Author Topic: Creating a Precision 25-30 mA Current Source.  (Read 382 times)

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Offline ELzekio

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Creating a Precision 25-30 mA Current Source.
« on: October 24, 2020, 09:21:29 pm »
Hello Everyone,

I had a question about potentially creating a precision 25 to 30 mA current source in a similar fashion to what Dave did for creating a 1mA current source in his video here:

You can see from this video that the REF102 is being used as the Precision voltage reference for the 1mA current source which works just fine because the current being generated (1mA) is well below the 10 mA maximum output of the REF102.

This of course is not going to work for a 25 - 30 milliamp current source because that is too much output for the REF102.

I’ve been looking up some potential voltage references on Mouser and it looks like they are few and far between if you want to go above 10 mA at high precision. Most only options for above 10 mA, are 5 and some 10v. I think either 5volt or 10 volt output would work just fine.

The following is an option I found on mouser:

It is a small SO-8 package with a good initial accuracy and it says it has a higher shunt output of 100mA. Seems quite high for such a small package. Other than that, it seems it could be a very good fit. My only concern is how hot it might get outputting 30mA and if that would affect its stability long term.

Basically, if possible, I would like to create a current source that is accurate enough for me to just tell a piece of data acquisition software (LabView) that it is 30 mA and not have to continuously measure the current via an analog input.  I would have this unit running for long extended periods of time.

Y’all think it would be worth giving this chip a try with an OPA277 and precision resistor? For the prototyping process I would probably just put the SO-8 chip on one of those SO-8 to through-hole adapters.

Most other through hole voltage references seem to have such high temperature coefficients and less accuracy, so I am not sure that they would be a good option or not.

Let me know if you have any suggestions, tips, or resources you can point me toward!
« Last Edit: October 24, 2020, 09:24:45 pm by ELzekio »

Offline Benta

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Re: Creating a Precision 25-30 mA Current Source.
« Reply #1 on: October 24, 2020, 10:13:44 pm »
Just buffer the output from the REF102 with another precision opamp capable of delivering the current. Watch out for the supply voltages that they fit the I/O range.

« Last Edit: October 24, 2020, 10:18:22 pm by Benta »

Offline MasterT

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Re: Creating a Precision 25-30 mA Current Source.
« Reply #2 on: October 24, 2020, 11:34:25 pm »
LM334 in TO-92 package

Offline David Hess

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Re: Creating a Precision 25-30 mA Current Source.
« Reply #3 on: October 25, 2020, 11:07:39 pm »
Add a pnp transistor into the positive supply line to bypass most the current around the REF102 to the resistor.  The base-emitter shunt resistor should be about 330 ohms for the REF102 to supply 2 milliamps.

Note that maintaining the inherent accuracy of the REF102 will be difficult because of self heating of the sense resistor.  Use a larger power part to minimize this.

Online Zero999

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Re: Creating a Precision 25-30 mA Current Source.
« Reply #4 on: October 26, 2020, 11:41:21 am »
LM334 in TO-92 package
The LM334 is only ratedd to 10mA, so no good on its own, but will work at higher currents with the additon of an external transistor: see figure 20 of the data sheet. It also has a temperature coefficient of +0.33%/°C, which can be virtually eliminated with an extra diode and resistor: figure 15.

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