2023-07-30, 09:00 PM
(This post was last modified: 2023-07-30, 09:01 PM by Adrian.
Edit Reason: added post-script
)
(2023-07-30, 08:01 PM)holgerw Wrote: i would modify R22 to half or put another 3K3 on top of it.
The lower Impedance of the circuit is better against Peaks.
ym2cents
With R21 being 20K, that would push the on-voltage (when SEATALK_DATA is at 12V) to 0.62V instead of 1.2V. I.e. I'd be worried that the transistor doesn't fully turn on, especially if the 12V supply is on the low side, or the transistor's VBE on the upper end of its specification.
An alternative would be to leave R21 at 10K and add 3K3 in parallel with R22. That would set the voltage range at the base of the transistor to 0.3-0.9V.
To calculate the voltage at the base, I look at the voltage divider formed by R20, R21 and R22. With SEATALK_DATA high, the voltage divider is from 12V through R20+R21 and R22. With SEATALK_DATA low, it is from the
diodes anode (~2V) through R21 and R22. We want the base of the transistor swing as far above and below 0.6V as possible.
Edir: Oops. I think the second paragraph is exactly what Holger suggests!