Tristated output to two common Base transistor issue

[RussMartin]

Saturation is not possible, here are the results

Thanks for showing us an in situ analysis. Your measurements should enable you to immediately infer what is wrong:

The circuit, as configured, doesn't permit enough voltage across the transistor! (What is Vf for the LEDs?)

If you remove the 22 ohm resistor to ground, you will get different results.

[Luciano]

If you remove the 22 ohm resistor to ground, you will get different results.

This is not possible, the circuit needs the voltage divider.



Roger will have to modify his design.

Best regards,

Luciano

[RussMartin]

If you remove the 22 ohm resistor to ground, you will get different results.

This is not possible, the circuit needs the voltage divider.

. . . and it is the voltage divider that is causing the problem.

I was not referring to your original circuit, I was referring to your test circuit, suggesting you compare your results by omitting one resistor.

Maybe we should start over. Let's go back to Roger's original circuit, which can be redrawn this way:

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1987&stc=1&d=118927164 6">

This is a first cousin to a half H-bridge.

You will recall the only problem he observed was that, when the input was tri-stated to high-Z, both LEDs were on, although not fully bright. This fault was traced to the resistors built into the transistors. Alain suggested, and I agreed, that the circuit be attempted with "classic" transistors (i.e., transistors without built-in resistors).

Has anyone tried that yet?

[Luciano]

Hi Russ,

The result for your circuit:



Best regards,

Luciano

[RussMartin]

Beg pardon, my friend; not my circuit. It is a generic redrawing of Roger's original circuit. Which transistors did you use? And did you toast them?

Thanks for the drawing! It illustrates the problem very nicely! (The red current path is through two diode junctions.)

[Luciano]

Hi Russ,

The transistors are still working.

With this circuit the LEDs are always ON.

NPN BC546
http://www.fairchildsemi.com/ds/BC/BC546.pdf

PNP BC558
http://www.fairchildsemi.com/ds/BC/BC558.pdf

Best regards,

Luciano

[Luciano]

Hi Russ,

My interest was to find a solution for the LEDs board that Roger has already made.

If there is no solution for the original LED board, then a new board with I/O port
expanders ICs + transistors is in my opinion the way to go.
(One I/O + 1 transistor for each 50mA LED). That way all the LEDs can be
ON at the same time and each LED can be individually controlled.

Best regards,

Luciano

[RussMartin]

The transistors are still working.

Well, that's good news!

If you have a few extra minutes, try the circuit this way:

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1992&stc=1&d=118928211 2">

Let us know how that comes out. You may want to tinker with the values for Rcl, maybe starting with something between 100 and 150 ohms.

Do you have some small-signal P-channel and N-channel MOSFETs laying around?