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Do I need to use zener clamping?
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kjr18:
Hi, I'm making something that uses attiny13. uC is used for detecting voltage on two inputs and depending on both inputs, outputting some different pwm signal. As this will be used in a car like environment, should I use clamping on both inputs as well as on voltage supply?
Benta:
Yes, you need protection. An automotive environment is one of the dirtiest you can operate in (electrically-wise).
I'd strongly suggest using an automotive voltage regulator that will survive a 60-V load dump. For the inputs, use double Schottky diodes for deflecting over/under voltage transients to V+ or ground.

Ian.M:
As Benta points out, you *need* clamping.   I'd clamp the taps of the input potential dividers with Schottky diodes as he suggests, then add 1K resistors between the divider taps and the input pins.  A few nF of capacitance to ground at the taps would go a long way towards making it less sensitive to transients and EMI.

As the whole circuit only needs a few mA at +5V for the ATtiny13 and the MOSFET gate drive, consider using a 5.1V Zener as a shunt regulator.  As long as it and its dropper resistor have adequate wattage ratings, it will be immune to typical automotive supply transients and load dumps.   I'd also upgrade the BC547B to something with a Vceo comparable to the MOSFET Vds breakdown voltage (100V) so it can survive the transients unprotected.
T3sl4co1l:
Toss on a MMSZ5231B and you're fine.

I wouldn't actually recommend clamp diodes, because there's nowhere for the current to go -- likely you don't have much load on the 5V supply, so it will be pulled up, and cause problems.  The solution to that, is a TVS across the supply.  Which isn't a bad idea, and usually comes for free in TVS array chips, which is not a bad idea here.

You may also want to replace Q1 with a protected switch device.  This is a MOSFET with an internal control circuit to handle level shifting (may not need Q2 and resistors), to protect against excessive voltage, current and temperature (but not all three at once, obviously(?)).

As-is, you still may want to protect Q1's gate with a zener, 5 to 15V rating is fine.  The zener would be wired from gate to source, in parallel with R5.  Q2 should then have a resistor added in series with its collector, maybe 1-10k, so it doesn't draw excessive current through the zener.  This protects against excessive input voltage (automotive load dump can swell up to 40V+, well outside of Q1's Vgs(max) rating).

Tim
Ian.M:
Good catch on the need for a gate-source Zener.
Yes, clamping to the +5V rail can cause it to rise, but if you use a shunt regulator (which is only possible because the operating current is so low), that's entirely manageable.    As the ATtiny13 has internal ESD clamping diodes, not fitting external Schottky ones wont prevent the rail rising if a series regulator is used.  You'd need clamps that divert current to ground to do that.
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