Just use a real 5 V adapter - "phone charger". 
You need 5 V. "Boffin" has spooked you to start with but corrected himself. Just check the voltage with your multimeter in case it is some piece of rubbish. A branded device such as Apple or Samsung should be the real deal or one you buy in a local shop. Ridiculously cheap ones direct from China may not only be poorly regulated, but unsafe.