# A analogue circuit that spit out the resistance desired

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#### henry kissinger

##### Member level 2
I am thinking about a analogue circuit that should work like this.
Right now I have 2 values known, these are the voltage of a node (38V) and a resistor (158.4ohm).
This circuit need to interpret 38V as 38ohm, and then tune the adjustable resistor to 50ohm (because the 158.4ohm and 50ohm in parallel give us the 38ohm resistance.
Any idea is appreciated.
The result circuit should work for all the values of known values of the voltage node and the resistor.

If you design a constant current source with a 1A value, you can measure this voltage then adjust the resistance to maintain this voltage as 38V.

Hi,

As far as I understand .. BigBoss´ idea is O.K.
On the other hand this means a lot of power dissipation = a lot of heat
38W on 38 Ohms ... or 158W on 158 Ohms.

Does this make sense?

What is it for?

Klaus

The function can be implemented as a controlled current source regulating the total current to Vin/Rset.

I am thinking about a analogue circuit that should work like this.
Right now I have 2 values known, these are the voltage of a node (38V) and a resistor (158.4ohm).
This circuit need to interpret 38V as 38ohm, and then tune the adjustable resistor to 50ohm (because the 158.4ohm and 50ohm in parallel give us the 38ohm resistance.
Any idea is appreciated.
The result circuit should work for all the values of known values of the voltage node and the resistor.

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Hi,

where does this 38 V requirement comes from? Only to intrepret the reading easy? Is the “parallel“ resistor your unknown one? If facilitating a constant current source, I would choose 100 mA or even 10 mA. At the moment, your compliance votage is 158.4 V at 1 A. And as @KlausST already mentioned, the power losses are huge.

BR

If you design a constant current source with a 1A value, you can measure this voltage then adjust the resistance to maintain this voltage as 38V.
right, the issue I am facing is how to generate this resistance automatically
If you design a constant current source with a 1A value, you can measure this voltage then adjust the resistance to maintain this voltage as 38V.
You are right!
but I am facing is how to tune this resistance to 50ohm so I can get the 38V at Va. This should be a automatic process. I can not simply change the resistance value manually.

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right, the issue I am facing is how to generate this resistance automatically

You are right!
but I am facing is how to tune this resistance to 50ohm so I can get the 38V at Va. This should be a automatic process. I can not simply change the resistance value manually.
If you cannot manually adjust the resistance value, you have to use discrete resistance bank so you will able to change the resistance values by adding or substracting resistances to fix the value. You cannot use small signal variable resistance like FET too. So, resistance bank is the most reliable solution.

It's needles to say to use ADC or similar control circuit.

Hi,

I still do not get the aim of the circuitry. Do have to keep a certain voltage "stable" or do you want to perform a measurement of an unknown resistor? As you are controlling the parallel resistor (Z Ohms) it does not seem like a measurement task to me, as the parallel configuration has only a single solution to fullfil the condition

38 V = 1A • (158.4 Ohm || Z Ohm) --> 49.9934 Ohm

If your parallel resistor is unknown (Z Ohms) and you are not able to control it, I would use the parallel configuration of the zwo resistors (Y || Z) to set the output current of a constant current source e.g. with an LM317HV . This tutorial might be useful .