Loosing PicAxe chips

[inglewoodpete]

The scenario for damaging a chip with a laptop being powered by an unearthed switchmode power supply goes something like this:

The PICAXE or potentially any microcontroller chip is being powered by a quality power supply, with the 0v line properly earthed. The laptop is powered by an unearthed power supply. The laptop's serial connection 0v line is not at earth potential unless it is properly earthed via the mains Neutral wire. With some unearthed power supplies, this may not occur: the 0v line can drift away from neutral (earth) potential. The 3.5mm stereo plug or the programming lead is inserted into the programming socket. Unlike typical audio leads, the tip of the programming lead's plug is at the computer's wandering 'ground' potential. The tip of the plug is likely to touch the SerIn connection, then the SerOut connection inside the stereo socket as it is inserted. In this situation, if the potential between the laptop "earth" and the PICAXE's power supply's real earth is high enough, the input or output pin's internal circuitry can be damaged.

If this was what caused the damage, then powering the circuit from a battery pack whenever connected to the laptop will practically eliminate this risk.

 

[Alan Sailer]

That is an interesting scenario. And it makes sense.

It's very unlikely in this situation though. The chips that died where in circuit, powered-up, connected to the laptop and working.
I was programming them over several cycles and then I was unable to program them. This happened three times. In addition
both the laptop supply and the power supply are three terminal devices and both plugged into the same power strip.

It is a mystery and will remain so since I am unwilling to destroy more chip while trying to solve the problem. I'm running a new
location, new power supply and a new laptop and have programmed an 18A about a dozen times. So far so good.

Cheers.

 

[neiltechspec]

FWIW, I would agree with IWP, suspect the laptop PSU. I have experienced similar (not with PICAXE)
with floating HV but very low current on a Laptop Gnd, that had a 3 pin mains plug.

Just because it has 3 pins for the mains plug doesn't mean the Earth does a great deal in some PSU's.

Some 2 pin mains devices are horendous - a certain well known (in the UK) companies Satellite Box springs to mind,
I received many a belter off some of these.
Certain installations required a separate good earth to the Sat Box to reduce risk to HDMI devices (Audio Amps & Projectors).

Neil.