Classic Circuits You Should Know: Constant Current Source
114,300
Published 2019-01-11
You can use a single NPN BJT to make a simple constant current source. The output of the BJT is a function of the input current to the base times the BETA (amplification). yes there are more stable and better ways to do this but this circuit if the foundation. You can set the output current by dividing the voltage at the base - .6 by the emitter resistance.
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FAQ:
Me: Paul, 49 from USA
Education: United States Navy, University of California at San Diego B.S. E.E., University of Pittsburgh M.S. E.E.
Experience: United States Navy STS, Bayer Intnl Process Engineer, C.C.A.C AP of EE
Current: Retired
Health: BAD (Congestive Heart Failure)
Hobbies: Electronics, flying, amateur radio, music (classic rock)
All Comments (21)
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I admire your willingness to teach but need to point out that a true constant current circuit will deliver the same current not just into a varying load but also from variances in Vcc. What you have is a current sink. If you vary Vcc even fractionally, the collector current (the load current) will shift accordingly. It's no longer constant. If you further expand your equation for Vb it approximates Vb = Vcc*(R2/(R2+R1)). This you can then substitute for Vb in your equation. If Vcc changes then Vb changes and so IL changes. Your circuit requires Vcc to be rock solid regulated for constant current to be realised. Not viable for loads that run directly off a battery for instance where terminal voltage falls over time as load is applied.
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Great video. If you wanted to make this circuit a variable current source, all you need to do is replace R1 and R2 with a potentiometer with the wiper blade going to the base of the transistor. To vary the current, just turn the knob of the potentiometer increasing or decreasing the current to whatever value you want. I would start with a 100K potentiometer first and see how it behaves. Other potentiometers are also possible.
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"Thank You", thank you very much for the diagram and formulas. That was REALLY great ๐๐๐
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Very usefull and helpfull video, thanks :). And Happy Birthday Paul with your 50th!!! anniversary!!!!!!!!!
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Great tutorial on constructing a constant current source. Theory then hands on labs are the only way to learn electronics. Thanks Paul and happy birthday.
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Now this is the content I'm looking for. Happy Birthday and keep up the good work.
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That helped to take some of the mystery out of CCS for me. Thanks!
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Another great video this is a circuit we all should all know deep down. Now I actually understand the math part of it. Thanks Paul you rock and have a great birthday ๐.
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Awesome circuit and well explained, thank you! I am trying to design my own current source to use in a zero temperature coefficient application. I was using the LM334 with a diode (opposite tempcos) but the LM334 is too large for my board. Hoping to try this design out - thanks again!
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Good stuff, thanks. And as many who commented before me have said, happy birthday sir! Also, by the bits of all this I am grasping, if you were to put a potentiometer in r2, that would give you the same scenario as the current adjustment potentiometer on a benchtop power supply? Thanks for the stimulation of mental growth in me and my kids as well.
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Happy Birthday Paul! Thanks for yet another very helpful video!
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Happy Birthday buddy and many thanks for these videos. They are both interesting and educational. Please keep them coming as they brighten my day when I see a new video from you. Many thanks and have fun, Joe
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Cool, been looking for just this circuit for use when playing with various high output LED's from China!
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I thought I had left my multimeter on.
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Your instructions are brilliant
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You really have this down. I can't seem to just dream stuff up like you. I feel I need to google circuits. It's best to understand it. Thanks
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Let my start by saying Happy Birthday But I will not give your a passing grade for that constant current source. Replace R2 with a red LED or two diodes would help because it will reduce its dependency on input voltage, but a even better solution is a JFET, it is a constant current source sometimes without external components, sometimes you need a single resistor. None of the above is perfect, you need something with a stable reference and more gain to be closer to perfect. In one of my designs I needed a a led (Opto coupler) to work from as low possible voltage to as high as possible voltage, a JFET did that nicely, my final design is rated from 4V to 25V and includes a polarity protection diode, a opto coupler, a red LED and a JFET with resistor to limit current. Low limit is due to voltage drops in the different part, high limit is due to the JFET.
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Really liked this video. Classic circuits are just what I need to know, please do more. Iโm going to build this circuit and have a play. Thanks ๐ ๐ด๓ ง๓ ข๓ ฅ๓ ฎ๓ ง๓ ฟ
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Great video! And happy birthday Paul! :)
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Trรจs bien merci ๐
