+5 volt logic to 3.3 devices?

Thread Starter


Joined Sep 12, 2011
I have a circuit that is using a PIC18f452 micro operating at 5 volts. I need to be able to communicate with a MAX31855 via SPI. My micro (because of other devices needs) operates on 5 volts but the MAX31855 at 3.3. What is the workaround for talking to devices operating at lower voltages? Will the MAX31855 blow up it I connect the comm pins directly to 5 volt logic and power it from 3.3?

Thanks for your thoughts,



Joined Mar 31, 2012
Look at the data sheet for the part you want to use and see if it's inputs are 5V tolerant.


Joined Dec 25, 2012
Hello ssutton
you can use zener diode a value of 3.3v
in picture uses 3.6 but you try to use a value of 3.3 and don't forgot to put resistor
connections :
MICRO out >>>> resistor and zener to ground >>>> MAX31855 input pin
and if the max ic pin is output and micro pin is only input you don't have to worry
cause 3.3V as i know is enough to be 1 logic
and if its not you should use a transistor to boost the voltage form max ic output to micro input
so there are many connections and its depending on the pins type
warning :
you must calculate zener resistor value by your self and it might be a good idea if you simulate it on a program like multisim or protues ....
you don't have to use microcontroller in simulation and either max ic just any type of ic's that have the same type of pins
another way is to use ic that converts 3.3v to 5 or 5 to 3.3
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