From: John Woodgate
Subject: Re: Sensing mains current
Date: Mon, 2 Dec 2002 16:50:12 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 2 Dec 2002 18:01:10 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that ted
wrote (in ) about
'Sensing mains current', on Mon, 2 Dec 2002:
>Thanks to all those who replied
>To summarise the various replies, I think I can do it by wrapping
>(one) mains lead around a ferrite (or toroid) and use a secondary with
>as many turns as possible to sense the output.
>This can be very cheap indeed. The only question I have is
>how do I select the ferrite?
>The toroids I tried give no better output than an air transformer
>(they must be RF toroids)
>Is there a specification for material that works down to 50Hz??
Yes, there are ferrites that work down to 50 Hz, but you can also use a
core from an old transformer. You don't need to put huge numbers of
turns on your secondary winding, but you should put a low-value resistor
(maybe 100 ohms) across it permanently. With no resistor, the core will
saturate and you voltage readings won't make sense.
You can also use a small mains transformer backwards, if you have one in
which the low-voltage winding will safely carry the mains *current* you
want to measure. In this case, you definitely MUST put a resistor across
what is normally the primary winding. Select the resistor to give you
about 5 V output when the current you want to detect is flowing.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
PLEASE do NOT copy news posts to me by E-MAIL!