6.3V with 5V between the GND and Vin pins on the MDBT42Q
I'm not sure I understand - so you're saying that the wires on your breadboard power supply that should supply 5v are actually supplying 6.3v?
yes, and up to 6.7V when the MDBT52Q is disconnected, even though I'm supplying less power than the supply is rated for.
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
I'm not sure I understand - so you're saying that the wires on your breadboard power supply that should supply 5v are actually supplying 6.3v?