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[SOLVED] op amp constant current thermal issues

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Zak28

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Is this particular form of a cc circuit vulnerable to the cc value being varied by thermal rise?

Op amp is lm358 please disregard the pot, its bound to have variation with thermals, Im more concerned about the opamp and fet varying current due to thermals.

opamp-cc.png
 

Hi,

* FET thermal drift is compensated by the OPAMP. No considerable error expectable.

* OPAMP input offset drift will cause errors. I_dtift = V_opamp_drift / R_shunt; --> Read OPAMP datasheet about thermal drift.
* R_shunt drift will cause error. Read resistor datasheet about thermal drift.

Klaus
 
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    Zak28

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KlausST said:
Hi,

* FET thermal drift is compensated by the OPAMP. No considerable error expectable.

* OPAMP input offset drift will cause errors. I_dtift = V_opamp_drift / R_shunt; --> Read OPAMP datasheet about thermal drift.
* R_shunt drift will cause error. Read resistor datasheet about thermal drift.

Klaus
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I noted the high value "shunt" is there because lower values such as 20R demand ~8mV with 25v supply. I suppose I can use lower voltages using a zener across the pot and cause it to accept ~20mV for 1mA but this is till quite low. Is the millivolt reference severely susceptible to noise thus distorting the constant current regulation?
 
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Hi,

First decide what current range you want, then decide which shunt to use.

Klaus
 
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    Zak28

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I had a misap with shunt power consumption apparently 1k works well also.
 

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