Thanks a lot for all your input.

What at first seemed like an easy task with a digitally controlled
pot has now become a bit more complicated.

I am going to have a closer look at the transformer, the h11fx part
and the 4066 approach. Plus a couple of ideas of my own. Initially I
was just going to replace the pot with a resistive circuit but now I
may have too look at other possibilities, to see if I can achieve the
same result by fiddling with other parts of the circuit.

Maby the simplest is to get a 10kohm digitally controlled put and
parallell it with a lower value resistor to get the currents down to
accepted levels.

Date sent:              Wed, 4 Oct 2000 06:08:37 +1000
Send reply to:          staff@blackrobotics.com
From:                   Roman Black <fastvid@EZY.NET.AU>
Organization:           BlackRobotics (Fastvid)
Subject:                Re: [EE]:Resistive circuit
To:                     PICLIST@MITVMA.MIT.EDU

> Ruben Jvnsson wrote:
> >
> > Forgot to mention that it has to work with both positive and
> > negative voltages (0V on one terminal and +- 1V oscillating at
> > 150kHz on the other terminal).
> >
>
> You have some special needs there Ruben, but I have a suggestion.
> About 18 years back in an industrial elec subject I learned about
> magnetic amplifiers. What you need is something reliable to adjust the
> impedance in your tuned circuit. Like a pot. Semiconductors are all
> going to be difficult to implement in that circuit.
>
> Imagine a small transformer like an audio coupling transformer etc.
> One winding is used to replace the pot. With the other (control)
> winding disconnected it will have a low impedance to your 150kHz ac
> signal. Now, if you connect DC (not ac) to the control winding the
> core of the transformer goes into saturation and it offers a high
> impedance to the ac going through the other winding.
>
> So by adjusting the simple DC supply to the transformer you
> can adjust the amount of core saturation and use that to control
> ac current through the main winding. I have seen huge ones of these
> control AC megawatts from quite a small dc input. Was common about 50
> years ago before things like thyristors came to save us all.
>
> Now you may have to purchase some small toroids or ferrite cores, and
> wind them yourself, because at 150kHz they won't need many turns. That
> is probably an advantage. It should be pretty easy to tune the thing
> by adding/removing turns. Keep the dc control winding with more turns
> to keep current to a more PIC-like level. You might also need to
> fiddle with some ac damping (snubbers, zeners etc) on the control
> winding but it shouldn't be too hard. Since you stated it was only
> milliamps through the pots you will only need tiny transformers and
> very small current to saturate a tiny core like that.
>
> This solution offers excellent industrial reliability and isolation
> and once you have done the homework on the first one will be very easy
> to wind the rest and has a fairly cheap parts count. If you are
> already working with metal detector coils you probably have good
> grounding in the magnetic formulas to give you rough turns to start
> with.

I'm afraid not. This is an old design, done long before I started my
electronic career.

>
> That's the way I would do it! 1940s magnetic amp theory. :o)
> -Roman
>
> --
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>


==============================
Ruben Jvnsson
AB Liros Elektronik
Box 9124, 200 39 Malmv, Sweden
TEL INT +46 40142078
FAX INT +46 40947388
ruben@pp.sbbs.se
==============================

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