Diodes of protection of transient voltages in microcontroller sensor

Thread Starter

curro92

Joined Sep 29, 2012
21
Hello,
I'm designing a weather station with a microcontroller. The first sensor is a rain gauge (with tipping bucket mechanism). Both microcotroller and the sensor go to 5V.
In a circuit I have seen that diodes of protection are used at the entrance of the rain gauge sensor. These diodes are 250mA/100V. I want to know if those values are critical, or they can vary within a few margins. Are they effective to protect 5V overvoltages?

 

Sensacell

Joined Jun 19, 2012
2,567
A slight improvement would be to change the resistor R11 to 100 ohms, then add an additional 4.7K in series with the input.
Then the network can withstand voltages beyond the rails without blowing the diodes or micro.

Without the 4.7K, a short to a higher external voltage could drive Vcc over 5 and blow the micro, via D5.
A negative transient could blow D6 if the source impedance is very low.
The 100 ohm resistor just keeps the main current flow away from the micro's internal diode clamping network.

The main consideration is speed, this idea works to about 200 Khz, doesn't seem like an issue here.
 

DickCappels

Joined Aug 21, 2008
6,386
One thing to consider is whether the type of transient against which you intend to protect your circuit could cause the +5V supply to exceed the ratings of parts in the circuit. One solution is to shunt the 5V supply with a 6.2 volt zener if your circuit can tolerate 6.2 volts, otherwise a 5.6V 5% or tighter tolerance zener can probably be used.
 

ian field

Joined Oct 27, 2012
6,539
One thing to consider is whether the type of transient against which you intend to protect your circuit could cause the +5V supply to exceed the ratings of parts in the circuit. One solution is to shunt the 5V supply with a 6.2 volt zener if your circuit can tolerate 6.2 volts, otherwise a 5.6V 5% or tighter tolerance zener can probably be used.
Back in the day - some full H 5 1/4" hard drives had a 5V protection diode in the same package style as the 1N5401.

Outdoors; there may be some justification for considering the gas filled transient clamps used in telecoms.
 

ScottWang

Joined Aug 23, 2012
6,898
In the circuit that the protecting range of voltage on the rain point are -0.7V ~ +5.7V, if the voltage over the range then it will be clamped, but that is assuming that the current does not that much and the diodes could afford the current pass through.

Normally I will use a 5.1V zener diode as shown in the links page on #3, the protecting range of voltage will be as -0.7V ~ +5.1V.
 
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