A simple cheap current transformer with good bandwidth...

Umm, how calibrated is the current transfer ratio? Just moved it around in the gray goo until it was "right"?

:-)

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On a sunny day (Mon, 29 Jun 2009 08:57:33 -0700) it happened Joerg wrote in :

Irrelevant. You glue the wire, so that part is stuck. You test the output, and calibrate in the software. You could use a trimpot too, if you really are lazy and have no idea on how to run a few tests. What counts is long term stability, and that does not change.

Much more interesting is your amplifier, or to be more precise how you terminate the inductor... for a straight frequency characteristic.

I like using these:

(follow the black twisted pair from the top left, inward)

Common mode filter ferrite (high mu) toroid, some wire (maybe 100 turns, can be wound as 20 * 5 by pentupling up the wire and connecting it all in series) and a burden resistor (like 1 or 10 ohms). And wrapped with masking tape. About as easy to put together as yours is plus calibration.

Incidentially, that's about 100T at 1uH/T^2 or 10mH, and saturation at maybe

10At is 100mA or 1mVs capacity... soooo, a burden of 10 ohms can carry 0.1A (e.g. 1Vpk square wave) down to 250Hz or so. Not quite line frequency, but fairly close. More turns, or less burden resistance, will easily include 60Hz with this puny little ferrite.

Speaking of current meters, today I put in a DC meter running from a shunt made of 8" of 18AWG. I enjoy meters with low DCR.

Tim

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"Jan Panteltje"  wrote in message 
news:h2ag8n$485$1@news.albasani.net...
> A simple cheap current transformer with good bandwidth...
> Been using this one now for many years, it measures the mains current.
> ftp://panteltje.com/pub/play_back_head_current_sensor_img_1153.jpg
> The sensor, or current transformer if you like,
> is a old audio playback head from a walkman IIRC.
> The twin wires glued to it are in parallel.
> The LM324 does some things with it, like amplification and rectification.
>
> I guess replacing the LM324 with a PIC, and adding a voltage tap would
> make it a good power meter....
>
> Just opened it up to show you how to use the PB head.
>
>
>

As long as the goo doesn't change :-)

Sure. But I can't use such open structures anyhow, too much RF around. I prefer looping one turn through a core. Can also be done with scrap parts if needed, such as modem transformers or audio transformers from old transistor radios.

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On a sunny day (Mon, 29 Jun 2009 10:26:18 -0700) it happened Joerg wrote in :

It is araldite, 2 component glue, gets hard as stone.

I do not think the PB head will pick up a lot of RF, at least I never noticed. Depends how high in frequency I guess. The main advantage is perhaps that it has good bandwidth, and you do not need to cut a wire, can glue it on anywhere. The big advantage for me, when I made that thing, was that it was in my junk box, so zero cost. Connect it to a scope, and you have a great current sniffer :-)

Ah, yes, I remember that from my time in NL. That was "the" glue over there when things had to be really sturdy. I think it's called Araldit without the "e" over there. A light blue and a white tube, right?

Not RF but switcher noise, big time.

to cut a wire,

box, so zero cost.

Shhhush! Don't leak all my EMI hunting tricks here ;-)

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On a sunny day (Mon, 29 Jun 2009 10:53:40 -0700) it happened Joerg wrote in :

It comes in different forms, I have something that looks like 2 injection tubes.. This has some metal in it I think.. hence the silver color.

Did you know you can even find the current path in a metal plate (like a chassis) with it? Great to find earthing problems....

tubes..

chassis) with it?

I mostly do that with an EMCO near-field probe kit because the chassis currents that bother my clients are mostly RF.

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Thanks. I've found a box of cassette deck playback heads I now know what to do with. :-)

--
Adrian C

rectification.

Do you have the gaps on the head set parallel with the wire? It looks like you have the wire going past the head like the tape would.

What's the phase shift at 50 or 60 Hz?

John

On a sunny day (Mon, 06 Jul 2009 16:54:04 -0700) it happened John Larkin wrote in :

Why dont you measure it?