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Constant Current sourc

SS_hwdesign
Prodigy 250 points
Other Parts Discussed in Thread: TL431, LM324

We are designing constant current source , circuit as shown in TI application note AN-1559 Practical RTD interface solution. (Refer Figure 2 page 4) . link : http://www.ti.com/lit/an/snoa481b/snoa481b.pdf

We are designing circuit using TL431 as  reference ( 2.5V refernce ) , LM324 OPAMP , R8:  2.49K .  

R6 = R10 = R9 = R7 = 22K . Single supply for OPAMP 5V , 100 ohm resistor is connected as Load .

Expected : Current through R8 is 1ma ,

Observation : Refernce Voltage generated : 2.496 V , Voltage Across R8 :  2.86V  :  Current through R8 1.15 ma ,

Question :

1 ) Why Voltage across R8 is not 2.5V ?  Is any changes required in value of  R6,R10,R9,R7 ?


2) On what basis the value of R6,R10,R9,R7 are desided ?

3) By only changing R8 what will be range of Constant current that we can design ? 

 

  • 0
    TI__Guru 61996 points

    Hello Saraubh,

    #1)  Are you using 1% or better resistors? Resistor tolerances will add to the error.

    The LM324 is not a precision op-amp, so each amplifier could add several volts of offset adding more to the error.

    #2) R6/7/9/10 form a differential amplifier around U1C. U1B buffers the sense voltage and provides a low impedance drive to R6.

    See AN1515 for more info about current sources.

    AN-1515 A Comprehensive Study of the Howland Current Pump (Rev. A) (PDF 165 KB)

    #3) The upper current limit is set by the positive output swing of U1C (5V) and/or the maximum output current of U1 (a few mA). The minimum current is limited by the bias current of U1B. Also observe tthe common mode input voltage limits of the buffer amp...

    Regards,