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newbie questions about transistors

ArchiverArchiver Posts: 46,084
edited 2003-07-19 18:43 in General Discussion
I'm putting a transistor on my prototype board to replace a reed relay
that's currently on it. I thought all that was necessary, was to apply
an output pin to the 'base' of the transistor, grounded at the
emitter, and then there were would be a path from the collector to the
emitter.

Well, this works, but ONLY when I put a resistor inline between the
output pin and the base. Without the resistor, the load current
that is allowed through the transistor is very weak, and 'pulses'
instead of remaining steady. I see now, that lots of the web-based
tutorials on transistors say you have to have that resistor.....I just
didn't read carefully enough before.

Will a resistor like that ALWAYS be necessary, or are there some
exceptions, based on the differences in voltage between the control
current and the load current? Can anyone explain why the resistor is
needed?

Thanks,
--Alex

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2003-07-19 16:21
    The resistor is absolutely needed to limit the current out of the BS pin and in
    the transistor base.
    Assuming that you want to limit to 10 ma and that the High voltage of your Stamp
    pin at this current is 4V, then
    (4-0.7)/10 = 0.33 Ko or 330 ohms .
    If the current flowing through the transistor is weak without resistor is
    because the base shorts the BS to 0.7V.
    Stop it, it can destroy the BS pin circuit.
    Regards
    ECO
    Original Message
    From: "atl_guy1138" <atl_guy1138@y...>
    To: <basicstamps@yahoogroups.com>
    Sent: Saturday, July 19, 2003 3:06 PM
    Subject: [noparse][[/noparse]basicstamps] newbie questions about transistors


    > I'm putting a transistor on my prototype board to replace a reed relay
    > that's currently on it. I thought all that was necessary, was to apply
    > an output pin to the 'base' of the transistor, grounded at the
    > emitter, and then there were would be a path from the collector to the
    > emitter.
    >
    > Well, this works, but ONLY when I put a resistor inline between the
    > output pin and the base. Without the resistor, the load current
    > that is allowed through the transistor is very weak, and 'pulses'
    > instead of remaining steady. I see now, that lots of the web-based
    > tutorials on transistors say you have to have that resistor.....I just
    > didn't read carefully enough before.
    >
    > Will a resistor like that ALWAYS be necessary, or are there some
    > exceptions, based on the differences in voltage between the control
    > current and the load current? Can anyone explain why the resistor is
    > needed?
    >
    > Thanks,
    > --Alex
    >
    >
    >
    >
    >
    > To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    > from the same email address that you subscribed. Text in the Subject and Body
    of the message will be ignored.
    >
    >
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    >
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2003-07-19 16:55
    Alex,

    Yes, the resistor is necessary. It prevents too much current flowing from
    the pin, and thus prevents the voltage from dropping too low, making the
    erratic behavior you saw.

    Doing it without the resistor is a sure way to burn out the pin.

    Jonathan

    www.madlabs.info
    Original Message
    From: "atl_guy1138" <atl_guy1138@y...>
    To: <basicstamps@yahoogroups.com>
    Sent: Saturday, July 19, 2003 6:06 AM
    Subject: [noparse][[/noparse]basicstamps] newbie questions about transistors


    > I'm putting a transistor on my prototype board to replace a reed relay
    > that's currently on it. I thought all that was necessary, was to apply
    > an output pin to the 'base' of the transistor, grounded at the
    > emitter, and then there were would be a path from the collector to the
    > emitter.
    >
    > Well, this works, but ONLY when I put a resistor inline between the
    > output pin and the base. Without the resistor, the load current
    > that is allowed through the transistor is very weak, and 'pulses'
    > instead of remaining steady. I see now, that lots of the web-based
    > tutorials on transistors say you have to have that resistor.....I just
    > didn't read carefully enough before.
    >
    > Will a resistor like that ALWAYS be necessary, or are there some
    > exceptions, based on the differences in voltage between the control
    > current and the load current? Can anyone explain why the resistor is
    > needed?
    >
    > Thanks,
    > --Alex
    >
    >
    >
    >
    >
    > To UNSUBSCRIBE, just send mail to:
    > basicstamps-unsubscribe@yahoogroups.com
    > from the same email address that you subscribed. Text in the Subject and
    Body of the message will be ignored.
    >
    >
    > Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
    >
    >
    >
    >
    >
  • ArchiverArchiver Posts: 46,084
    edited 2003-07-19 17:57
    Thank you both for your help! I have it working just as expected now,
    and I managed to correct it before ruining the Stamp.

    Thanks again,
    --Alex



    --- In basicstamps@yahoogroups.com, "Jonathan Peakall" <jpeakall@m...>
    wrote:
    > Alex,
    >
    > Yes, the resistor is necessary. It prevents too much current flowing
    from
    > the pin, and thus prevents the voltage from dropping too low, making
    the
    > erratic behavior you saw.
    >
    > Doing it without the resistor is a sure way to burn out the pin.
    >
    > Jonathan
    >
    > www.madlabs.info
    >
    >
    Original Message
    > From: "atl_guy1138" <atl_guy1138@y...>
    > To: <basicstamps@yahoogroups.com>
    > Sent: Saturday, July 19, 2003 6:06 AM
    > Subject: [noparse][[/noparse]basicstamps] newbie questions about transistors
    >
    >
    > > I'm putting a transistor on my prototype board to replace a reed
    relay
    > > that's currently on it. I thought all that was necessary, was to
    apply
    > > an output pin to the 'base' of the transistor, grounded at the
    > > emitter, and then there were would be a path from the collector to
    the
    > > emitter.
    > >
    > > Well, this works, but ONLY when I put a resistor inline between
    the
    > > output pin and the base. Without the resistor, the load current
    > > that is allowed through the transistor is very weak, and 'pulses'
    > > instead of remaining steady. I see now, that lots of the web-based
    > > tutorials on transistors say you have to have that resistor.....I
    just
    > > didn't read carefully enough before.
    > >
    > > Will a resistor like that ALWAYS be necessary, or are there some
    > > exceptions, based on the differences in voltage between the
    control
    > > current and the load current? Can anyone explain why the resistor
    is
    > > needed?
    > >
    > > Thanks,
    > > --Alex
    > >
    > >
    > >
    > >
    > >
    > > To UNSUBSCRIBE, just send mail to:
    > > basicstamps-unsubscribe@yahoogroups.com
    > > from the same email address that you subscribed. Text in the
    Subject and
    > Body of the message will be ignored.
    > >
    > >
    > > Your use of Yahoo! Groups is subject to
    http://docs.yahoo.com/info/terms/
    > >
    > >
    > >
    > >
    > >
  • ArchiverArchiver Posts: 46,084
    edited 2003-07-19 18:43
    What no one has said yet is:
    A transistor is a current-switching device.
    A 'small' amount of current into the Base,
    allows a larger amount of current to flow
    from the Collector to the Emitter. Yes, it
    does have to have a larger voltage on the
    base than on the emitter for this to happen.

    Thus the purpose of the 10 Kohm or so Base
    resistor is to limit the amount of current
    from the BS2 to the Base. This protects
    both the BS2 and the transistor, which also
    has a limit on how much current can pass
    through it without damage. So yes, you
    always need this resistor.

    Of course, you also need a resistor in the
    Collector circuit, to limit the amount of
    current from Collector to Emitter in the
    transistor.

    --- In basicstamps@yahoogroups.com, "atl_guy1138" <atl_guy1138@y...>
    wrote:
    > I'm putting a transistor on my prototype board to replace a reed
    relay
    > that's currently on it. I thought all that was necessary, was to
    apply
    > an output pin to the 'base' of the transistor, grounded at the
    > emitter, and then there were would be a path from the collector to
    the
    > emitter.
    >
    > Well, this works, but ONLY when I put a resistor inline between the
    > output pin and the base. Without the resistor, the load current
    > that is allowed through the transistor is very weak, and 'pulses'
    > instead of remaining steady. I see now, that lots of the web-based
    > tutorials on transistors say you have to have that resistor.....I
    just
    > didn't read carefully enough before.
    >
    > Will a resistor like that ALWAYS be necessary, or are there some
    > exceptions, based on the differences in voltage between the control
    > current and the load current? Can anyone explain why the resistor
    is
    > needed?
    >
    > Thanks,
    > --Alex
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