I am using this DC to DC Step Up Voltage Converter. It is powered by 12VDC adapter and it is adjusted to output 35VDC. The output is measured with Multimeter and the voltage is correct. Onboard Led is on.
Output of this converter goes to L298N to pins (+12VDC and GND). Onboard Led is on. Two extra wires from +5VDC pin and common GND pin (shared with +12VDC) go to Uno R3 pins Vin and GND (next to Vin pin).
Uno R3 is off. Not enough voltage. Multimeter on pins (+5VDC and common GND) shows 2VDC. Other functionality of L298N and Uno R3 are not used at this time.
The 7805 on the L298 board has probably a max input rating of about 25volt.
A 30volt drop (35 to 5) at the ~50mA an Uno draws would dissipate 30*0.05= 1.5watt.
Probably way to high for an smd 7805. Does it get hot?
The V-in pin of an Uno needs 7volt minimum to make a stable 5volt VCC for the Uno.
If you have a stable 5volt, then connect it to the 5volt pin.
Not recommended in this case, because the 5volt regulator on the L298 is used outside it's comfort zone.
Leo..
There is a warning on that L298 page you linked to. It says to use an EXTERNAL 5V supply not rely on the regulator if you have over 12V as a drive voltage. 35V is a lot over 12V!
It sounds like you have hardware problems galore. Post a schematic,not a frizzy thing showing all power supplies, converters, etc as well as all interconnections. Include links to each hardware item that shows technical information, what you have is a start. You need to calculate your power requirements, there is a lot more to this then voltage. You are using a 12VDC adapter, what are the ratings of it, it sounds like you modified it to output 35VDC. Please define your problem better with what you expect and what you are getting.