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convert input resistance to different output, but how?!

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pcoghlan

New Member
Really hope I articulate this properly!

I have a piece of equipment to which an earth connected to its input. This earth has a resistance which is variable. i.e there may be 50 ohm to 500 ohm resistance between the input and earth. So, measuring at the input there would be a resistance between it and the earth.

What I want to achieve is to be able to make a black box which sits between the earth and my equipment and allows me to convert the resistance ‘on the fly’.

So, for example I know that for a certain case the input will be between 50 and 500 ohms but I want to convert this to being 90 and 900 ohms at the equipment. Each 50 ohms difference on the input of the black box will result in a 90 ohm change on the output to the equipment.

I ideally want this to be something I can change fairly easily. Ideally by dip switches although I suspect that as I don’t know how many combinations there will be this isn’t feasible. So, in addition to what the circuit will look like I guess what are the options for dictating how flexible this can be? Can it be programmable, which would be perfect! If not maybe can I change the input/output ranges with a single change of component?

I suspect I will have to set limits for both the input and output ranges. Is this something that can be accomplished by a knowledgeable novice!

Any help much appreciated!

Paul
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I don't really have any idea what you are wanting to do?, it sounds rather bizarre!.

Perhaps you should tell us what it is you are trying to connect together?, and why you need to make it variable?.

Assuming you are talking about input and output impedances, the methods vary greatly depending on the actual use - for example RF is normally matched, and AF isn't matched - the first gives power transfer, and the second only voltage transfer.
 

pcoghlan

New Member
how about this?

Nigel,

Gotcha! You need to understand the real life application to know the problem and hopefully the solution!!

The ‘equipment’ is a display gauge which shows fluid level in a tank.

A sender unit, which varies from application to application in terms of its resistance to earth, changes the resistance to earth which changes throughout the travel of its arm within the fluid tank.

What I am trying to build is a generic gauge ‘system’ which will allow me to plug into various sender units and test them throughout their travel to ensure they work properly.

At present I need to have a gauge ‘matched’ to the sender unit. My idea is a black box which will allow a single gauge to be used on multiple sender units.

Hope this is clearer?

Paul
 

grrr_arrghh

New Member
hmm, i'll leave this one ot Nigel, however, I did once see a resistor box, that had about 6 rotary switches on the front, with a resistor connect to each leg of the rotary. The first switch would set 0R, 1R, 2R up to 9R. The second switch would set 00R, 10R, 20R etc, alowing you to choose any resistance you wanted.

Not sure if this is any use to you at all, but just in case it helps.
 

bmcculla

New Member
Variable resistances are not easy to do. Instead you could build a circuit that measures the resistance and outputs a voltage that has been scaled to be generic for the guage. This circuit could be built with a couple resistors and opamps and should be quite easy.

If you absolutly have to have resistance as an output you could probably build a circuit to emulate a resistance if you know a bit about how the guage is measuring resistance.

Brent
 

Styx

Active Member
its prolly best to keep such a thing a passive box.

say a load of resistors in parallel and then you switch them in when you need a lower resistance.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: how about this?

pcoghlan said:
Nigel,

Gotcha! You need to understand the real life application to know the problem and hopefully the solution!!

The ‘equipment’ is a display gauge which shows fluid level in a tank.

A sender unit, which varies from application to application in terms of its resistance to earth, changes the resistance to earth which changes throughout the travel of its arm within the fluid tank.

What I am trying to build is a generic gauge ‘system’ which will allow me to plug into various sender units and test them throughout their travel to ensure they work properly.

At present I need to have a gauge ‘matched’ to the sender unit. My idea is a black box which will allow a single gauge to be used on multiple sender units.

Hope this is clearer?
Yes, that makes far more sense - essentially this is a fuel gauge as used in a car - a crude variable resistor attached to a float that drives a simple ammeter.

As has already been suggested, it's not an easy task to provide an electronically variable resistance. A far easier method would be to generate a variable voltage from the reading, and use that to feed a voltmeter (as also already suggested).

For 'grrr_arrghh', the device you mentioned is called a 'decade box'.
 

ChrisP

Member
Something else to consider when looking at tank units and level gauges...

There are two basic designs in use. In one design, a decreasing resistance through the tank unit drives the gauge towards its "full" indication, while the second design uses the opposite relationship. In that design, a decreasing resistance through the tank unit drives the gauge towards its "empty" indication.

All that is really needed for testing tank units is a decent ohmmeter... that and some information regarding the specs on the tank unit. Most tank units use a linear taper. Minimum and maximum resistances through the tank unit will vary with manufacturer and model, and the actual gauge deflection produced will depend upon the gauge itself and the supply voltage applied.

What all of this means is that it may well be impractical to expect one device to allow you to easily test a number of different tank unit/gauge combinations.
 
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