Constant current source

Discussion in 'General Electronics Chat' started by nikomas, Apr 6, 2010.

  1. nikomas

    Thread Starter New Member

    Mar 31, 2010
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    Hi everyone.

    I need constant current source in range 50-100mA which i can turn on/off with signal from 30 second timer. Operating voltage is 5V.

    Than you for advice.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    That is a little open ended, so I'll give an open ended schematic.

    [​IMG]

    R2 could be broken into a resistor network to do what you want it to. Bring the base to Vcc and the circuit shuts down.

    I thought about using a LM317, but you don't have much range there on the power supply. The LM317 will use about 3V just to power up.
     
    Last edited: Apr 6, 2010
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If your load is more than about 42 Ohms, you won't be able to get 100mA current through it using a 5v supply and a pass transistor.

    Tell us more about your load.
     
  4. nikomas

    Thread Starter New Member

    Mar 31, 2010
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    I have bridge connection with pt100 resistor in it. Through that bridge connection must pass constant current.
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    231
    Assuming a half bridge with a PT100 and 100Ω resistor with a 50mA source, you will need (100Ω+100Ω)*0.05A=10V plus the overhead for the constant current source. ~15V. ~25V for 100mA. That's also 1/4W of heating in the PT100.

    Ken
     
  6. nikomas

    Thread Starter New Member

    Mar 31, 2010
    11
    0
    Thank you for answering,
    sorry because i didn't think on basic electronic (that i couldn't have that current with that voltage and resistors). I am trying to make constant current hot wire anemometer (overheat this

    http://uk.farnell.com/jsp/displayProduct.jsp?sku=8598525&CMP=e-3e6f

    pt100 resistor with constant current) and actually i don't know how much current i need to measure wind speed in range 0-15m/s.

    If anyone has advice about this i will be grateful.
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Part of the problem is the low voltage. The regulator I put would work well with a 1Ω emitter resistor. At 100ma it would output 4.8-4.9 volts max, and still be a decent regulator. BJT transistors make decent constant current sources.

    Turning it off on demand is a trick though, but it could be done by taking the base to Vcc,
     
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