Clamping digital input to 3.3V rail

M

msadie

Well-known member
I'm working on an automotive project that uses a T4.0 to read a digital signal that is either 0V (off) or 11-15V (on). Rather than just using a simple voltage divider to reduce the voltage to under 3.3V, I thought I would improve the protection by using clamping diodes tied to the GND and 3.3V rails. More specifically, I'm using a BAT54S dual Schottky diode and a 3.3k resistor to limit the current.

My concern: The input may provide 15V at the same instant (or slightly before) the T4.0 gets its 5V supply. Until the time when the 3.3V regulator is online, is it safe to clamp to this rail? Is this method even effective at protecting the digital input in this scenario?

Thanks for the help!
 
If you have a divider and the clamping diodes you should be ok - note that an opto coupler would be more robust especially against variations in ground potential - depends how fast a signal though.
 
Rezo said:
Why not use an optocoupler?
Click to expand...
I briefly considered an optocoupler but I was trying to keep it simple, plus no galvanic isolation is required. But yes, I agree that would be a good solution for the next pcb revision.

MarkT said:
If you have a divider and the clamping diodes you should be ok - note that an opto coupler would be more robust especially against variations in ground potential - depends how fast a signal though.
Click to expand...
The signals are quite slow, the project is just reading some dash rocker switches and a brake pedal switch and sending via CAN.

When you say it would be ok with clamping diodes - do you have an idea of a limit to how much current the 3.3v rail can sink while the 5v is disconnected? I want to avoid some weird lock-up situation during boot caused by powering up devices in the wrong order.

Appreciate the help
 
The divider should limit the current to way less than needed to power up a T4 - say 47k : 15k divider will limit to 0.3mA from 15V
 
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