Shop OBEX P1 Docs P2 Docs Learn Events
Looking for elegant solution: Have an output capable of 4mA but I need at least 10mA — Parallax Forums

Looking for elegant solution: Have an output capable of 4mA but I need at least 10mA

MicksterMickster Posts: 2,721
edited 2012-05-02 10:52 in General Discussion
@24vDC.

I know there are many ways to achieve this but is there some sort of device out there that can do this in-line without having to install another circuit board (because there are several outputs), etc?

Cheers!

Mickster

Comments

  • Mike GMike G Posts: 2,702
    edited 2012-05-02 08:37
    A transistor? Your post is a bit vague.
  • MicksterMickster Posts: 2,721
    edited 2012-05-02 08:43
    Mike G wrote: »
    A transistor? Your post is a bit vague.

    Yes, a transistor or opto-coupler would work but it's a permanent installation and it needs to look professional. I'm trying to avoid soldering flying leads to transistors and also trying to avoid having to install another board with, say, SSR's. I just thought I'd throw the question out there in case someone knew of a simple ready-made in-line solution.

    Cheers!

    Mickster
  • Mike GMike G Posts: 2,702
    edited 2012-05-02 08:55
    2.5 * 4mA source = 10mA.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-05-02 09:10
    I guess Mike G is saying is to use 3pins as outputs and change the software to run these pins in tandem.
    Are there extra unused pins available?
  • Mike GMike G Posts: 2,702
    edited 2012-05-02 09:28
    Ohm and Kirchhoff have something to say on the matter. Basically, given the problem, "I have a fixed 4mA source but need 10mA". We must add 6mA which means adding another current source; Kirchhoff current law. Otherwise I = V/R
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2012-05-02 09:41
    Mickster,

    You didn't specify a voltage or whether the output has to source 10 mA, sink 10 mA, or both. Whatever the case, though, you're simply not going to get there with an inline (i.e two-terminal) device. IOW, the extra current has to come from somewhere beside the 4mA-limited pin.

    For sourcing current, an NPN emitter follower is probably the simplest non-inverting add-on; for sinking, a PNP follower will work. If inversion is permitted, a common-emitter arrangement will give you a wider voltage swing. Or you could use a small SMT buffer chip.

    If the pin in question is on a DIP chip, and if you can get a small (i.e. TSSOP) equivalent, you can make an adapter PCB that includes the extra buffering.

    -Phil
  • MicksterMickster Posts: 2,721
    edited 2012-05-02 10:01
    Mike G wrote: »
    Ohm and Kirchhoff have something to say on the matter. Basically, given the problem, "I have a fixed 4mA source but need 10mA". We must add 6mA which means adding another current source; Kirchhoff current law. Otherwise I = V/R

    Yeah, I've been aware of that for 35+ years, thanks. I'm unable to simplify the question any further so I'll explain a different way. On many occasion I have had to improvise a design only to find later that a ready-made, off-the-shelf solution existed. For all I know, there exists the equivalent of an encapsulated, maybe transistor-based, device that would solve this problem rather than me having to concoct something that is not an off-the-shelf item. Oh well, I sent my guy to Radio Shack anyway.

    Mickster.
  • tonyp12tonyp12 Posts: 1,951
    edited 2012-05-02 10:28
    You not gonna find a already made pin-out-booster at radio shack.

    Get a transistor (probably a pnp if it's a high side drive) solder a 2k resistor to it's base.
    Get some heat shrink wrap, put a little hole in the side on it for your center pin to poke out.
    heat up and you're done.

    But as you don’t say much what voltage you need or have ( I guess the pin-out is 24v dc? )
    high/lowside drive
    sink,source or both it’s hard to help you.
  • MicksterMickster Posts: 2,721
    edited 2012-05-02 10:52
    tonyp12 wrote: »
    You not gonna find a already made pin-out-booster at radio shack.

    Get a transistor (probably a pnp if it's a high side drive) solder a 2k resistor to it's base.
    Get some heat shrink wrap, put a little hole in the side on it for your center pin to poke out.
    heat up and you're done.

    But as you don’t say much what voltage you need or have ( I guess the pin-out is 24v dc? )
    high/lowside drive
    sink,source or both it’s hard to help you.


    LOL....Already done...but using an NPN (2N3904)...

    Many thanks!

    Mickster

    P.S. The "assistant" at RS was insisting that my guy needed and ohm value and a wattage (for the transistor) or he couldn't be of help...LOL
Sign In or Register to comment.