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I need a discrete, voltage-independent, current limiting circuit — Parallax Forums

I need a discrete, voltage-independent, current limiting circuit

Bobb FwedBobb Fwed Posts: 1,119
edited 2010-07-27 09:04 in General Discussion
I am looking for a (simple as possible) voltage-independent, current limiting circuit, preferably a schematic of discrete components, but I may be able to manage with an IC or two.

Any suggestions would be great, I can't seem to find anything just by googling it, so I am hoping someone knows a circuit, or where to find it.

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Comments

  • tdlivingstdlivings Posts: 437
    edited 2010-07-26 16:52
    Are you looking for inrush current limiting?

    http://www.ametherm.com/inrush-current/inrush-current-faq.html
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-07-26 17:03
    An LM317 (TO220) or LM317L (TO92) can be configured as a simple current limiter (one 3-pin semiconductor and one resistor). See the app schematics in the National Semiconductor datasheets for these parts.

    -Phil
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2010-07-26 17:34
    The Lm317 is likely the best choice if it fits your voltage range, but something similar can be done with a transistor.
    http://www.radio-electronics.com/info/circuits/transistor_current_limiter/transistor_current_limiter.php

    I used a single transistor to limit charging NiCad packs to 1/10th of the maH rating. You might find more about doing so in NiCad charge schematics.

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  • LeonLeon Posts: 7,620
    edited 2010-07-26 18:23
    A BJT as an emitter follower with the base held at a constant voltage makes quite a good constant current source. I used that arrangement once for charging NiCd cells.

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  • Bobb FwedBobb Fwed Posts: 1,119
    edited 2010-07-26 18:36
    I guess I should describe what I am doing. I am making a standalone circuit board. The inputs on the board go to transistors that have a particular threshold current on the base. So I can put a resistor in place, but that only reliably works for a 2V (or so) window. If I am able to put a current limit on it, I can accept a much wider range. I want at least 2.5V to 5.5V, but 1.5V to 7V would be much better. I forgot about the regulator thing. I will look into that.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    April, 2008: when I discovered the answers to all my micro-computational-botherations!

    Some of my objects:
    MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
    Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
    String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
    Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!
  • ercoerco Posts: 20,260
    edited 2010-07-26 21:52
    As previously mentioned, an LM317 is a cheap & quick current limiter. It's a linear regulator, so it acts as a self-adjusting resistor to regulate the current. As such, it will get hotter as more current flows, and also hotter as the input-output voltage difference grows. It has built-in thermal shutdown (hopefully!) if it gets too hot. Heat sink it as best you can.

    See http://diyaudioprojects.com/Technical/Voltage-Regulator/·at the bottom for a current regulator:·a LM317,·a cap·and·a resistor. Doesn't get much simpler than that.

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    ·"If you build it, they will come."

    Post Edited (erco) : 7/26/2010 9:59:20 PM GMT
  • Tracy AllenTracy Allen Posts: 6,666
    edited 2010-07-27 04:51
    Hi Bob,
    Bobb Fwed said...
    ...The inputs on the board go to transistors that have a particular threshold current on the base. So I can put a resistor in place, but that only reliably works for a 2V (or so) window. If I am able to put a current limit on it, I can accept a much wider range. I want at least 2.5V to 5.5V, but 1.5V to 7V would be much better.


    I don't understand why the window is limited in that way. If the input signals are voltages that turn the transistor on and off, the acceptable window with an appropriate resistor in place sould be much larger, easily 1.5 to 7V.

    A current limiting diode may be the part you are looking for.

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    Tracy Allen
    www.emesystems.com
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2010-07-27 09:04
    Voltage regulator should work fine as long as you want 1 amp or less of current regulation, otherwise you need to go to something like a 2N3555 transistor or a darling like a TIP127.

    These won't work all the way down to zero volts without quite a bit of additional circuitry. It has something to do with the 'diode drop' involved. To get to zero, I suspect you need a negative power source as well as positive. But you should easily be able to go up to 30 or so volts.

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    Ain't gadetry a wonderful thing?

    aka G. Herzog [noparse][[/noparse] 黃鶴 ] in Taiwan
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