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mambrino · 11-20-2008, 03:53 AM

This is my first actual post so Hi! to everybody.

I've been following this thread for a while now and I'm planning to build something similar to MPGuino (the main reasons, too much time

and a litres/(100 Km) "european" presentation instead of MPG).

To the point... I've been thinking for three days now what's the purpose of the 5.1 V zeners in the VSS and injector inputs.

At first I thought it was a "voltage regulator" configuration. The idea is, Vin is 11-14 V, but the zener limits it to 5.1 V, a somewhat valid TTL "high" level, so the input pin reads a "1".

However, a zener MUST have a minimum inverse current to work properly, a 5.1 zener, about 50 mA. The 100k resistor limits the current to (13 V - 5 V = 8 V; 8V / 100k = 0.08 mA), a fairly lower value than the ~50 mA required. In theory, the zener would not give the 5.1 V but another value between that and 0 V.

What I'm missing? Maybe it's there for surge protection? Then how is the level conversion done?

Also, anyway, why are we not using a more "normative" input level conversion, like this ones (link below), suggested in the Arduino website?

http://www.arduino.cc/playground/upl..._4_arduino.pdf

Please, tell me! I need to sleep!

Thanks!

11-20-2008, 03:53 AM	#749 (permalink)
mambrino EcoModding Lurker Join Date: Nov 2008 Location: Spain Posts: 5 Thanks: 0 Thanked 0 Times in 0 Posts	This is my first actual post so Hi! to everybody. I've been following this thread for a while now and I'm planning to build something similar to MPGuino (the main reasons, too much time and a litres/(100 Km) "european" presentation instead of MPG). To the point... I've been thinking for three days now what's the purpose of the 5.1 V zeners in the VSS and injector inputs. At first I thought it was a "voltage regulator" configuration. The idea is, Vin is 11-14 V, but the zener limits it to 5.1 V, a somewhat valid TTL "high" level, so the input pin reads a "1". However, a zener MUST have a minimum inverse current to work properly, a 5.1 zener, about 50 mA. The 100k resistor limits the current to (13 V - 5 V = 8 V; 8V / 100k = 0.08 mA), a fairly lower value than the ~50 mA required. In theory, the zener would not give the 5.1 V but another value between that and 0 V. What I'm missing? Maybe it's there for surge protection? Then how is the level conversion done? Also, anyway, why are we not using a more "normative" input level conversion, like this ones (link below), suggested in the Arduino website? http://www.arduino.cc/playground/upl..._4_arduino.pdf Please, tell me! I need to sleep! Thanks!