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: 50mA-500mA constant Dc current source

  1. Mar 5, 2009 #1
    URGENT: 50mA-500mA constant Dc current source

    Hi everyone,

    I am trying to design a 50mA-500mA constant current source for my project. I found the following link. But I think it would give slightly less than the required or designed value.
    http://www.ecircuitcenter.com/Circuits/curr_src1/curr_src1.htm
    SO I change the transistor to a 2N3055 and the opamp to a LM348. I want to know what can i use to simulate this circuit before i can order nay parts.
    Also please suggest if I can use any better circuit.
    I will have to supply two different voltages for this. So I will have to use voltage regulators for the same I understand.

    Its very urgent for me and Can someoen please help me.

    Thanks in advance
    Kari
     
  2. jcsd
  3. Mar 6, 2009 #2
  4. Mar 6, 2009 #3
    Current amplifier

    Hi I am trying to build an adjustable current supply using an opa547, so the thing is the output current is dependent on the load that i am attaching. I was wondering as to what I could do to get rid of this dependence?

    Thanks in Advance
    Kari
     
  5. Mar 6, 2009 #4
    Re: Current amplifier

    It shouldn't. An op-amp circuit doesn't work if you exceed its current or voltage range, in which case there will be a measurable voltage difference across its two inputs. Could that be the case? Is the circuit design a proven one or one of your own? Current supply circuits should be easy to find, e.g. http://ecow.engr.wisc.edu/cgi-bin/get/ece/340/schowalter/opampckts.pdf [Broken] .
     
    Last edited by a moderator: May 4, 2017
  6. Mar 6, 2009 #5

    berkeman

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    Re: URGENT: 50mA-500mA constant Dc current source

    (Merged the two threads on the same subject...)
     
  7. Mar 6, 2009 #6
    Re: URGENT: 50mA-500mA constant Dc current source

    So the output of the opamp should be independent of the load you mean right? please correct me if i am wrong.

    Thanks in advance
    Kari

    << txt speak edited out by berkeman >>
     
    Last edited by a moderator: Mar 6, 2009
  8. Mar 6, 2009 #7

    Redbelly98

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    Re: URGENT: 50mA-500mA constant Dc current source

    In theory, yes. In practice, yes as long as the load current is kept below the maximum current for the op-amp.

    When is your deadline?
     
  9. Mar 6, 2009 #8

    berkeman

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    Re: URGENT: 50mA-500mA constant Dc current source

    No, the output voltage will not be independent of load. Opamps have an output impedance and a maximum current drive specification. Check the datasheet.

    To get more current, you need to include some power amplification circuit after the opamp, either an open-loop amp, or include the power amp in the feedback loop of the opamp (being careful about your phase margin at unity-gain crossover....!).
     
  10. Mar 9, 2009 #9
    Re: URGENT: 50mA-500mA constant Dc current source

    Hi the maximum rating on the opamp is 500 mA and I am not exceeding it. I am worried that I am not able to work properly with the inputs and supplie sto the opamp. I am using an OPA547 . I am reffereing to teh datasheet and there is also a application note on current source design. I am not able to understand it i guess. Please someone help me
    Kari
     
  11. Mar 9, 2009 #10
    Re: URGENT: 50mA-500mA constant Dc current source

    do you have a basic sense of how the circuit works?

    you won't come anywhere near the current limits of the opamp using that circuit you referenced in your first post. all the current to your load is coming directly from VCC. the opamp is basically just outputting a small signal to control that transistor. the transistor does the heavy lifting in the design, so you need to make sure it meets specs.
     
  12. Mar 9, 2009 #11

    berkeman

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    Re: URGENT: 50mA-500mA constant Dc current source

    As Proton says, it's the external pass transistor that is passing the load current, not the opamp. There is no reason to use a 500mA opamp in a current source/sink circuit.

    Have you read the explanation of the circuit that you linked to in Post #1? It's pretty clear how this kind of circuit works. Post specific questions about that explanation if it still doesn't make sense.
     
  13. Mar 9, 2009 #12
    Re: URGENT: 50mA-500mA constant Dc current source

    So let me start from the begining. I am using an OPA 547 high precesion high current opamp to build a power supply. I used the application note for the opamp 547 in its datasheet. SO now the problem is that I am not able to control the output current. I am using TINA Ti simulator for simulating the same. All my issues are with the load. it has to operate over a load of 10-100OHMS and current of 50-500mA.
    1. I used a V+ of 30V
    2. V- of 0V
    3. RCL calculations as shown in the circuits in the datasheet.

    Problems:
    1. Current is not remaining constant
    2. I am worried that i am not understanding teh datasheet specifications like the input signals to be given.
     
  14. Mar 9, 2009 #13

    Averagesupernova

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    Re: URGENT: 50mA-500mA constant Dc current source

    Without knowing anything about the circuit other than what you have posted I don't think there is any way to help you. I would like to know resistor values, power supply voltages, etc.
     
  15. Mar 9, 2009 #14
    Re: URGENT: 50mA-500mA constant Dc current source

    well, yeah, that's your problem. think about what the beta of your transistor is and i bet you need a lot less than 50mA to drive the base. like others are telling you, you need to choose another opamp. a less optimal solution might be to put a dummy load from the output of the opamp to ground of 100 ohms. and then it might be able to drive the output at the correct voltage. but you're wasting a bunch of power by kludging it that way.
     
  16. Mar 9, 2009 #15

    Averagesupernova

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    Re: URGENT: 50mA-500mA constant Dc current source

    Just because the OP doesn't need the current that the op-amp is able to source doesn't mean that the circuit won't work. A 2N3055 doesn't have a very good beta to start with. I want to know resistor values.
     
  17. Mar 10, 2009 #16
    Re: URGENT: 50mA-500mA constant Dc current source

    Hi

    Please c the following circuit. how can I decide the specifications of the opamp?
    I will be choosing a BC 328-40 pnp transistor for the output stage. It will be bale to take a maximum collector current of 800mA, a little more than my requirement. the input to the opamp is through a Zener.

    Will the zener essentially be doing in this circuit? how do i calculate the resistances at the input terminal?

    Please help me. I tried to replace the previous circuit with this now to source more current.

    Thank You
    Kari
     

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  18. Mar 10, 2009 #17

    berkeman

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    Re: URGENT: 50mA-500mA constant Dc current source

    Why is there an alpha in your Iout equation?
     
  19. Mar 10, 2009 #18
    Re: URGENT: 50mA-500mA constant Dc current source

    that was how the circuit was described. it means the transistor parameters change teh load current rt? would that be a bad design?
     
  20. Mar 10, 2009 #19

    berkeman

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    Re: URGENT: 50mA-500mA constant Dc current source

    The opamp is there to take out the variabilities of the transistor's parameters. The only thing you need to worry about is that the transistor can handle the heat being generated as it drops a voltage and passes the current.
     
  21. Mar 10, 2009 #20
    Re: URGENT: 50mA-500mA constant Dc current source

    but what abt the alpha paramenter? that changes with the transistor currents rt? which is teh load current? so not just the value of thr emiter resistance but the alpha value is also varying while we try to vary the load current. rather is it that the alpha value is stablized by the opamp feedback for the comman emitter configuration?
     
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