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Basic Stamp to 74HC595 (serial to parallel) to NPN transistor — Parallax Forums

Basic Stamp to 74HC595 (serial to parallel) to NPN transistor

shmowshmow Posts: 109
edited 2009-07-08 17:37 in BASIC Stamp
Hello All,
is there something I've overlooked (or connected incorrectly).

the Stamp and 74HC595 are working.
the output pin from 74HC595 gives the correct HIGH state voltage 5.0V when true,
but when I connect that output pin of the 74HC595 to the base of the transistor there is a short (or no voltage and the 74HC595 gets hot to the touch).
I tested the transistor separately, and it works, continuity signal sounds between collector and emitter when 5.0V sent to base.
I have a separate voltage supply (24VDC) for the collector connection.
The grounds of both supplies are connected.
Any suggestions would help.
Regards,
Shmow

Comments

  • Mike2545Mike2545 Posts: 433
    edited 2009-07-07 23:04
    Can you provide a schematic?

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    Mike2545

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  • shmowshmow Posts: 109
    edited 2009-07-07 23:10
    Hello Mike2545,
    no, I don't have a schematic. I'm thinking that because I haven't used transisters before there might be a problem.
    If you can suggest an alternative device short of a relay then I'd be much happier.
    Shmow
  • Mike GreenMike Green Posts: 23,101
    edited 2009-07-07 23:15
    You have to have a current limiting resistor between the 74HC595 output and the transistor base. Whenever the input voltage on the transistor base is greater than the base-emitter diode voltage (about 0.6V), that diode conducts and will take as much current as is available (until something melts).

    A 74HC595 output probably can supply 15-20mA without complaining. I'd plan on 10mA. With a 5V output from the 74HC595, less the drop across the output MOSFET (about 0.3V) less the base-emitter voltage (about 0.6V), you've got to drop 4.1V across the resistor at 10mA. Using Ohm's Law, that's R = 4.1V / 0.01A = 410 Ohms. You could probably manage with anything from 330 Ohms to 470 Ohms.
  • shmowshmow Posts: 109
    edited 2009-07-07 23:38
    Hello Mike,
    thanks for the information - very helpful.
    I am curious about one thing. I did connect a 470 ohm resistor in series and it works better. When I checked for continuity, the connection wasn't maintained (between emitter and collector) as long as the signal given to the base lasted. I connected a LED w/ a resistor (470 ohm). The LED lights up when the signal is sent to the base, but similar to the continuity test the LED stays on for about half the amount of time the base is powered.
    Not sure whats going on.
    Shmow
  • shmowshmow Posts: 109
    edited 2009-07-07 23:40
    For clarification, the LED and resistor are in series with the emitter.
  • shmowshmow Posts: 109
    edited 2009-07-08 17:37
    Thanks all for your advice!
    I figured out the sudden interruption of current flow through collector/emitter even when the base is energized. The base was de-energized even though the test lamp (indicating power to the base was high) remained on.
    It all came down to the programming on the stamp - which I corrected and now everything works as I had hoped.
    Thanks very much.
    Cheers,
    Shmow
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