vendredi 11 novembre 2011

Possible Big design prob found

Argh ...
After reviewing schematics (to adapt them to the teensy) i've found a possible design bug in the schematics.
If my tought are right, i would not be possible to stop the transistor to saturate.

So imagine few values :
Speak and Spell battery + 5V (when Batteries are not new)
Speak and Spell battery : - 12V
Arduino out 1: 5v / 0 : 0V

So when Arduino output =0 , the transistor will saturate and you will 5V on the Speak and Spell BUS
When Arduino output is 5V, the transistor will stop saturate (VBE-0V)and  you will get -12V on the S&S Bus
Everything is fine..

BUT if you put new batteries on the Speak and Spell then the VBATT + is ~ 6.5 V. So  to stop saturate the Transistor we should have min 5.8 V on his base. (VBE 0.7 V).

We have few option to correct this :
- Use darlington transistor (VBE 1.4) 
- Connect Transistor emmiter to regulated 5V supply
- add a diode + a 5V1 zener on emitter of the transistor.

The PCB has been sent to Prod and they are not arrived yet.. and i''ve already spotted a design error.
Why i didn't spotted this during the testing .. ? because i'm using old batteries .. and Speak and spell Battery voltage is 5.3 V. Grr.......

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