Hi guys,

I have inputs from a microcontroller that can be configured as either digital or analog inputs. I would like to protect them from direct 12V voltages. The controller operates at 3.3V. I had thought of using an opto-isolator, but by doing so I would no longer have the option of reading the analog signals.
Has anyone had the same need as me?
It would be nice to be able to use one terminal and switch between the two modes with a jumper....

Does anyone have any ideas?

 

There are more sophisticated ways to do it, using transistors or op amps (or opto coupler as you mentioned), but just to protect it from voltages over 3.3V you could probably put a zener diode between the input and the GND?

 

Yes, I could do that, but I would prefer the two circuits to be separate, even if only when acquiring a digital signal. A zener diode, near its conduction, would affect the analog measurement.

 

Consider that diode clamping is a common and effective scheme for both analog and digital ports, although recently I have been reminded that the voltage supply connection also needs to be protected. THAT can be achieved with series resistors and zener diodes.

 

A common solution is a voltage divider and a Schottky diode.
Tell us the MCU part number.

 

Hi guys,
@MisterBill2 @MrChips @crutschow

Sorry for my delay.

I took tried to put your advice into action and designed the attached circuit.
Does it make any sense in your opinion?

My goal is to have an opto-isolated input , but also maintain the capability of using the internal ADC when optoisolated mode is off.

 

I'd simplify it.
Delete the switches.
Connect the opto transistor so that the collector is at 3.3V and the emitter connected to the input, with a 10k pull-down resistor.
If the opto-isolated input is not being use, the transistor will be off, and you can connect the analogue input either straight to the input, or via a divider, and the 10k resistor forms one half of the divider.
I would not recommend the use of D19. If there is an over-voltage on the input, then current will flow into the 3.3V power supply and may raise the voltage by enough to damage the microcontroller.

 

Thank you for the advice. Maybe I confused the drawing. ADC1 is not a terminal it is just a label. That is why I had to introduce a switch

 

Thank you for the advice. Maybe I confused the drawing. ADC1 is not a terminal it is just a label. That is why I had to introduce a switch

I would suggest that you have extra terminals instead of the switch. I think it would eliminate misconnection, or having the switch in the wrong position for the inputs that have been connected.

 

Hi @Ian0,

I have modified everything according to your suggestions.
However, I have a question. If I connect a signal to the opto-isolated input, I would have voltage on the ADC1 connector.
What do you think about the configuration in the image below with two jumpers?

 

Hi @Ian0,

I have modified everything according to your suggestions.
However, I have a question. If I connect a signal to the opto-isolated input, I would have voltage on the ADC1 connector.
What do you think about the configuration in the image below with two jumpers?

Yes. That's good.
3.6V zeners are not good zeners. They will probably start conducting well before the input gets to 3.3V and will mess up the divider.
Are your ADC inputs 5V-tolerant? If so use a 5.1V zener which will have a much sharper curve.
This is another way to clamp the input. It doesn't completely eliminate current into the supply, but it reduces it by a factor of Hfe.
If the fault current into the supply is lower than the load current on the supply it does no harm.

 

Yes. That's good.
3.6V zeners are not good zeners. They will probably start conducting well before the input gets to 3.3V and will mess up the divider.
Are your ADC inputs 5V-tolerant? If so use a 5.1V zener which will have a much sharper curve.
This is another way to clamp the input. It doesn't completely eliminate current into the supply, but it reduces it by a factor of Hfe.
If the fault current into the supply is lower than the load current on the supply it does no harm.View attachment 349847

No, my inputs do not tolerate 5v, they only work at 3.3v

 

That's a shame. Either use the transistor clamp, or try this one.

V1 is some higher voltage supply you might happen to have. 5V or 12V etc. Select R1 to give the rated current for the zener. That way you might stand a fighting chance that it might bias up pretty close to 3.3V.