AVR-Chat

Got it.  Thanks.   But just to make sure I truly get it, in your equation
"(7.5V-5V)/0.1ma", "7.5V" is the breakdown voltage and "5V" is the working
voltage?   In my case, with 3.3 V Transguards and Vcc = 3.3 V, the equation
would be (5.0-3.3)/0.1 = 17K?  

 

From a practical point of view, what is the significance of the "clamping
voltage"?

 

Clearly the Transguards are just fine for 50 KHz signals.

 

Steve

 

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of wagnerj@proaxis.com
Sent: Thursday, January 06, 2011 10:08 AM
To: AVR-Chat@yahoogroups.com
Subject: RE: [AVR-Chat] AVX Transguard

 

  

If data rates will allow, simply add a resistor between the AVR port and
the transient protector. I think (but may be wrong) that communication
with Atmel has established a limit of 0.1ma forward current in the
internal over-voltage protection diodes inside an AVR. Thus, if the
transient protector will practically limit the input voltage to 7.5V, then
a series resistor of (7.5V-5V)/0.1ma = 25K would be appropriate; I would
use 27K or 33K.

The thing to watch out for is rise/fall time at the port pin. A typical
CMOS input has 5-7pf. Lets assume 5pf for the small geometry recent
generation chips. So, in combination with 27K, the time constant would be
around 140ns and the rise/fall time would be about 300ns. So, as long as
the data rate on any protected input is less than 1bit/300ns, you would be
OK.

The transient protector is still important in your environment, especially
with the possibility of lightning. It will limit the input voltage to
something under 12V instead of 100s of volts.

Jim






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