AVR-Chat

At 06:30 PM 3/14/2004 -0500, John Johnson wrote:

>Thanks for the reply!
>
>No, we aren't all EEs. Some of us do this for fun :-)
>
>So, I have about 4 m of 24 awg wire going to a Genie garage door opener  
>with unknown circuitry inside. How would I determine the inductance?  
>What if I sold one of these, and the customer had 1 m or 10 m of wire?
>
>In the heater circuit, I have about 2 m of 14 awg wire to the heater,  
>unknown inductance in the heater, and unknown length of wire in the  
>walls. How would I determine the inductance of that? Do snubber work on  
>AC circuits? Or should I just stick a MOV across the contacts and hope  
>for the best.

In these examples, the wire inductance is trivial, compared to the other elements in the circuit. There is a formula for wire inductance, but it's rarely used in this domain. 

Here's a handy reference for you: On a ground planed 2 layer PCB, a 104 mil track has 50 ohms impedance. An 8 mil, IIRC is 104 ish.(?)  So in the domain where bypass caps operate, think of the tracks as resistors. We aren't much concerned with energy stored in the fields. 

When subbing a relay coil, then you have some serious energy to get rid of. 
Either provide it a path to discharge through, or it will make one.