Circuit Protection Scheme

[J_M_B]

Hi

I'm looking at adding protection to a small battery powered device.
The battery terminals will be touched by user.
The micro controller programming interface will be able to be touched by the user.

Its easy to google descriptions of varistors/MOVs/TVS diodes but I find it difficult to get an answer I'm happy with about what will be an effective protection scheme.

Essentialy my spec is:
- System must survive user touching battery terminals throughout the products life.
- System must survive user touching micro controller programming interface pins throughout its life
(Pins are system power, system GND, 3x micro controller pins)

I would really appreciate any thoughts on which components I should use to achieve the above specification.
Also any advice on resources App notes/textbooks I could use to educate myself so I could answer this question myself.

Thanks
Jamie

 

[KlausST]

Hi,

specifications need values..

The problem is not "touching" a pin, but there may be an ESD voltage with a specific capacitance and a series resistor.
There are several models like "human body model" that fits best your needs with "touching". Charged device models and different machine models...

So you need to know:
* voltage
* capacitance
* resistance

With a human body model one typically uses 100...300pF of capacitance and about 1500 Ohms as series resistance.
And the (withstand) voltage is often the result (or the variable) of the ESD Test.

Klaus

 

[J_M_B]

Hi Klaus

Thanks for the message.
Ok so based on what youve said, what forms of protection would you add to this system?

After reading I have settled on Unidirectional TVS diodes on the positive battery terminal and on each of the microcontroller data lines.

- Does this sound reasonable to you?
- I'm still not clear why everything I have read tends towards TVS rather than MOV based solution, any thoughts on this?
- The system 3V3 is also exposed to the user on the programming header, would you add TVS on the 3V3 net aswell?

Thanks for your help
Jamie

 

[BradtheRad]

Zener diodes are another component which is used to limit voltage across terminals.

 

[KlausST]

Hi,

For the relatively sensitive signal pins (in opposite to the power supply pins) I often use two resistors.

One resistor is used to limit the current and dissipate the power. It should be one with a higher voltage rating and it should withstand power bursts. Here carbon resistors have an improvement against metal oxide resistors.
Then there should be a fast voltage limiting device (to Gnd). If you look into the datasheet you will see that the limiting voltage is much higher than 3.3V + 0.5V. So a microcontroller internal protection diode may be conductive.
Because of this I use a second resistor between voltage limiting device and microcontroller.
Maybe the limiting voltage is 8V then you could use a resistor of about 47 Ohms to keep peak current below 100mA.
Voltage rating and power rating is not critical.

Klaus

 

[J_M_B]

Hi Klaus

Thats great, thanks for the explanation

Jamie