amplify the output by about 1.8.  I then take that output and feed it
into my PICs A/D.  I have a "calibrate" button that takes the A/D
reading at zero current and stores that value in EEPROM. All subsequent
readings subtract out that value.
No pots, no adjustments and 2% accuracy to 100A.

Charles Linquist



PicDude wrote:
{Quote hidden}

2005\07\17@143102 by Dave VanHorn

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At 01:26 PM 7/17/2005, Charles Linquist wrote:
>Really low-ohm resistors are difficult to do, because even the
>solder connections to the part change the resistance. Individual
>calibration of each sensor becomes mandatory if you want any decent accuracy.
>
>Have you thought of using the Allegro hall-effect sensors (like the
>ACS750SCA-100) instead? I use them all the time with great results.
>I use one op-amp to move the normal Vcc/2 offset to near gnd, and
>amplify the output by about 1.8.  I then take that output and feed
>it into my PICs A/D.  I have a "calibrate" button that takes the A/D
>reading at zero current and stores that value in EEPROM. All subsequent
>readings subtract out that value.
>No pots, no adjustments and 2% accuracy to 100A.

I was just going to suggest this :)
No kelvin connections, no messy analog high side sensing, no <1%
precision resistors.

2005\07\17@160506 by PicDude

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Hmmm...  can't believe I forgot about those, especially since I have a few of
the 75A versions here (ACS750LCA-075), but haven't tried them yet.  Very
nifty devices.  The thing about these is that they're ~$6 each, plus the
op-amp is required, which I would think would be best kept near the sensor.  
Then I'd need regulated 5V there as well.  I'm guessing about $9 per station,
without counting the PIC, etc.

However, I'm now re-thinking that I could eliminate the op-amp and use a 50%
voltage divider for Vref-.  And Vref+ would be fed with Vdd directly.  This
would let my full-scale output (for a 100A sensor) be (2.0/2.5) * 1024 = 819
steps, which is pretty good for my requirements.  Also, whereas storing the
0A output level in EEPROM would be easy (I already do this in a
pressure-sensing application), I'd prefer to avoid this if possible.

Still not the few bucks per station I was hoping for, but the simplicity is
very appealing.

BTW, I remember thinking (ages ago when I picked these up), that the holes for
the large terminals would be a pain to drill since slots add a significant
amount to the PCB cost.  Not sure if they'll let me put a few holes right
next to each other, especially since it needs to have the pad around it.  How
do you handle this?

Cheers,
-Neil.



On Sunday 17 July 2005 01:26 pm, Charles Linquist scribbled:
{Quote hidden}

2005\07\17@163159 by Charles Linquist

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PicDude wrote:
> Hmmm...  can't believe I forgot about those, especially since I have a few of
> the 75A versions here (ACS750LCA-075), but haven't tried them yet.  Very
> nifty devices.  The thing about these is that they're ~$6 each, plus the
> op-amp is required, which I would think would be best kept near the sensor.  
> Then I'd need regulated 5V there as well.  I'm guessing about $9 per station,
> without counting the PIC, etc.

I use a $1 quad National Semi Rail-Rail I/O Quad op amp to handle 4
sensors.  They don't really have to be close to the Allegro parts, since
I use an R/C filter ahead of each one to filter out any garbage.

>
> However, I'm now re-thinking that I could eliminate the op-amp and use a 50%
> voltage divider for Vref-.  And Vref+ would be fed with Vdd directly.  This
> would let my full-scale output (for a 100A sensor) be (2.0/2.5) * 1024 = 819
> steps, which is pretty good for my requirements.  

I'm sure this will work.  I couldn't do this in my application since I
had to measure ground-referenced DC voltages with the other channels.

Also, whereas storing the
> 0A output level in EEPROM would be easy (I already do this in a
> pressure-sensing application), I'd prefer to avoid this if possible.

Don't know why this would be something to avoid.  Works great. You could
even make it automatic by checking for a flag in EEPROM. If the flag
wasn't set, calibrate, then set the flag.  This way, first-time power-up
would calibrate the unit automatically.

>
> Still not the few bucks per station I was hoping for, but the simplicity is
> very appealing.
>
> BTW, I remember thinking (ages ago when I picked these up), that the holes for
> the large terminals would be a pain to drill since slots add a significant
> amount to the PCB cost.  Not sure if they'll let me put a few holes right
> next to each other, especially since it needs to have the pad around it.  How
> do you handle this?

We had a couple of hundred boards made, and the incremental cost of the
obround holes was insignificant.

Charles Linquist



2005\07\17@172852 by olin piclist

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PicDude wrote:
> I need to measure DC current up to 100A (at 12V) at multiple points and
> planning to use the ZXCT1010 current monitor and a sense resistor for
> this. A drop of 100mV or even a bit more is acceptable; few percent
> error/accuracy preferred.

At that current there will be a reasonable magnetic field around the wire.
How about a "clip on" type ferrite with embedded hall effect sensor?  This
won't cause any additional voltage drop and therefore extra power
dissipation, and it is automatically DC isolated so you don't have to worry
about what voltage it is floating at.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\07\17@210733 by PicDude

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On Sunday 17 July 2005 03:31 pm, Charles Linquist scribbled:
> I use a $1 quad National Semi Rail-Rail I/O Quad op amp to handle 4
> sensors.  They don't really have to be close to the Allegro parts, since
> I use an R/C filter ahead of each one to filter out any garbage.

In my app, the display unit is already done (a board/enclosure used for
another project), and I want to add this functionality.  Keeping the same PCB
is a *big* plus.  So I'd put the op-amp on the sensor end anyway.  But the
Allegro chip requires 5V power, so I'd need a regulator for that anyway.


> > However, I'm now re-thinking that I could eliminate the op-amp and use a
> > 50% voltage divider for Vref-.  And Vref+ would be fed with Vdd directly.
> >  This would let my full-scale output (for a 100A sensor) be (2.0/2.5) *
> > 1024 = 819 steps, which is pretty good for my requirements.
>
> I'm sure this will work.  I couldn't do this in my application since I
> had to measure ground-referenced DC voltages with the other channels.

Couldn't you also switch A/D modes when sampling different channels?


BTW, would anyone here have an Eagle library already made for this, before I
go create it?

Cheers,
-Neil.


2005\07\17@211019 by PicDude

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On Sunday 17 July 2005 04:28 pm, Olin Lathrop scribbled:
> At that current there will be a reasonable magnetic field around the wire.
> How about a "clip on" type ferrite with embedded hall effect sensor?  This
> won't cause any additional voltage drop and therefore extra power
> dissipation, and it is automatically DC isolated so you don't have to worry
> about what voltage it is floating at.


Not familiar with this component ... any links?  Wouldn't this solution
require individual calibration as well?

Cheers,
-Neil.


2005\07\17@212821 by Charles Linquist

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>
>
> Couldn't you also switch A/D modes when sampling different channels?

You are correct. I could do that, and I don't know why I didn't think of
that.
>
>
> BTW, would anyone here have an Eagle library already made for this, before I
> go create it?
>
I have a PROTEL library, if Eagle can deal with that.


Charles Linquist


2005\07\17@220444 by Jinx

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> > How about a "clip on" type ferrite with embedded hall effect sensor?
>
> Not familiar with this component ... any links?  Wouldn't this solution
> require individual calibration as well?

http://www.siliconchip.com.au/cms/A_30551/article.html

2005\07\18@074310 by Alan B. Pearce

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> > How about a "clip on" type ferrite with embedded hall effect sensor?
>
> Not familiar with this component ... any links?  Wouldn't this solution
> require individual calibration as well?

Is this not what the Allegro units are? LEM also make hall effect
transformers. http://www.lem.com/

2005\07\18@075754 by olin piclist

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PicDude wrote:
> Not familiar with this component ... any links?  Wouldn't this solution
> require individual calibration as well?

I think I'm talking about the same thing others have mentioned too.  I've
never used such a Hall effect sensor, so I'm not that familiar with the
mechanical details or exact model numbers.


*****************************************************************
Embed Inc, embedded system specialists in Littleton Massachusetts
(978) 742-9014, http://www.embedinc.com

2005\07\18@143652 by alan smith

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Isotek or Caddock are sources I have used in the past.



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2005\07\18@143926 by alan smith

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Another passive sensor is made by Senseron....believe its the same guys that do the SHT7x sensors.  I've got some samples around here...never did use them because the design changed.  But looked promising


               
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