Digital Input Range Select

[ric_vas]

Hi,

not sure if this is the right forum, anyway..

I need to design a circuit to support three levels of "digital" input to a microcontroller (PIC) working out of a 5Vdc power supply.

The input levels: 0-125Vdc, 0-48Vdc, 0-24Vdc.

I can use jumpers to select from them.

What you recommend in terms of protection circuit? Are series resistance, opto-couplers, maybe zener, enough?

Thanks in advance.

Ric

 

[Jepeto]

You can make voltage divider to 4v - 5v for each input and to drive it with FET transistors like 2n7000 with pull-up resistor connected to the board logic supply
Its only 6 resistors no jumpers needed
Take care for the 125v input resistors maximum voltage rating and power

 

[ric_vas]

Hi Jepeto,

Thanks for helping.

I didn't understand the "no jumper needed" part. Maybe I was not so clear, there will be only one physical input conector which can receive a 125, 48 or 24 Vdc (not at the same time obvioulsy).

I'm also considering to use an opto before the microcontroller. Will I still need the FET you suggested?

Best regards,

Ric

 

[andre_luis]

If you use a Zenner diode to avoid over-voltage, take care with resistor value, due the behaviour of this type of diode is non-linear under nominal secified courrent.

 

[Jepeto]

Zenner diode or standard diodes and opto coupler will be good
Just need to calculate the right resistors value for 125v and 24v inputs
No FET needed

 

[ric_vas]

Thanks guys for the inputs!!

Let's see if I got it:

Is that correct? Then I will select R1 and R2 so that the Zener and Opto receive the expected current to operate.

Ric

 

[unkn0wn]

Did it work???

 

[ric_vas]

That project is standing by I didn't have a chance to test it but I think that's the way to go about it.

I would make use of jumpers to select between different voltage divider nets for each input range needed.

Regards,

Ric