Tristated output to two common Base transistor issue

[Luciano]

Hi Russ,

The circuit is for the brake light of a car, so the circuit gets the power
only when the brake pedal is pressed. Also consider that power is generated
by the alternator, so this current will also decelerate the car when the driver
activates the brakes ...

Best regards,

Luciano

[RussMartin]

The circuit is for the brake light of a car, so the circuit gets the power only when the brake pedal is pressed.

Aha! Thanks! In that case, Roger should go ahead and give your circuit a try!

[Luciano]

Hi,

There are ony two 22 ohm resistors for the 20 LEDs.


Best regards,

Luciano

[RussMartin]

BrianT advised me by private message that I had oversimplified my earlier analysis, and he is right. I looked only at the voltage divider created by the resisitors built into the transistors and neglected to consider the diode drop across the base-emitter junctions of the transistors. The circuit, redrawn, looks like this:

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1980&stc=1&d=118913663 8">

With the diode junctions in place (a nominal 0.7 VDC drop across each), two voltages change: At A, 4.3 volts; at C, 0.7 volt. At B, it remains 2.5 volts. Both transistors are still "turned on" if there is no voltage present (a high-impedance state) at B.

[Luciano]

Hi Roger,

After testing my circuit on a breadboard I have found out
that the circuit works, but the current available for the LEDs
is too low for your application where the forward current of
the LEDs is 50mA.

Best regards,

Luciano

[flotulopex]

So,

Seems I have no other choice but to buy a 20 Outputs PIC.

Thank you all anyway for all your propositions and the time you spent on my problem.

BTW Luciano, I made the circuit too and yes, available current is far to low...

[RussMartin]

Luciano: After testing my circuit on a breadboard I have found out
that the circuit works, but the current available for the LEDs
is too low for your application where the forward current of
the LEDs is 50mA.

Roger: I made the circuit too and yes, available current is far to low...

Is it possible that the series base resistor (2.2K) is too large? The "brute force" method to assure that the switching transistor saturates is to give the base about ten times as much current as it needs to turn on.

Remember, the base current controls the collector current: Not enough base current means not enough collector current. Try 820 ohms, then on down (680, 560, and so on) until it works.

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I did a little looking on line. Is this the kind of thing you are doing?

http://www.customdynamics.com/knight..._light_bar.htm

and the cool video:

http://www.customdynamics.com/videos...derz_video.htm