Electronics Design 50A DC Current Sensing

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Subject Author Date
50A DC Current Sensing Brian 02-12-06
Posted by Brian on February 12, 2006, 9:21 am
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I have both a 50A DC hall effect IC with with the same specifications
as Allegro's ACS750LCA-050 ( http://www.allegromicro.com/sf/0750/ ) and
an old analog simpson guage with a 0-50A plate which I presume to be a
50mV meter. The output on the IC is linear enough for my purposes
outputting 2.5V @ 0A and 4.5V @ 50A with a Vcc of 5V. Is it even
possible to mate both of these? If so, what would be the easiest
way/design? If any more information is needed, feel free to ask.

Thanks,
Brian


Posted by Winfield Hill on February 12, 2006, 10:56 am
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Brian wrote...
>
> I have both a 50A DC hall effect IC with with the same specifications
> as Allegro's ACS750LCA-050 http://www.allegromicro.com/sf/0750/ and
> an old analog simpson guage with a 0-50A plate which I presume to be
> a 50mV meter. The output on the IC is linear enough for my purposes
> outputting 2.5V @ 0A and 4.5V @ 50A with a Vcc of 5V. Is it even
> possible to mate both of these? If so, what would be the easiest
> way/design? If any more information is needed, feel free to ask.

Brian, this is an easy task, and I imagine you'll get plenty of
people describing various ways to do it. For example, the Simpson
meter could be placed between the 750LCA-050 IC's output and a
resistive divider connected between 5V and ground, plus a trimpot
to remove the 5% zero error. We need to know the meter's internal
resistance, which you can measure with a multimeter. And just in
case it was really meant to work with 100mV shunts, etc., it would
be helpful to know its full-scale current (or voltage), which you
can measure with a multimeter, 1k resistor and a bench supply.


--
Thanks,
- Win

Posted by Winfield Hill on February 13, 2006, 8:25 am
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Winfield Hill wrote...
>
> Brian wrote...
>>
>> I have both a 50A DC hall effect IC with with the same specifications
>> as Allegro's ACS750LCA-050 http://www.allegromicro.com/sf/0750/ and
>> an old analog simpson guage with a 0-50A plate which I presume to be
>> a 50mV meter. The output on the IC is linear enough for my purposes
>> outputting 2.5V @ 0A and 4.5V @ 50A with a Vcc of 5V. Is it even
>> possible to mate both of these? If so, what would be the easiest
>> way/design? If any more information is needed, feel free to ask.
>
> Brian, this is an easy task, and I imagine you'll get plenty of
> people describing various ways to do it. For example, the Simpson
> meter could be placed between the 750LCA-050 IC's output and a
> resistive divider connected between 5V and ground, plus a trimpot
> to remove the 5% zero error. We need to know the meter's internal
> resistance, which you can measure with a multimeter. And just in
> case it was really meant to work with 100mV shunts, etc., it would
> be helpful to know its full-scale current (or voltage), which you
> can measure with a multimeter, 1k resistor and a bench supply.

OK, now you know the meter is a 50mV type meant for use with shunts,
as you thought, with 1mA full scale and a 50-ohm resistance. You've
calculated that you need 1950 ohms of additional series resistance,
which is correct, except you'll ant the +/-15% as Fred suggested.

Start with two 1k resistors from Vcc to gnd, to establish the Vcc/2
midpoint with a 500-ohm Thevenin output resistance. Add a 1k pot
from Vcc to gnd, and take its wiper through 47k to the Vcc/2 node,
to provide a +/-26mV zero-trim range from the pot's +/-2.5V swing.
Your meter goes from the Vcc/2 node to the 750's output through the
1950 ohms you calculated, less 495 ohms for the 1k dividers and zero
adj ckt, and less about 300 ohms to provide for a 1k Fred's gain pot.
You can use a 1k series resistor with the 1k gain pot, that's close
enough. Have fun calibrating your 50A Hall-effect electronic meter.

. ________________________________________________________ Vcc
. | Allegro | |
. __|__sensor ,------, 1.0k |
. | | _ V | | 1k
. | |---|_|--- 1k --- 1k pot ---+-----+--- 47k ---> ZERO
. |_____| meter GAIN | pot
. | 1mA 1.0k |
. ____|____________________________________|______________| gnd


--
Thanks,
- Win

Posted by Jamie on February 12, 2006, 2:41 pm
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Brian wrote:

> I have both a 50A DC hall effect IC with with the same specifications
> as Allegro's ACS750LCA-050 ( http://www.allegromicro.com/sf/0750/ ) and
> an old analog simpson guage with a 0-50A plate which I presume to be a
> 50mV meter. The output on the IC is linear enough for my purposes
> outputting 2.5V @ 0A and 4.5V @ 50A with a Vcc of 5V. Is it even
> possible to mate both of these? If so, what would be the easiest
> way/design? If any more information is needed, feel free to ask.
>
> Thanks,
> Brian
>
yes it is..
for the - side of your meter, use a Voltage divider with a
small range pot on it in the middle of the divider to calibrate
the zero point..
just a pot with Resistors from each outer leg for the voltage
divider.
what you need to do is set this divider to 2.5 volts from the
5 volt supply.
next.
you have 4.5 volts for full 50Amps, since you already have 2.5
volts gone, your net voltage would be 4.5 - 2.5 = 2 volts.
now you know that you have only a 0..2 volt range in reality for
the + side of your meter.
you need to know the current of your meter at full scale.
or simply use an ohm meter to test the meter coil and do the math.
I := E/R = 0.050/R = Im ( Im= meter current).
now, since we have 2 volts to work with you now need to subtract
the meter voltage, Es (voltage sum) = 2.00 - 0.050 = 1.950 volts.
now the series Resistor needed for this from the output of the
hall detector to the + input of the meter would be 1.950/Im

P.S., when selecting resistors for the voltage divider on the
- side of the meter, use low value ones so that you don't get
a noticeable effect in the math.
something like maybe a 470 ohm on each outer leg of a 100 ohm pot.
the center tap of the pot is what goes to the - meter side
one wing of the R goes to common, and the other to your 5 volt supply.
happy camping and i hope it is close enough for your needs.



--
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5


Posted by Fred Bloggs on February 12, 2006, 3:05 pm
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> I have both a 50A DC hall effect IC with with the same specifications
> as Allegro's ACS750LCA-050 ( http://www.allegromicro.com/sf/0750/ ) and
> an old analog simpson guage with a 0-50A plate which I presume to be a
> 50mV meter. The output on the IC is linear enough for my purposes
> outputting 2.5V @ 0A and 4.5V @ 50A with a Vcc of 5V. Is it even
> possible to mate both of these? If so, what would be the easiest
> way/design? If any more information is needed, feel free to ask.

Obviously you need to buck that Vcc/2 zero current output with the same.
Then you need to two trims: one for the 1-5& sensor + components error,
and another for possibly 15% gain error deviation from 40mV/A.


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