16F input Diode clamp

[Mosaic]

The 16F886 carries diodes which clamp/protect the inputs from overvoltage.
I have 2 questions:

1) Will this protect against inductive transients from direct driving an inductive load like this:
https://www.electro-tech-online.com/custompdfs/2011/04/2008661155.pdf

2) Will a 9.1K resistor inline with a digital input safely accept 14VDC as a high signal?

 

[Diver300]

The current that sounder takes is probably too much for the output of the 16F886. I would not rely on the clamping diodes, but it would probably be OK.

A 9.1k resistor will probably be OK. However there would be a current of about 1 mA dumped into the 5 V supply, and if your current consumption is ever less than that, the 5 V power supply could rise.

However, a larger resistor would still allow you to read the input. Also if you are detecting 14 V or 0 V then you could have a potential divider which would keep the voltage below 5 V. You could also use a zener diode.

 

[Pommie]

The problem with relying on the clamping diodes is that the spike is transfered to the 5V line and will affect all the circuit. Much better to put a diode from the pin (anode) to ground as the ground clamping diode is the other way around. I'm assuming load is from pin to ground.

One thing to be aware of with the 14VDC is that it may stop the pic from resetting properly when power is removed as it may keep the 5V line above the brownout level. If this is the case then a resistor across the power lines should fix it.

Mike.