1mA Constant Current Source?

Discussion in 'General Electronics Chat' started by tracecom, Jan 5, 2016.

  1. tracecom

    Thread Starter AAC Fanatic!

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    What are the pitfalls of the 1mA constant current source shown below?

    Thanks.

    1mA Constant Current Source.PNG


    1mA Constant Current Source.PNG
     
  2. OBW0549

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    1. R2 is way too large, as it will only allow about 12 μA through ZD1; while ZD1 will "kinda-sorta" function as a Zener diode at that current, it won't be very stable or predictable. I'd make R2 about 1.2 kΩ which will give you about a mA of bias current for ZD1.

    2. Since Vbe of a transistor has a negative temperature coefficient of about -2 mV/°C, there will be a small positive temperature coefficient to the 1 mA output current (800 nA/°C, roughly).

    Other than those two things, I can't see anything that stands out unless you're out to achieve very high precision.
     
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  3. RichardO

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    It only gives a couple of volts of compliance (voltage swing) on the load.
     
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  4. hp1729

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    See attachment for an example of how much the voltage across a Zener changes with current through it. At that low a current you may only get about half the rated voltage. So about 0.5 mA out and only about 1 Volt (?).
     
  5. JohnInTX

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  6. cmartinez

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    Wow... I didn't even know those existed. Kind of expensive, though, at $2.28 a pop.
     
  7. RichardO

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    Current regulator diodes are J-FET's with the gate tied to the source. Because of this the minimum voltage allowed across the "diode" is the gate-source threshold voltage of the FET. The 1ma diode in the link looks like it needs around 3 to 4 volts minimum. :(
     
  8. #12

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    Last edited: Jan 5, 2016
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  9. RichardO

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    And for that money you get a very inaccurate current. The 1ma diode in the link can vary between 0.88 and 1.32 ma!
     
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  10. dl324

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    As previously noted, the zener current is too low for the zener to regulate voltage.

    What are the requirements? Will supply voltage vary? Do you require temperature stability? Do you need exactly 1mA? How much headroom do you need? ...
     
  11. JohnInTX

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    Fair enough. Just thought I'd throw it out.
     
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  12. RichardO

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    Glad that you did. You just hit a sensitive spot with me. :rolleyes:

    I was frustrated when I wanted accuracy and compliance that current regulator diodes could not deliver. But that's how we learn. :)
     
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  13. cmartinez

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    Don't we all? ... and there are no shortcuts!
     
  14. crutschow

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    You can use a TL431 voltage reference to make a fairly low-cost and stable current sink as shown here (Figure 14).
    Unfortunately there's no easy way to make it into a high-side current source to a grounded load.
     
  15. Bordodynov

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    May 20, 2015
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    See
    1mA.png
     
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  16. OBW0549

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    Another approach you might consider is a simple current mirror, as in the attached. R1 sets the output current, while emitter ballast resistors R2 and R3 mitigate the effects of any Vbe mismatch between Q1 and Q2. Assuming the +4.5V supply doesn't vary, output current will be constant within a couple percent over a load voltage range of 0V up to within a couple of hundred millivolts of the supply voltage.

    Mirror.png

    Mirror.png
     
    Last edited: Jan 6, 2016
  17. tracecom

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    Lots of good suggestions. Thanks to all for the input.
     
  18. atferrari

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    Maybe not what you have in mind.

    I played with several variations. This is one, bipolar, from the Walt Jung's book, with buffered Vsens.

    Sure it must be simplified and could be to a high degree. The unconnected resistor is there to play with two ranges of input signal. Worked OK. Sensible to spikes in Vdd. Solved with good filtering.

    Para tracecom 01.png

    /EDIT to add:

    Load is R31 which for tests I made 4 Ohms.

    EDIT/
     
  19. dl324

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    If supply voltage will vary, you could combine the TL431 reference from @crutschow with the current mirror from @OBW0549; replacing the ballast resistors and 2 transistors with a dual transistor that would give better matching.
     
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