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-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]On Behalf
Of Russell McMahon
Sent: Saturday, 30 July 2005 12:21 PM
To: Microcontroller discussion list - Public.
Subject: Re: [OT] Circuit for measuring mains voltage


> 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

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