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'[EE]: (Followup) Having difficulty interfacing a c'
2001\07\31@015146 by Sanjay Punjab

picon face
Thanks everyone for all of your advice.
Unfortunately for cost reasons, I must stick with
using a low-ohm resistor as a current sensor. A
hall-effect current sensor, a magnetoresistive device
and
a reasonably accurate current transformer, cost $6 and
above in volume. It looks like these are my only
options:

1. Use a 1:1 transformer to isolate the voltage across
the low-ohm resistor (current sensor), from the
interfacing analog circuitry. On the secondary side,
the voltage can be referenced to ground, making it
quite easy to get reasonable accuracy out of an
inverting op-amp circuit with .1% resistors.
This will work, since I am only concerned with AC
current. The problem is finding a 1:1 transformer with
truly accurate winding ratios (within 1%). Or else, I
will have to perform some type of testing in each unit
during manufacturing and come up with a normalization
look-up table in software.

2. Use some type of optocoupler. Again the same
problems as solution #1. I am wondering how difficult
it would be to get an monolithic optocoupler
that has consistant voltage/current transfer ratios
between device samples.

Perhaps some of you can comment on these potential
solutions. Thanks for all of your help.





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{Quote hidden}

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2001\07\31@022955 by Vasile Surducan

flavicon
face
On Mon, 30 Jul 2001, Sanjay Punjab wrote:

> 1. Use a 1:1 transformer to isolate the voltage across
> the low-ohm resistor (current sensor), from the
> interfacing analog circuitry.

For me this have not too much sense if you could built yourself the
transformer. How many pieces do you need ( more than 100 ? )
Why a resistor and a transformer when you could just a transformer ?


On the secondary side,
> the voltage can be referenced to ground, making it
> quite easy to get reasonable accuracy out of an
> inverting op-amp circuit with .1% resistors.
>  This will work, since I am only concerned with AC
> current. The problem is finding a 1:1 transformer with
> truly accurate winding ratios (within 1%).

Why you need 1% accuracy if you will use an operational amplifier which
gain could be slighty ajusted ? Using a trimmer you could use 5%...10%
accuracy transformers quite well.


Or else, I
> will have to perform some type of testing in each unit
> during manufacturing and come up with a normalization
> look-up table in software.

No, a simple fixed signal generation ( by pressing two buttons known only
by yourself in testing mode ) and one potentiometer adjustment is cheapest
than a variable table read

>
> 2. Use some type of optocoupler. Again the same
> problems as solution #1. I am wondering how difficult
> it would be to get an monolithic optocoupler
> that has consistant voltage/current transfer ratios
> between device samples.

 Will be expensive. Search at Burr-Brown for example and you'll like to
kill yourself. You'll need 2 common optocouplers ( photodiode +
phototransistor ) with at least 20 KHz bandwith and 2 common
operational amplifiers. The problem will be another floating power supply.
This version is not doable for large production bacause is much expensive
like the first one.

Cheers, Vasile

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2001\07\31@023207 by Vasile Surducan

flavicon
face
I forgot something important. How looks your power amplifier final stage ?
I have doubts you can't measure there the output current by simply
measuring the potential on protective resistors ( in emiters of both
final transistors ) one for positive alternation and the other on negative
alternation. Thus you'll be able to find the real current and knowing the
output voltage you'll find the output power ( if that's you need ).

Cheers, Vasile

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2001\07\31@035243 by Alan B. Pearce

face picon face
>1. Use a 1:1 transformer to isolate the voltage across
>the low-ohm resistor (current sensor), from the
>interfacing analog circuitry. On the secondary side,
>the voltage can be referenced to ground, making it
>quite easy to get reasonable accuracy out of an
>inverting op-amp circuit with .1% resistors.
> This will work, since I am only concerned with AC
>current. The problem is finding a 1:1 transformer with
>truly accurate winding ratios (within 1%). Or else, I
>will have to perform some type of testing in each unit
>during manufacturing and come up with a normalization
>look-up table in software.

Use a Bifilar wound transformer. I would also put the resistor on the
secondary, but that decision would also depend on getting a transformer with
suitable core and windings.

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2001\07\31@070128 by Spehro Pefhany

picon face
At 10:39 PM 7/30/01 -0700, you wrote:
>Thanks everyone for all of your advice.
>Unfortunately for cost reasons, I must stick with
>using a low-ohm resistor as a current sensor. A
>hall-effect current sensor, a magnetoresistive device
>and
>a reasonably accurate current transformer, cost $6 and
>above in volume. It looks like these are my only

I don't think so.. but as you haven't said whether "volume"
means 5 or 50,000, it's hard to tell. ;-)

{Quote hidden}

Have you heard of trimpots?  It's often cheaper and
more practical to put an adjustment in rather than
try to make everything fit into your error budget.
Of course, with proper design you can sometimes
use a multiplier stored in EEPROM or whatever, but
there are disadvantages to this.

>2. Use some type of optocoupler. Again the same
>problems as solution #1. I am wondering how difficult
>it would be to get an monolithic optocoupler
>that has consistant voltage/current transfer ratios
>between device samples.

Forget it, even optos designed and spec'd for linear
operation have +/-10% or more variation from unit to
unit (--talking about closed loop with an op-amp).
You can buy isolators that will work, but they cost
a lot of money (>> $6 US).
Linearity of +/-0.1% F.S. is achievable, however,.
But you'd still need a floating supply.

P.S. You might get better suggestions if you
disclosed:

1)      Target quantity range
2)      Frequency response range
3)      Accuracy requirements and dynamic range

Of course if you are just seeing what's possible
this may be challenging...

Best regards,


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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Embedded software/hardware/analog  Info for designers:  http://www.speff.com
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2001\07\31@073521 by SkinTech

flavicon
face
If you are cost-sensitive, why not use an instrumentation opamp, some can
sense diff voltages with 200V+ common mode. The supply (+- 15 v or so) can
probably be taken from the amp. Must be cheaper, smaller etc than xformer.

Cheers, Jan Didden

{Original Message removed}

2001\07\31@073923 by Spehro Pefhany

picon face
At 12:43 PM 7/31/01 +0200, you wrote:
>If you are cost-sensitive, why not use an instrumentation opamp, some can
>sense diff voltages with 200V+ common mode. The supply (+- 15 v or so) can
>probably be taken from the amp. Must be cheaper, smaller etc than xformer.

Because the CMRR of these amplifiers is nowhere near good enough.

Best regards,
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

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2001\07\31@081448 by David VanHorn

flavicon
face
At 10:39 PM 7/30/01 -0700, Sanjay Punjab wrote:
>Thanks everyone for all of your advice.
>Unfortunately for cost reasons, I must stick with
>using a low-ohm resistor as a current sensor. A
>hall-effect current sensor, a magnetoresistive device
>and
>a reasonably accurate current transformer, cost $6 and
>above in volume. It looks like these are my only
>options:

Try different sourcing. Magnetics are very inexpensive in China.

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differentiate a signature line from the text of an email, I am forbidden to
have it.

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2001\07\31@100341 by Olin Lathrop

face picon face
> If you are cost-sensitive, why not use an instrumentation opamp, some can
> sense diff voltages with 200V+ common mode. The supply (+- 15 v or so) can
> probably be taken from the amp. Must be cheaper, smaller etc than xformer.

We've been thru this already.  Such a setup would require 106dB common mode
rejection to get 1% common mode noise.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2001\07\31@162011 by George Tyler

flavicon
face
A firm I consult for makes current transformers for metering applications,
give me the spec. and I will see if they can do anything.

{Original Message removed}


'[EE]: (Followup) Having difficulty interfacing a c'
2001\08\01@093134 by SkinTech
flavicon
face
So?

Witness the INA163, 100dB+ CMR and PSRR to close to 20kHz at gains of 100,
more at higher gains. If your sense resistors are small, which they will be,
you need the high gain anyway.

Cheers,

Jan Didden

{Original Message removed}

2001\08\02@023503 by Vasile Surducan

flavicon
face
INA 163 it's a BURR-BROWN product isn't it ?
It cost twice than the best taiwanese 150W car amplifier...[final product,
in aluminium case, 1V input]
I have doubts it will be cheap, but maybe I'm wrong, you have some
discounts there ?
I've buy an OPA128 a while ago, with that money I sould buy two kilograms
of other operational amplifiers...

Vasile


On Wed, 1 Aug 2001, SkinTech wrote:

> So?
>
> Witness the INA163, 100dB+ CMR and PSRR to close to 20kHz at gains of 100,
> more at higher gains. If your sense resistors are small, which they will be,
> you need the high gain anyway.
>
> Cheers,
>
> Jan Didden
>
> {Original Message removed}

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