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'[OT] Circuit for measuring mains voltage'
2005\07\29@205335 by Groups

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I am looking for an isolated method of measuring mains voltage (230VAC).

First thoughts are a Opto coupler running in its linear region ?
My concerns are that the transfer ratio degrades over time.

The circuit needs to provide a fairly linear response from 150VAC to 270VAC.
The output required for the Micro is 0-2.5V.

Any ideas for existing circuits much appreciated

Thanks in advance

Regards

David


2005\07\29@211635 by Jack Smith

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I've had good results with a transformer feeding a Maxim MX536A
RMS-to-DC converter, which I then read with the 10-bit ADC in a 16F877A.

The MX536A converts an AC input into a DC output equal to the RMS of the
input, e.g., a 1 V RMS input provides 1V DC output. Hence, in your case,
to have a 2.5V DC output correspond to 270VAC, you will need a suitable
step down transformer, which will provide the isolation you need as well.

Jack



Groups wrote:

{Quote hidden}

2005\07\29@214350 by Groups

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Jack,

How large is the transformer ?

I should also have mentioned, I have limited space (around 25mm x 25mm). If
I could locate a very small
transformer it may work.

My main concern for isolation is that if the Phase and Neutral are reversed,
I do not want 230 VAC on the ground
as it is common to an RS485 interface and some other IO circuitry.

There is no problem conditioning and processing the signal once I can ground
reference it.


{Original Message removed}

2005\07\29@225726 by Russell McMahon

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> How large is the transformer ?

A transformer is a very eledgant solution, technology notwithstanding.
You can get very small main transformers - certainly under 25mm^2
footprint.
Jinx may be able to comment due to recent similar experience.

An alternative is a purpose build double tx, single rx opto made for
this sort of task. There are a number available. The sender uses a
feedback loop to contioj the sending signal and the receiver slaves
this. Opto tx drift not a problem.

Infineon HCNR200, HCNR201
"High-Linearity Analog Optocouplers"
0.01% non linearity
The light side of the force at work.

Clare AN107 Linear Opto Couplers (Google knows)
LOC110. LOC111. LOC112
Techniques apply to other manufacturer's products.

I have often thought that you could probably do an OK version of this
using a std dual opto IC.
Run both TX diodes in series and replicate techniques above.


       RM

2005\07\29@232248 by Groups

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Russell

Thanks for the feedback.

I think I will look into small transformers. Farnell list one from "Block"
that is 22mm x 22mm
that looks suitable.

I looked at the linear opto solution and it seems I would need to place an
op amp etc also on the primary side to
implement the feedback.

I was also thinking that maybe I could print a transformer on the PCB. ie.
Place a resistor in series with a track to develop
a mA or 2 in the primary and pick up a small portion magnetically (bit like
a directional coupler). The current limiting then allows
the low impedance track to be used without shunting the motor (The circuit
is to have 3 inputs that monitor the voltage
across motors).  A 270 V MOV would be placed across the input for transient
protection.

What are your thoughts ?

Thanks


{Original Message removed}

2005\07\30@002321 by Jinx

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> I think I will look into small transformers. Farnell list one from
> "Block" that is 22mm x 22mm that looks suitable

Sounds like the one I got from RS. Be aware that it saturates
and you need a series resistor. Mine was nominally two 6V
secondaries but put out 73Vp-p. The lowest picture shows
two primaries in series to give the approximately expected V
out

http://home.clear.net.nz/pages/joecolquitt/block-tx.html

Russell suggested, during [EE] Block miniature transformer
in May '05, that

"Placing a 0.5 watt load on the transformer secondary and a 24k
resistor in series with the primary MAY produce approximately the
desired result"

Block themselves confirmed that the small Tx would generate
less heat with a load

I regret that that particular part of the project got sidelined
during my battle with the F88 and I2C, but it is back on the
agenda and I hope to do some experimenting ASAP


2005\07\30@020408 by Groups

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Jinx,

Yes, that is the device (VB0.35XXX) I had noticed from Farnell.

The 230VAC input voltage monitoring part of my application is to be used as
a low voltage alarm to indicate
when the voltages supplying a motor fall below a programmable value. It will
have 270VAC MOV on the primary
to limit transients.
The secondary will be fed via bridge to a smoothing cap and burden resistor.
Next a series R to Op-Amp with a smaller
cap for additional smoothing and Zener for over voltage protection.
The op-amp gain will be adjustable to set a specific Vout at 230VAC.

As long as the DC voltage recovered is reasonably linear from 230VAC down to
about 150VAC we can get the results
we require.

Regards

David

{Original Message removed}

2005\07\30@032125 by Wouter van Ooijen

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> I am looking for an isolated method of measuring mains
> voltage (230VAC).
>
> First thoughts are a Opto coupler running in its linear region ?
> My concerns are that the transfer ratio degrades over time.

Do the measuring at the HV side and transfer a digital signal via an
optocoupler.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\07\30@033752 by Groups

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Wouter,

Most of the pins on the PIC18F6720 are committed, I can bring out the SPI
and an enable to
talk to say a ADC with SPI interface but it would add quite a bit of cost
and PCB real estate for ADC IC, power supply
and Opto Isolators on the HV side.

The current provisional design is using AN1,AN2,AN3.

Maybe another micro on the HV side could be added to do the conversion but
this also needs a PSU,
ICSP for programming and opto isolators.

What was your idea ?

Thanks

Regards

David


{Original Message removed}

2005\07\30@041747 by Howard Winter

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On Sat, 30 Jul 2005 13:17:59 +1000, Groups wrote:

>...<
> I was also thinking that maybe I could print a transformer on the PCB. ie.
> Place a resistor in series with a track to develop
> a mA or 2 in the primary and pick up a small portion magnetically (bit like
> a directional coupler). The current limiting then allows
> the low impedance track to be used without shunting the motor (The circuit
> is to have 3 inputs that monitor the voltage
> across motors).  A 270 V MOV would be placed across the input for transient
> protection.
>
> What are your thoughts ?

The isolation requirements would mean that you're unlikely to be able to do this in a 25x25mm space.  Using a
transformer with the mains side pins on the edge of the PCB gives you some "automatic" clearance by the
transformer's own size, and putting a slot in the PCB can increase this, but you can't do that with a printed
coil - it, its current-limiting resistors, and the MOV are all live so you need clearance all round, and it
sounds like you don't have the space.  I'd go for a "proper" transformer and as you've found, Jinx has some
experience of these, and has made a lot of the mistakes for you!  :-)

Cheers,


Howard Winter  G1BYY
St.Albans, England

2005\07\30@043204 by Wouter van Ooijen

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> Maybe another micro on the HV side could be added to do the
> conversion but
> this also needs a PSU,
> ICSP for programming and opto isolators.

You have a voltage on the HV side, so deriving 5V could be simple if you
can afford to dissipate some heat. The PIC would do just measurement and
sending the result, I see no real need for ICSP (compare it to an analog
solution: you must get that right first time too!).

I don't say this is the ideal solution, just compare it to other
solutions.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\07\30@050124 by Groups

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Wouter,

Thanks for your comments.

The ICSP would be required to program the PIC on the HV side. The SMD ICs'
are soldered directly onto the PCB and
I use a small transition board that enables the Microchip ICD2 to plug in
via 5 way 2mm pitch header.
This also has a Schotky diode and resistor.

Then I would need crystal, caps for decoupling and crystal. An analogue
reference for the ADC inputs.
A voltage regulator with smoothing caps and suitable dropping resistors to
enable current for the PIC and
opto coupler. (Assuming I use a TX pin only). Then I need to have bit-bashed
code in the main processor
to receive the data. (Both UARTS are taken). I would also need a timer in
the main micro and interrupt
input to detect the start bit (time so that the micro can service the other
interrupts without too much disruption).

I could use the SPI bus but this would take more hardware interfacing.

To be able to probe the side that is reading the 3 x 230 VAC inputs and
converting to Digital, I would need an isolating
transformer for my scope.

So far from this thread, it seems the transformer option would be the
simplest solution and offer good isolation at the cost
of PCB area, but as Howard mentioned the clearance requirements for
isolation prevent the circuitry from being too close
anyway.

The 22mm x 22mm form factor x 3 will make layout a bit tight, I may need to
investigate stacking more PCBs and placing
the micro and other logic type circuitry on another board. Then leave all
the IO interface devices on the lower PCB near the
terminals and bring up the control signals via some DIL headers.

Thanks everyone for your input. I appreciate the variety of issues that have
been raised.

Regards

David

{Original Message removed}

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