AVR Freaks

There are many ways to protect the input( s ) to the micro.

With the design shown, a TVS isn't required.

JC

Two resistor, one transistor, maybe cap, you don't have to worry if your system is disconnected from power supply and input still powered. Transistor can be almost 'what ever' small signal tr.

Depending on how critical the data is, you might want some backup strategy; completely unexpected sudden power loss can happen.

I sighted a part number for the zener.
That was a compromise between what was available and what I "wanted".
The heftier the zener the greater the transients it can withstand.

The cap should not see much more than 5 V across it, if the zener is doing its thing.

Since small SMD caps come in higher voltage ratings for the same physical size a little over spec'ing here is reasonable. If you are doing the entire PCB with 0.1 uF caps at 25 V rating that should be fine. My original design used 100 V caps, but that was a carry over from another project, and I had lots of them in stock. The by-pass caps were then from a different lot, 16V or so.

R8, 10K, won't see much more than 5 V, so its power idssipation is about 2.5 mWatts. Pick whatever you want.

R7, 18K, has to withstand a significantly higher voltage drop across it, and hence a higher power rating.

There are many references for Load Dump characteristics. Wiki, for example, sites 120 V. Some references go significantly hgiher than that.

With the zener clamping the right side of R7 to about 5V, one could have 120-5 V across the resistor, briefly, (not steady state).

The power dissipation is therefore around 300 mW.

The good news is that this is for brief transients, not continuous power dissipation.

How safe do you want to be?

500 mW would be reasonable.

Some would say that is overkill.

It is all a matter of how "mission critical" and bullet proof you want the design to be. An F-16 has different requirements than a moped... ;)

Improved redundancy is easily obtained by adding a second zener in parallel with the first one, and dividing R7 into two resistors, in parallel. A failure of any one of the four components will alter the operating point for the components, but won't zap the micro input, for example.

This, (rather old), through hole project photo shows my implimentation. Zbasic AVR, GPS, MMC card, Bluetooth module (to upload the SD Card data), 3-Axis accel. The power monitoring, (Ignition and Lightbar/Siren), input clamps are just to the right of the 4 diodes on the left side of the PCB.

There are a few of these out in the real world, doing there thing, without a failure, (yet).

JC

replying to an old thread here -- but wondering if the answer to the above wish has changed in the intervening years.  i'd like to do some very simple automation / intelligent control in my RV, and i confess that the electronics necessary to protect power/inputs/outputs is daunting enough to make me use relay logic, instead.  :-)

does anyone make automotive-to-5V interface devices, or even (major fantasy here) a breakout board (for just about any uC) that has all that stuff on board already?