Re: QMX: Requiescat in Pacem
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I have to say, that I completely lost the trust into the CPU controlled switching regulator in the QMX.
My guess is, that under certain situations during the handshake between the QMX and the PC via the USB port, the timing of the switching regulation loop is negatively impacted and the output voltage might raise uncontrolled.
In any firmware version from?1_00_009 onward, the switching regulation loop runs in the highest priority interrupt process. NOTHING can stop it. It certainly doesn't get delayed by any USB handshaking between QMX and PC. The USB handling actually happens in the lowest priority interrupt, it is the least critical thing and everything else is prioritized above it. In any system fault that crashes (resets) the processor, the PWM outputs float, which considering they are pulled low in hardware, means the SMPS converter switches are OFF. So the hardware defaults into a safe state.
The SMPS are one of the complex and daring parts of the QMX and it's too tempting to blame this part of the circuit if things go wrong. I believe (contrary even to my own initial expectations) that the actual reliability of this circuit is very high, much better than we would intuitively feel; our intuition is ruined by a fear of its complexity.?
I wonder if you might have had ground potential differences between multiple power supplies. This is a quite easy trap to all into (don't ask me how I know). In your case it sounds like you had three power supply domains. The 12V battery powered QMX, your laptop, and whatever power supply was powering your QSE. Your QMX "ground" wouldn't have been your shack ground, and most probably your laptop ground wouldn't either (laptop power supplies normally "float"), and I don't know about what was powering your QSE. I've seen situations where I had an ungrounded lab variable supply and half mains voltage appeared?on its "negative" terminal. Sure, at very high impedance, it's not like you feel anything when you touch it. But I do wonder if you might have inflicted some kind of discharge when you connected these various power supply domain grounds together and thereby forced them all to the same level? I don't know, just speculating.
73 Hans G0UPL
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