VAGCOM Adapter: Power Consumption & Requirements

Hello again.

Today, I temporarily modified the K-Line output of my adapter by adding an output transistor (following the Noxon/Stautz principle, but using resistor values from version 2.0 of the construction guide).

It was already clear from yesterday's discussion (in the thread "Need Input...") that an adapter without these transistors places slightly higher demands on the control currents of the PC/laptop.

I just performed some comparative measurements with and without an output transistor. The criterion was achieving an output voltage of 0.5 volts on the K or L line, as specified by the vehicle, using a pull-up resistor of 2.2 kOhms (e.g., R 7 in the construction proposal).

Without an output transistor, the input (RTS or TXD) needs to be supplied with approximately 3 mA. This could pose a problem, especially for laptops with weak COM ports (since 5 volts across a 3 kOhm resistor results in a current delivery of only about 1.7 mA).
This can also explain the partial need to significantly reduce the value of R2 in construction proposal 1.1 in order for the adapter to function properly.

A simple transistor can achieve the same result with as little as 0.3 mA, which most PCs/laptops should easily be able to provide.

Therefore, for the sake of operational safety, it is recommended to use output transistors.

If a computer can provide at least 5 mA on the COM port, version 1.1 may be sufficient.