Hello forum viewers!
If I want to control transient voltages that might occur on an input or output pin on a microcontroller externally with a schottky diode (reverse biased) of 5v (my logic level is 5v), does the "forward current" need to be a consideration if the anode is connected to ground and cathode is to the 5v logic input/output (reverse biased) or is that only for forward biasing? I just want to clamp any transients over 5v. Should I only be concerned with "inputs" to the microcontroller as these will be going "to" the microcontroller or do I need to also think about "outputs" from the microcontroller?

Thanks again for all your help!!

Lisa

 

does the "forward current" need to be a consideration if the anode is connected to ground and cathode is to the 5v logic input/output (reverse biased) or is that only for forward biasing?

wind77, I(f) rating is important for breakdown operation to extent that you don't exceed maximum power dissipation rating! But transients have very low d/c

Should I only be concerned with "inputs" to the microcontroller as these will be going "to" the microcontroller or do I need to also think about "outputs" from the microcontroller?

wind77, It depends on application. If you are switching inductance with output it needs to be protected but that's better done with snubber topology than avalanche!

 

You cannot rely on the reverse breakdown voltage of the schottky diode. Use 2 schottky diodes in series, reverse biased (cathode No.1 to +5V, anode No.2 to 0V) with the input connected to the junction of the two. Or use a single zener diode (4.7V) reversed between input and ground. In most applications, leakage should not matter.

Best to do outputs as well as inputs because micro-controller I/O pins are often all inputs until set to be an output in software.

You can get diode arrays especially designed for this but not cheap.

 

I just want to clamp any transients over 5v.

In this case, you should use the diode to clamp the input voltage to a diode drop above 5V by connecting anode to the input and cathode to the 5V supply. The schottky diode has a lower forward voltage than the parasitic diode on a CMOS input and will prevent that diode from becoming forward biased; which could cause latchup and/or electromigration.

Whether you need a series resistor to limit current in the clamp diode depends on what you mean by transients.

You might also want to clamp negative going transients.

Should I only be concerned with "inputs" to the microcontroller as these will be going "to" the microcontroller or do I need to also think about "outputs" from the microcontroller?

Depends on whether the inputs the microcontroller is driving are also protected against transients.

 

Diode connected jfets work a treat too and give you very low leakage loading!
Here's example of protection against transient excursions of >5v and <0v, look on components D5 & D6:

 

To protect the analog inputs I use dual diode with low leakage BAV199. If you need to protect digital input and there are much interference, use the dual Schottky diode BAT54S and series resistor 100 ohms.

 

To protect the analog inputs I use dual diode with low leakage BAV199. If you need to protect digital input and there are much interference, use the dual Schottky diode BAT54S and series resistor 100 ohms.

Could you please explain to me why I would use a BAT54s schottky with Vr of 30v when I have a digital input of 5v? I assumed I would use a 5v schottky for this.

TY
Lisa

 

• BAT54S is low cost.

 

Could you please explain to me why I would use a BAT54s schottky with Vr of 30v when I have a digital input of 5v? I assumed I would use a 5v schottky for this.

TY
Lisa

The diode is NOT being used in reverse breakdown mode, the diode clamps the input to the 5V power supply rails.
The 30 V rating is the maximum it can withstand in reverse bias without suffering breakdown.

It's like you are safe driving 55 MPH on tires rated for 120 MPH.

 

Thank you to everyone who helped me to understand better schottky diode use for protect digital input 5v.

Lisa