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PICList Thread
'[PIC]: AC current'
2001\06\13@205900 by Tony Nixon

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Hi all,

Does anyone have any ideas of measuring a small AC current with the PIC
A2D?

A friend of mine has a hydroponic garden and wants to measure the
current flow between 2 probes which are fed with 16VAC.

I would imagine the current would be in the order of milliamps.

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Best regards

Tony

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2001\06\13@211736 by Paul Hutchinson

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Can you stick a resistor in series with the probes, in the range of 1 to 100
ohms?

If you can, then amplify the AC voltage measured across the resistor and
rectify it. If you need good linearity you can use an op-amp based active
rectifier.

Paul


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2001\06\13@215752 by Brent Brown

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> Does anyone have any ideas of measuring a small AC current with the PIC
> A2D?
> A friend of mine has a hydroponic garden and wants to measure the
> current flow between 2 probes which are fed with 16VAC.
> I would imagine the current would be in the order of milliamps.

Sounds like a soil moisture sensor or water nutrient tester? You
could add a series resistor as suggested (to convert the current into
a voltage), capacitively couple this into a rectifier circuit of some
sort then feed into an ADC input. The 16VAC would need to be more
regulated than just the output of a transformer if you want accurate
results.

I have a small and simple circuit idea that I could post if it's of
interest. It uses the PIC to generate the AC as well.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brownspamKILLspamclear.net.nz

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2001\06\13@220211 by Tony Nixon

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Brent Brown wrote:
>
> > Does anyone have any ideas of measuring a small AC current with the PIC
> > A2D?
> > A friend of mine has a hydroponic garden and wants to measure the
> > current flow between 2 probes which are fed with 16VAC.
> > I would imagine the current would be in the order of milliamps.
>
> Sounds like a soil moisture sensor or water nutrient tester? You
> could add a series resistor as suggested (to convert the current into
> a voltage), capacitively couple this into a rectifier circuit of some
> sort then feed into an ADC input. The 16VAC would need to be more
> regulated than just the output of a transformer if you want accurate
> results.
>
> I have a small and simple circuit idea that I could post if it's of
> interest. It uses the PIC to generate the AC as well.


Yes please :-)

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2001\06\13@220427 by Spehro Pefhany

picon face
At 09:15 PM 6/13/01 -0400, you wrote:
>Can you stick a resistor in series with the probes, in the range of 1 to 100
>ohms?
>
>If you can, then amplify the AC voltage measured across the resistor and
>rectify it. If you need good linearity you can use an op-amp based active
>rectifier.

Or, more crudely, just allow a bit of voltage drop. With 16VAC to play
with there's
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2001\06\14@005745 by Brent Brown

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Hi,

Circuit idea attached with sketches of waveforms to show how it
works.

The micro toggles a port pin to generate a square wave. It is
capacitively coupled to the probe as DC is not a good thing on a
probe for electrolysis reasons.

The probe is an uknown resistance to ground. This forms a voltage
divider with the series resistor. The amplitude of the waveform is
now a function of the resistance of the probe.

Two diodes and two caps make a rectifier / voltage doubler. The
input to the ADC is now in the range of 0 to about 3.8V. The parallel
resistor (large value) discharges the filter capacitor when the probe
is open circuit again.

Hope this gives you some ideas.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile/text: 025 334 069
eMail:  brent.brownspamspam_OUTclear.net.nz

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2001\06\14@034439 by Alan B. Pearce

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>A friend of mine has a hydroponic garden and wants to measure the
>current flow between 2 probes which are fed with 16VAC.

>I would imagine the current would be in the order of milliamps.

depending on how accurate you want to measure, I would look seriously at
using something like an old transistor radio output transformer in reverse
(i.e. current to be measured through speaker winding) so you get a step up
that you may be able to put straight on a rectifier. If you are just
interested in the amplitude (rather than calculating "true RMS") because you
assume a true sinewave then this may well give a minimum parts solution. On
the other hand if you need to calculate a "true RMS" then you will need to
go the opamp way to get good rectification.

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2001\06\14@061509 by Vasile Surducan

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On Thu, 14 Jun 2001, Tony Nixon wrote:

> Hi all,
>
> Does anyone have any ideas of measuring a small AC current with the PIC
> A2D?
>
> A friend of mine has a hydroponic garden and wants to measure the
> current flow between 2 probes which are fed with 16VAC.
>

 I'm thinking like a pic beginner. Measuring precisely RMS is difficult
( for me ) without some electronics. But if you'll use chopped rectifier
tehnique ( same as operational amplifier precisely rectifier ) and square
wave pulses, as someone mentioned before, will be more easy.
A stepup transformer can supply a stable 16V square wave. And can be
easily driven on - off. When the pulse is on you can measure soil
resistance. And averaging sounds much simple. When is off you can do
something else, like offset correction of the input measuring amplifier.
The only real problem is avoiding sensor electrolysis effect which will
change all measured results.
Vasile

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2001\06\14@093237 by Douglas Butler

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Letting the PIC be the AC source lets you choose an operating frequency
to avoid outside interference.  Also consider synchronous rectification.
You can do it easily with a OP amp and a FET switch.  It will reduce
all sorts of interference.

Of course if you just want to know if the plant needs to be watered it
may be overkill.

Douglas Butler
Senior Engineer
Imetrix Inc.
1235 Route 28A
P.O. Box 152
Cataumet, MA 02534-0152
tel. (508) 564-6460
Fax (508) 564-6860
spamBeGonedbutlerspamBeGonespamimetrix.com


> {Original Message removed}

2001\06\14@112453 by Harold M Hallikainen

picon face
On Wed, 13 Jun 2001 22:03:23 -0400 Spehro Pefhany <TakeThisOuTspeffEraseMEspamspam_OUTINTERLOG.COM>
writes:
> At 09:15 PM 6/13/01 -0400, you wrote:
> >Can you stick a resistor in series with the probes, in the range of
> 1 to 100
> >ohms?
> >
> >If you can, then amplify the AC voltage measured across the
> resistor and
> >rectify it. If you need good linearity you can use an op-amp based
> active
> >rectifier.
>
> Or, more crudely, just allow a bit of voltage drop. With 16VAC to
> play
> with there's

       Or more crudely, put a current limit resistor between the shunt resistor
and the PIC analog input. The PIC clamp diodes will rectify the AC such
that only the positive side gets to the A/D. If the current is
symmetrical, you can just measure this positive side. No diode
nonlinearities to deal with!
       Another trick, though I haven't used it for this low a current, is to
use a current sense transformer (I use stuff from Toroid Corporation of
Maryland) to drive a diode bridge driving a terminating resistor. Within
limits, the current transformer secondary acts like a current source,
forcing current through the terminating resistor no matter how much
voltage it takes, so the diode drops do not affect the voltage across the
terminating resistor.


Harold


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2001\06\14@180545 by Olin Lathrop

face picon face
>         Another trick, though I haven't used it for this low a current, is
to
> use a current sense transformer (I use stuff from Toroid Corporation of
> Maryland) to drive a diode bridge driving a terminating resistor. Within
> limits, the current transformer secondary acts like a current source,
> forcing current through the terminating resistor no matter how much
> voltage it takes, so the diode drops do not affect the voltage across the
> terminating resistor.

If you are willing to use a current sensing transformer, then you can tie
one side of the secondary to 2.5V and read the other end with the PIC A/D as
long as the transformer output does not exceed 5V peak to peak (or is
attenuated so that it does).


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

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