Searching \ for '[OT] Isolation measurement' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: techref.massmind.org/techref/index.htm?key=isolation+measurement
Search entire site for: 'Isolation measurement'.

Exact match. Not showing close matches.
PICList Thread
'[OT] Isolation measurement'
1998\04\25@231741 by Gavin Jackson

flavicon
face
Hi there

I need to measure the output from a circuit
that has a floating power supply. The circuit
that does the measurement has a 16C71
and its supply is referenced to ground.

The circuit with the floating power supply
produces a voltage in the range of 3V to 9V
and I need to isolate it from the PIC and
bring it down to within the limits of the PIC's
AD converter.

Is there a simple way of using OP-Amps to
provide the isolation or another method that
does not cost much?

Regards

Gavin
--------------------------
spam_OUTvulcanTakeThisOuTspamihug.co.nz
www.geocities.com/TheTropics/Cabana/2625
--------------------------

1998\04\25@235934 by Dwayne Reid

flavicon
face
>I need to measure the output from a circuit that has a floating power
supply. >The circuit that does the measurement has a 16C71 and its supply is
referenced >to ground.
>
>The circuit with the floating power supply produces a voltage in the range
of >3V to 9V and I need to isolate it from the PIC and bring it down to
within the >limits of the PIC's AD converter.
>

How much voltage seperates the floating supply from ground?  Is it a limited
range (12v or so) or is it much more than that?  Can your floating circuit
tolerate any leakage to ground?  For that matter, why is it floating?

1998\04\26@042627 by g.daniel.invent.design

flavicon
face
dear Gavin,

farnell catalog may/october shows a "precision voltage to frequency
convertor" by EXAR code 4151, dil 8 pin. farnell code 562-701, $3.78 NZ
1 up price. if you have a small difference in supply voltage of your
floating PSU to the PIC's GND then you may be able to output this to an
input to the pic (with suitable resistance) if a large difference then
you will need an optoisolator.

note that the TMRO/PA4 pin only has one internal protection diode due to
single sided output driver.

of course this way of testing the voltage would be applicable to a
pic12c508 or 16c84 etc as uses any pin for "tacho" input.

if you want to skip tacho S/W routine then the frequency can be back
converted to a voltage with a resistor/capacitor on the opto output.
(lesser accuracy)

regards,
Graham Daniel.

Gavin Jackson wrote:
{Quote hidden}

1998\04\26@043042 by Russell McMahon

picon face
I can supply outline circuit diagrams for the following if
desired and some references but will start with a word
outline as others may have better suggestion.

If the floating signal is not "isolated" per se and only
floats within the power supply range of a standard op-amp
(say about +-15 volts ) then a standard 2 input differential
amplifier will give you the ground referenced result with as
little as 1 op-amp (3 for a full instrumentation amplifier).
If the floating voltage floats at much higher voltages or
true isolation is needed then "telemetry" style approaches
may be needed.

Two of many approaches are:

1.    Siemens (and others) make opto couplers which have 2
receivers so that one can be wired in a feedback loop and
allow a linear voltage to be transmitted across the opto.
Siemens IL300-X016 - RS sell for $NZ11.54 in ones. Farnell
probably also sell them.

2.    A "reasonable" method is a voltage to frequency
converter and use of an opto isolator to send the pulses to
the PIC for counting. Could be a 555! or a purpose built IC
such as LM331 (Nat Semi) , TC9402C (Burr Brown) AD654 and
others (RS 1998 catalog page 1-1133)


As you are a "local" (to me anyway :-)  you could ring me on
(09)8372999 if any of this sounds useful.

{Original Message removed}

1998\04\26@043910 by Gavin Jackson

flavicon
face
part 0 1062 bytes
I've not measured the voltage between the supply and ground,
but I don't think the difference is all that much.

Regards

Gavin
--------------------------
vulcanspamKILLspamihug.co.nz
www.geocities.com/TheTropics/Cabana/2625
--------------------------


{Original Message removed}

1998\04\26@155606 by Tom Handley

picon face
  Gavin, while I'm no expert on pH probes, you are probably looking at an
impedance of 10^9 to 10^13 ohms. Analog Devices has an example pH probe
buffer in their 1992 "Amplifier Applications Guide" which is based on their
AD645. However, I'd take a look at their AD549 which has better specs. You
are basically looking for a JFET-input amp with an input bias in the fA
range and a high input impedance. By the use of guarding, shielding, high-
resistance insulation, etc, you may not need to isolate the circuit. In
anycase, I would use the buffer on the probe side and then an isolated amp
if needed.

  Also check out Burr-Brown. Their INA116 Instrumentation amp has an input
bias of 3fA and supports guard pins on the inputs. Actually this would
be a better choice...

  Omega has a good introduction to pH sensors in their "The pH and
Conductivity Handbook". For more info, contact:

     Omega
     http://www.omega.com

     Analog Devices
     http://www.analog.com

     Burr-Brown
     http://www.burr-brown.com

  - Tom

At 08:38 PM 4/26/98 +1200, Gavin Jackson wrote:
{Quote hidden}

>{Original Message removed}

1998\04\27@201202 by paulb

flavicon
face
Gavin Jackson wrote:

> The reason the circuit is floating, is that it comes from a
> pH probe with a very high output impedance.

 Ah!  It becomes clear now!  That is true, but the trick is that only
*one* electrode (the glass one) has a high impedance, the other is quite
low.

>  I was told that if a supply referenced to ground was used, many
> problems could be introduced.

 Quite so, but not half as many as if you *don't* reference it to
ground.  That suggestion was made in ignorance of the need of telemetry!

 My suggestion:

 1) Ground the solution(!) with a third electrode of whatever material
is practical.  With some luck, it may already be in a metal pipe or
container which can be grounded.  Otherwise *shield* the whole measuring
area with a grounded surrounding shield, either solid metal or mesh.
Reason:  It's far easier to ground out interference (mains) than to
balance it out in the amplifiers or isolate it.

 2) Determine the glass electrode.  You presumably already have a
circuit to measure its "signal" (voltage) at the ultra-high impedance.
Only the buffer part is required.

 3) Provide a medium-high impedance buffer (unity gain) for the
reference electrode.  Frequency compensation must be identical to the
previous buffer.  In fact, the components are not too expensive
nowadays and it will be just as easy to use the same circuit.

 4) Provide a differential amplifier to compare the two with precision,
temperature compensation and the necessary gain.  This should operate
from the standard "instrumentation" amplifier supply of +/- 15V and the
common-mode signal with the test solution grounded/ shielded should fall
well within this range.

 The last three parts should in fact already be part of the design,
being a standard triple-amplifier "instrumentation" design.  Such are
readily available from Analog Devices or the like, already assembled for
the purpose.

 Well, my suggestion anyway.  Any holes to pick?

 Now Russell McMahon <EraseMEapptechspam_OUTspamTakeThisOuTclear.net.nz> wrote:

> If the floating signal is not "isolated" per se and only floats within
> the power supply range of a standard op-amp (say about +-15 volts )
> then a standard 2 input differential amplifier will give you the
> ground referenced result with as little as 1 op-amp (3 for a full
> instrumentation amplifier).

 Actually, if the floating signal is reasonably low in impedance (say,
less than 1 megohm) and reasonable level (a volt or so), it may still be
measured in this way using resistive dividers up to a few hundred volts.

 If a low-impedance common-mode reference is available (such as the
third electrode/ container previously suggested), a floating buffer may
measure and amplify a higher-impedance signal and relay it to the
"ground" reference via such resistive dividers or a current driver.

> If the floating voltage floats at much higher voltages or true
> isolation is needed then "telemetry" style approaches may be needed.

 I have to agree.

 Cheers,
       Paul B.


'[OT] Isolation measurement'
1998\05\01@032030 by Josef Hanzal
flavicon
face
Tom Handley wrote:
>   Gavin, while I'm no expert on pH probes, you are probably looking at an
>impedance of 10^9 to 10^13 ohms. Analog Devices has an example pH probe

Gavin,

those numbers are probably 20 years old, todays ordinary pH probes fall into
the 10^8 ohms class (the one I use is specified for 200 Mohms maximum
internal impedance). OA with input bias current of several pA (like
TL061...) will make at most few mV voltage drop. And since your pH probe is
always calibrated with buffers, the same error is encountered during both
the calibration and the measurement, so it actually cancels out. Althouhg OA
featuring units of fA Ib current will also work, it will bring very little,
maybe insignificant improvement.

On the other hand, there could be special high impedance pH probes, for
special purposes like high pressure, complicated shape, microprobes, for
strongly alkaline solutions and I don't know what else. But you probably do
not have any of these, do you ?

If you could describe your circuit in better detail, I may be more helpful.
Is it powered from a battery or from a wall socket ? What is the whole
instrument connected to ?

Josef

======================================================================
Electronical devices for chemical laboratory, custom electonics design
----------------------------------------------------------------------
Snail Instruments                     Josef Hanzal
Vojanova 615                          phone/fax: +420-311-24433
266 01 Beroun                         e-mail: euroclassspamspam_OUTpha.pvtnet.cz
Czech Republic                        URL: http://www.vitrum.cz/snail/
======================================================================

More... (looser matching)
- Last day of these posts
- In 1998 , 1999 only
- Today
- New search...