Solution for a high current digipot (maybe)

Thread Starter

reconchris

Joined Aug 2, 2017
4
An application I am working on consists of varying the resistance between two points precisely. The max current flow through the resistive load would me 50mA and minimum current flow would be 10mA. However every digipot I have seen usually can’t handle more than 3mA of through put through the wiper. In an attempt to fix this problem I looked at DAC units and this is what I found.

In the circuit diagram the upper voltage source can be considered the constant 8v supply. The lower dc source can be thought of as a variable DAC. The resistor in the middle provides current regulation.

The idea is that by increasing or decreasing the ADC output voltage the voltage differential across the resistor is controlled and the current flowing from the 8V source can be precicly regulated.

I’m just a dumb EE student however and although on paper and in simulations this looks good I still would like an expert opinion. upload_2018-7-22_10-53-20.png

HOW DO THEY NOT HAVE HIGHER THROUGHPUT DIGIPOTS BY NOW?!?
 

Vytas Klyvis

Joined Dec 5, 2016
47
A DAC will probably give you the same problems as most DAC's will probably not allow you to sink the 50mA that you want.


This circuit can also be used to control current. By varying the voltage at the non-inverting input, you can vary Iout. I don't know however which circuit better fits your needs, this one or the one provided by jpanhalt.
 

Thread Starter

reconchris

Joined Aug 2, 2017
4
To answer some questions.
I did further testing on this project and it is the resistance that needs to be effected not necessarily the current. The only thing I have found that would be a one component design for regulating resistance via an IC is by using a N channel Mosfet. I saw one that is designed to have its gate run by logic level voltage FQP30N06L. However the difference between 100K and 10 ohms is 0.4V of gate voltage increase. That's a very sharp and almost uncontrollable amount of gain. I need to control resistance from about 250 ohms to 10 ohms. How do I do that easier? I don't want to use Op Amps because driving their - + rails is a complicated process and this device is designed to work in the field not on a lab bench. There has to be something easier for controlling resistance using an IC.
 

Vytas Klyvis

Joined Dec 5, 2016
47
Could you elaborate on why you have to control the resistance and not the current. Normally the load will not care if there is a current source sinking the current or a resistor that sinks the current.

Vytas
 

crutschow

Joined Mar 14, 2008
23,814
Here's the LTspice simulation of an opamp-MOSFET circuit that exhibits a resistance which can be varied by a pot (U3).
The opamp just requires a single plus supply voltage, V2, which is not complicated.
A wall-wart DC supply can be used

For the values shown, the equivalent resistance (bottom trace - looking from the output of V1 to ground) varies from 10Ω to >250Ω with pot U3 adjusted from 0% to 100% of its rotation.
The pot could likely be a digipot if you want electronic control of the equivalent resistance.

Does that do what you want?

Edit: Modified circuit for more linear adjustment.
upload_2018-7-23_11-1-37.png
 
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