Question about NPN darlington pair transistor array

Discussion in 'The Projects Forum' started by sintau.tayua, May 17, 2009.

  1. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
    0
    Hello,
    As part of the project I am working on, I need to use a microcontroller to control 6 small motors. The microcontroller I am using is the PIC16F819, and it can output 25mA maximum at 3.3V (when sourcing current, that is). The motors use 120mA @ 2.6V. Therefore, the output current from the microcontroller needs to be increased: I need a transistor array, one transistor per motor.
    The transistor array I was planning to use is the uln2003a, from texas instruments. This is a series of 7 npn darlington pairs, with the collectors connected to the load being controlled, and the emittors connected to ground (battery negative).

    My question is, what happens when the base voltage (3.3V) is higher than the collector voltage (close to zero, because it's after the load, not before it)? Will it hurt the transistors? Do I need to put a resistor in each signal between the microcontroller and the transistor array? I have actually planed on doing this, but if I can get away with just one resistor between the uln2003a and ground, that would advantageous in my situation.
    I realise that NPN transistors are 'source' transistors (conduct when high voltage is applied to base), but nowhere can I find what happens when you 'super-source' a transistor.

    Any ideas?

    And in advance, I want to say thank you very much for any assistance you give. It's greatly appreciated!:)
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If you connect the output of the PIC directly to the base of the transistor you will find out which one blows out first. :p

    You have to use a resistor between output of the PIC and the base of the transistor. Its values depends on the gain of the transistor as to drive it into full conduction.

    From the datasheet I saw that the gain is 1000, thus a 1K resistor will saturate the transistor.
     
  3. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    Actually, it's not needed with the ULN2003A or ULN2004A. The ULN2003A and ULN2004A employ an integral 2.7K and a 10.5K Base resistor respectively. They also contain Diode protection for inductive loads on both the Base and Collectors. They are excellent chips.
     
  4. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
    0
    That sounds very nice!

    So, just to make sure I've got this right: all npn transistors must not be supplied with a base voltage greater than the collector voltage. If a microcontroller outputs a greater voltage, it must be 'dropped down' with a resistor. Luckily, the uln2003a has a 2.7K resistor in series with the base input, inside the chip itself. Have I got this right?

    Honestly, the news of the integrated resistors makes me very happy - This particular part of my project is very size-sensitive, so being able to remove the resistors will be a big help.

    Thanks again for all your help,
    Sintau.langer
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Thanks CDrive for the correction. I missed that on the datasheet.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    A forward biased BJT is treated like a diode junction. The voltage between the base and emitter can't exceed ~ .7 volts. The base - emitter junction does this naturally as long as you provide sufficient series resistance. Remember always that BJTs are current sensitive devices.

    Common Emitter Amplifier/Switch:
    You can have (for example) a 5V collector voltage and a base control voltage of 25 volts as long as you use the appropriate value series base resistor to limit the base current below the manufacturers specified max value.

    The NLN2003A & NLN2004A are Darlington transistors so the Base-Emitter voltage will not exceed ~ 1.4V. Because the base resistors are internal you can't measure this directly. You can feed the output of your Micro directly to its inputs with no fear of damage to either.
    This forum has tutorial sections. I would suggest reading through some of them for more detailed info. ;)
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
    99
    No problem. It happens! ;)
     
  8. sintau.tayua

    Thread Starter Member

    May 11, 2009
    11
    0
    Thanks guys!
     
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