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Constant current Source clarification

Discussion in 'General Electronics Discussion' started by PRIYADHARSHINI, Feb 11, 2015.

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  1. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    Hi all,
    I m trying to Design the Constant current source for 1mA,10mA and 0.1mA...
    I have doubts.Here i have enclosed my circuit.
    When i Apply VCC as +5V
    TL431 provides constant 2.5V at the output.
    This can be applied to the opamp (LM358) non inverting input terminal
    So the inverting terminal of the opamp try to compensate the 2.5V of the non inverting terminal.(+Vin--Vin=0)
    Hence my current will be (5-2.5)=2.5V/2.5K=1mA as constant current when i continously changes the resistor 100 ohms..
    But the invering terminal has 3.3V..so i get 5-3.3=1.7V/2.5K=0.68mA as the constant current instead 1mA.
    I have Replace this LM 358 with OPA2340 which also provides the same output current.

    But When i applied VCC as 12V to LM358 it provides the exact results...
    What is the reason for this?
     

    Attached Files:

  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    You have no power to the current source transistor. Move the connection of pin 2 to the source of the FET Q1 and the connect VCC to the top of R1.
    Adam
     
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    11,007
    2,506
    Nov 17, 2011
    Your circuit cannot work. There is no power source for driving a currrent through the output stage.

    This is what a current source looks like (usingh a bipolar transistor but will work with a MOSFET, too).
    You should also refer the TL431 to GND, putting a Pull-Up to Vcc (Swap D1 and R2). This is because GND is the common reference.
     
  4. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    sorry..i forgot to draw that power source.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    The op-amp's inverting input is connected to the wrong place in the circuit. Assuming R1 is the shunt resistor and R3 is the load resistor, you need to connect the inverting input to Q1's source, i.e. the bottom end of R1, and connect the top end of R1 to the positive supply.

    This circuit won't work very well from a 5V supply because (a) the LM358's output has pretty limited voltage range at 5V supply voltage - try a rail-to-rail op-amp; (b) Q1 needs several volts between its gate and source before it will start to conduct - use a Darlington PNP instead.

    Edit: Harald, this is a current source not a current sink; VCC is actually the reference rail.
     
    PRIYADHARSHINI likes this.
  6. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    Thanks for your reply.

    extremely sorry..its my careless mistake...this is my circuit.And i have used Rail to rail op amp OPA2340 .it also provides the same result as LM358..
    when i use 12V VCC it provides exact results for 10,1,0.1 mA
     

    Attached Files:

  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Have a look at this graph from the data sheet.

    IRF9540N Vgs vs. Id.png

    This graph tells you how much voltage you need between the gate and source of the output MOSFET (the -VGS value) to make it conduct properly.

    At VCC=5V and with 2.5V across the source-to-VCC resistor (R1), even with the op-amp output all the way at 0V, there will only be 2.5V of -VGS available for the MOSFET.

    As you can see, this amount of -VGS voltage will not even bias the MOSFET into conduction strongly enough for it to pass 100 mA. It is beyond the left edge of the curves on the graph.

    If you need to run your circuit from 5V you need to do one of these things:
    • Use a MOSFET with a lower -VGS requirement OR
    • Use a lower reference voltage and adjust R1 accordingly
     
    PRIYADHARSHINI and Harald Kapp like this.
  8. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    Thanks for your reply.
    May i use
    BSS84 OR BS250?
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    I don't know offhand. Are you asking me to look at the data sheets for you? I explained the reason for the problem and showed you which graph to look for on the data sheet. You're a grown-up now, you need to be able to do these things for yourself!

    No-spoon-feeding-4kb.jpg

    BTW another option that I mentioned in post #5 is to use a Darlington PNP such as MPSA64 instead of a MOSFET. The accuracy will be a tiny bit less, since even a Darlington with a minimum current gain of 10,000 will require some base current. But that will solve the main problem.
     
  10. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    I have chosen the Mosfet with lower Vgs Requirement..
    To confirm this I have asked to you.
    I will post a reply after check the result using this MOSFET

    thanks for your reply
     
  11. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    I have got the output (9.5mA,0.09mA and 0.9mA) using BSS84 mosfet

    thanks for your reply
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    Good. You know you need to check the guaranteed worst case specifications of the components you use, right? Confirming proper operation with a simulation, or with the particular BSS84 you have in your breadboard, doesn't guarantee it will always work, and if it won't always work, it is not properly engineered. This is why manufacturers provide the minimum and maximum columns in the data sheet.
     
    PRIYADHARSHINI likes this.
  13. PRIYADHARSHINI

    PRIYADHARSHINI

    137
    8
    Feb 6, 2014
    I m using the same circuit which i mentioned above..
    i need to get 4 v to the micro controller when the R3 reaches 390 ohms,3.9K and 39K respectively.
    So i have designed the constant current source for 1mA,10mA and 0.1mA.
    I have increased the resistance linearly .
    but when it reaches above 29 ohms ,1.5K and 29K the current has decreased...
    such that when it reaches 390 ohms , 3.9K and 39K it should provide 3.9V across R3.
    but it results only 2.9552V for 3 Ranges...
    what is the wrong with this circuit?
    can any one explain me?
    Thanks in Advance
     
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