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When I was at work the other day I was pulling apart an LED fault detection unit. I noticed they were using an Atmel ATtiny13A as the brains.

The interesting thing is this unit was multi-voltage and so they were connecting 9-33V to the microcontroller's Vin and then using a zenor diode to reduce the Vin by 5V then connecting the microcontroller's ground pin to Vin - 5V. Lastly they dissipated the remaining voltage with a resistor connected to ground.

The reason for them doing this was so it could interface Vin on its digital pins without any additional circuitry.

My question is, is this a normal thing to do (The microcontroller still had 5V between Vin and Ground pins)?

Is there some limiting factor stopping you from using the same logic and connecting the micro to 1000V at Vin and 995V at ground?

At what point does the voltage become too high to do this?

Is the only thing stopping this at higher voltages arcing out to other components etc

Thanks! :)

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Is there some limiting factor stopping you from using the same logic and connecting the micro to 1000V at Vin and 995V at ground? No there isn't as long as the uC cannot "see" it. Ground, the point which you call zero volts is just a reference for you. A chip does not care about ground it cares about voltage differences. As long as these voltage differences do not exceed the ratings of the chip, it will survive even if your ground reference is at - 1000000 Volts DC. \$\endgroup\$ Commented Aug 29, 2017 at 6:39
  • \$\begingroup\$ Draw a complete circuit. In 99,99% of all cases you will end up with another real ground/PE referenced signal - like USB for example. This will not allow you to put the MCU ground to an arbitrary high potential. \$\endgroup\$
    – Turbo J
    Commented Aug 29, 2017 at 12:02
  • \$\begingroup\$ Note that the ISP/TPI programming adapter is usually referenced to the real gound(PE), too. The OP should absolutely not work at high voltages, as the question in itself reveals a suspicious lack of required knowledge to work safely. \$\endgroup\$
    – Turbo J
    Commented Aug 29, 2017 at 12:10
  • \$\begingroup\$ Yeah in this case though, the micro was already programmed and used in 12V circuitry (automotive application) interfacing 12V signals on its ADC pins. Other than the two pins connected to 12V and 7V (like above) there was only the 12V ADC :) \$\endgroup\$
    – OpAmpMan
    Commented Aug 30, 2017 at 10:07

3 Answers 3

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This is equivalent to deriving the 5V from a much higher voltage, and the same problems apply. For the power, any fault in the voltage-reduction circuitry can destroy the uC. For the logic, it must be within the 0-5V range as seen by the uC.

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This is unusual, since it complicates what other circuitry you connect to the microcontroller.
But from the perspective of the microcontroller, it's within specification.

This is typically done to cheap out on parts, but you have to pay attention when designing your circuit.

Is there some limiting factor stopping you from using the same logic and connecting the micro to 1000V at Vin and 995V at ground?

The 1000V (above mains earth as 0v) will be common mode for the microcontroller. Microcontrollers don't care about this because nothing has to connect to 0V.
If you do connect something that has 0V, like your programmer with mains earth, magic smoke will be released if you do not use isolators (eg: optocouplers).
Optocouplers do care about this, since they connect to both 1000-995V and 0-5V.

Voltage is always relative. You can flip the labels and use -5V and 0V to supply your microcontroller. It will be an unusual circuit diagram, but the electrons don't care.

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  • \$\begingroup\$ Thanks so much for your answers. I'd never seen something like this in my reletively short time playing with electronics. Seems like a good way to save cost in certain applications \$\endgroup\$
    – OpAmpMan
    Commented Aug 29, 2017 at 7:24
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There are quite a few high-voltage highside switches. Those are put in high voltage connections and derive power by a small voltage drop between connectors. The control signals are electrically decoupled, so you can use voltages on completely different potential to switch the unit.

A simple example of such a switch would be an optocoupled transistor.

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