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A better constant current source. Bench supply

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by diy didi, Aug 18, 2017.

  1. diy didi

    diy didi Member

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    I built 2 bench power supplies a couple of years ago. One of the sections of the circuit uses a conventional constant current source with a zener and npn transistor. Textbook stuff.
    They used a 3,3V zener which isn't great and varies with temperature along with the Vbe junction of the transistor.
    Is there s better way to reference the 3,3V?
    Maybe a TL431?
     
  2. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    TL431 is good.
    Watch, it needs a minimum current. (max to....too much heat will cause trouble)
    TL431 is much better than a Zener.
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Actually a 3.3V zener is the best zener to use, as it's got pretty close to zero temperature co-efficient - which is probably why it was used.

    Below that the co-efficient is one way, higher then that it's the other - 3.3 is the 'sweet spot'.

    Obviously a TL341 would be far superior though, and is easy to use as a zener replacement (with a couple of resistors).
     
  4. dave

    Dave New Member

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  5. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    I used a zener as a reference once... I don't think the zener is the problem as much as the current limiting resistor... I had to use very low PPM/°C ±5... The same as I use in the 4~20mA convertor... The normal resistors are the temperature sensors...
     
  6. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    I like the 5.1 and 4.7 Zener. +/- 0.03%/C
    upload_2017-8-19_7-13-34.png
    I must not understand what Mr. Rogers said. I can not see how a small change in the resistor will effect the voltage (much). The left hand two curves are for the 4.7 and 5.1 Zener. A 1% change in resistor will cause a 1% change in current and maybe a 0.1% in voltage.
    It is hard to find a data sheet on a old 5% resistor (through home). The first one I found has 500ppm/C.
    upload_2017-8-19_7-24-32.png
    I went back and found a data sheet from 1950 showing a CC resistor with 0.2%/C temp. A Carbon Comp resistor should never be used where you care about temperature. LOL So a resistor change of 0.2% could cause a voltage change of about 0.02%.
     

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  7. Ian Rogers

    Ian Rogers Super Moderator Most Helpful Member

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    I'm not the best when messing with analogue... But when using a zener diode place your fingers on the limiting resistor and the voltage does move enough to annoy!! when I swapped it out for the low PPM type, it was far more stable... It must have been a carbon type, maybe I wasn't driving the zener with enough current!!
     
  8. dr pepper

    dr pepper Well-Known Member Most Helpful Member

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    Zeners are used as noise sources so dont forget to put a cap across it, I seem to recall a 5v1 gives a good stability.
    A good current source just feeding a resistor would give a reasonable voltage ref, it depends how accurate you want to be.
    I agree with the Tl431, its a very popular device.
     
  9. crutschow

    crutschow Well-Known Member Most Helpful Member

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    If you want to compensate for the BJT Vbe change of about -2mV/°C then you could use a 5.2V zener which has about a +2mV/°C change with temperature.
     
    • Agree Agree x 1
  10. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    Please, what part number? 5.1V Zener -2mV/C
     
  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    That was a typo.
    It should have said 5.6V such as the 1N5232b shown in your post #5.
     
    • Informative Informative x 1

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