Easy question re: transistors

[Alex Bryant]

I'm trying to switch a series of 6V lamps using a microcontroller and
a battery pack, and a bunch of transistors. Does it make any
difference whether the lamps are upstream or downstream (for the lack
of a better term) of the transistors? If not, is there a general
convention for this in designing a circuit?

Thanks
--Alex

 

[KevinR]

On 21 Jul 2004 08:12:51 -0700, alex1138@charter.net (Alex Bryant)
wrote:

I'm trying to switch a series of 6V lamps using a microcontroller and
a battery pack, and a bunch of transistors. Does it make any
difference whether the lamps are upstream or downstream (for the lack
of a better term) of the transistors? If not, is there a general
convention for this in designing a circuit?

Thanks
--Alex

My prefference would be to put the transistors up-stream of the bulb,
then you have some resistance in the emiter of the transistor which
means you need far less current to drive the transistor. I am under
the impression that grounding the emiter is generally frowned upon

But then again, I would rather use PNP transistors and hook the emiter
up to the supply rail and pull the base down via a resistor. Also put
a resistor between the supply and the base to keep the transistor off
when it's meant to be, otherwise leakage in the transistor can
partialy turn it on (or something like that).

And then again, if I was driving several bulbs, I'd be more inclined
to use ULN2803 or ULN2804 driver IC. These are open collector drivers
and will allow the switching of lamps driven from a 6V supply while
keeping your micro on a 5V supply. They pin-out nicely too, with
all the inputs down one side and the outputs down the other.
the 2803 version is driven from 5V TTL and the 2804 from 12V CMOS

 

[Gareth]

KevinR wrote:

On 21 Jul 2004 08:12:51 -0700, alex1138@charter.net (Alex Bryant)
wrote:


I'm trying to switch a series of 6V lamps using a microcontroller and
a battery pack, and a bunch of transistors. Does it make any
difference whether the lamps are upstream or downstream (for the lack
of a better term) of the transistors? If not, is there a general
convention for this in designing a circuit?

Thanks
--Alex


My prefference would be to put the transistors up-stream of the bulb,
then you have some resistance in the emiter of the transistor which
means you need far less current to drive the transistor. I am under
the impression that grounding the emiter is generally frowned upon

I do not think this is correct.

The easiest thing to do is to use an NPN transistor to switch the ground
side of the bulb. Connect the emitter to ground and the collector to
the bulb. Connect the other side of the bulb to +6V. Connect the base
of the transistor to your microcontroller output via a resistor. You
should also connect the grounds of the 5V and 6V supply together.

Your resistor should be low enough that it allows enough current to
saturate the transistor (i.e. turn it on fully).

But then again, I would rather use PNP transistors and hook the emiter
up to the supply rail and pull the base down via a resistor. Also put
a resistor between the supply and the base to keep the transistor off
when it's meant to be, otherwise leakage in the transistor can
partialy turn it on (or something like that).

That can work but it is a bit more difficult since the base of the PNP
transistor needs to be within 0.6V of the +6V supply to turn it off
fully, this means it needs to be higher than the output of the 5V
microcontroller. This can be done but the NPN transistor on the ground
side is easier and more common.

And then again, if I was driving several bulbs, I'd be more inclined
to use ULN2803 or ULN2804 driver IC. These are open collector drivers
and will allow the switching of lamps driven from a 6V supply while
keeping your micro on a 5V supply. They pin-out nicely too, with
all the inputs down one side and the outputs down the other.
the 2803 version is driven from 5V TTL and the 2804 from 12V CMOS

This last bit is good advice.

--
-----------------------------------------------------------------------
To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[Jamie]

i think what your referring to is using a emitter follower which
will only output the amount of voltage - aprox 0.6 to the lamps because
the collector would have to be fixed to a supply no more than what can
be driven voltage level wise from the output signal of the processor.
thus if the process only outputs lets say 5 volts on its io lines then
aprox 4.5 volts at best is all you will get to your lamps, this signal
ofcourse needs to drive the base of the transistor.
the other way is the open collector, this way your output signal needs
to drive the transistor base using a resistor to force the transistor
into saturation to reduce heating effects in the transistor.
the lamp is connected to the collector of the transistor and the
other side of the lamp can be connected to a source up to the max Vcbe etc.
this is the way that most go since one could use higher voltage lamps
and only need a low voltage signal to drive the transistor base via a
bias resistor.



Alex Bryant wrote:

I'm trying to switch a series of 6V lamps using a microcontroller and
a battery pack, and a bunch of transistors. Does it make any
difference whether the lamps are upstream or downstream (for the lack
of a better term) of the transistors? If not, is there a general
convention for this in designing a circuit?

Thanks
--Alex