Hello and thank you for your question.
According to the brother.ch website, the adapter for the PT-P300BT printer is the AD-24ESEU. If you search for this model on the digitec website, you will come across the following article:
Brother AD-24ES Power Supply UnitUnfortunately, there is no indication of the maximum voltage and current available.
Ideally, you should consult the user manual for the printer and power supply unit to find out this information in detail.
Sorry I can't be of more help.
Sincerely