4 replies to this topic

[stotech]

I'm using the .net micro framework toolbox to multiplex a bunch of inputs. Can some one tell me if the input voltage of a line into a 74HC165 can go above the vcc or reference. For example. Will either of the following 2 examples work.

 

Or will I get "Great Balls of Fire!"

 

A.

 

B.

 

Thanks in advance!

[CW2]

The NXP datasheet says maximum input voltage is V_CC, so in your case 3.3V.

 

The absolute maximum voltage for 74HC series is V_CC = 7V; recommended operation conditions max 6V. It will probably not burn under 12V, the inputs are (most likely) protected with clamping diodes, but I don't see any reason to feed inputs with such high voltage, when there is already 3.3V supply...

[stotech]

It is a strange idea isn't it. I need to tap into and monitor a large group of cables that are moving rams. The entire system runs on 12 and 24 volts. I'm thinking I'm going to end up isolating the inputs with opto's to be on the safe side. But I was trying to avoid it seeing as I've already got the spi and power isolated using a RF digital isolator. adding the opto's seems like a double isolation waste.

[CW2]

Ok, if you don't want to use isolator, then you could probably use simple resistor voltage divider, V_IN = 12 V, R₁ = 10 k?, R₂ = 3.9 k? -> V_OUT = 3.367 V.

[stotech]

Thanks. Still pondering all the options.