How can I make a current look like a resistor?

[wabiz]

I've two circuits, one measures a resistor using a series of pulses. Another separate circuit outputs a current in proportion to a given input. I can't modify the first circuit but I'd like to use it to monitor my current circuit. Is this possible? Can I use a fet or transistor in a common base configuration or something similar?

 

[itsmoked]

Your description has left me... confused.

Try again?

 

[VEBill]

Use the current to light up a light bulb or LED (assuming the values are in the right range). Use the light bulb to illuminate an old fashioned CDS photocell. The photocell resistance will vary with the amount of light falling on it. Maybe 2k ohms in bright light to 100k ohms in darkness (I think that's the way it goes - please double check).

Current -> light bulb -> CDS photocell - Resistance

There's lots of adjustments including spacing. Keep the pair in a dark can (black 35mm film canister). Depending on YOUR variables, it might actually work.

That'll be a dollar. ;-)

 

[Skogsgurra]

There is such a thing as a magneto-resistor (Philips and Siemens used to make them and several Japanese manufacturers exist, try Murata).

A magneto-resistor is just that, a resistor that changes its resistance when a magnetic field is present. All you need to do is send your current through a coil and put the MR in the field. It will probably not be absolutely linear - but you have turned your current into a resistor.

There are other ways of doing that. A gyrator circuit may be the answer. Google "gyrator". At least it works for AC. Never seen an application for DC.

Gunnar Englund

http://www.gke.org

 

[VEBill]

Use a solenoid working against a spring to move a potentiometer. This is applicable if the available current is high (amps).

 

[ScottyUK]

What is the actual purpose? Is this to interface two transducers with incompatible inputs and outputs?



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[nbucska]

Corvert i to V with an R.
Convert V to dogital with an ADC
Use this to as input to a digital pot.meter.

<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[ScottyUK]

nbucska -

Excellent idea. I'd forgotten all about the synthesised pots. Well done.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[logbook]

So far all the ideas presented suck big time.

What is needed is a more thorough system review. If you really can’t interface any other way than you have suggested then it will be helpful to discover how the resistance measuring circuit is actually working. If, for example the resistance measuring device puts out a fixed current and measures the voltage then:

1/ feed the current output of your new circuit into a resistor to get a voltage
2/ buffer the voltage with an opamp unity gain follower

When the resistor measuring circuit pulses the current the opamp will just absorb the current and deliver the same output voltage as before. If your resistor measuring circuit is not a linear ohmmeter then the complexity will be further increased.

 

[VEBill]

What if his 'resistor measuring gadget' has a bigger op-amp than your 'unity gain follower'?

 

[wabiz]

First, let me thank all of you for the tremendous response.
Excuse the cofusion but I don't have the particulars on the measurement circuit but it works fine with a real resistors and is very accurate. I was thinking of the digpot idea, thanks nbucska, and implementing an R stick/comparator binary converter for an input then I needed a clock .... . Yes Scottyuk, thats exactly my case here. I'll be investigating that gyrator too skogsgurra. You've all helped trigger the grey stuff again and I should get it from here, thanks.