@cnwdon posted:Arriving late to this party, but hopefully the answer to my questions will help others of us less sophisticated circuit tweekers. Thanks as always for your shared expertise.
1. Can the SA5.0A "primary" TVS alternate be installed across the external terminals of the TIU channel? If so, the output side (or input)?
2. Can the added "beefy clamp" be the two 1n4148 diodes, in parallel but opposite polarities, likewise be across external channel terminals, output (or input) ?
3. Finally, do these steps all apply to any Rev L TIU, or only certain ones after a certain production date?
Hi there,
1. So the transient events the TVS protects against from all the layout sparks and stuff are like 1ns events, which means they are physically about 2 inches long in air (considering 1/4wavelength), and more like one inch long in a cable where the dielectric is 2,3,4 ish (depending on exactly what wire).
What this means is in a 1ns world (not DC and not 60 Hz, but up at GHz land where this stuff is propagating) the TVS is only clamping/limiting the voltage around it within 1 inch of the device, so if you put it out at the terminals you're not doing an amazing job of protecting the other parts on the PCB (upstream driver and logic). You really want it right at the output of the ACT244 where the track connection (where the transients originate) meet the electronics of the TIU board.
2. The beefy clamp is actually a set of series 1n4148 diodes (Stacked to get the right threshold (n x Vf) with two strings in parallel but opposite directions "anti-parallel". You need to cover both directions since a transient may have a positive or negative polarity relative to the ground in the TIU. TVS are also like this, some are + only and some are - only, and some like the SA5.0A has two inside in anti-parallel to cover both directions.
3. Protection is protection. It's not really about rev. You have the angry spark machine that is a train layout connected to the TIU electronics. The better you stop over-voltage stress from being applied to the electronics the less likely and less frequent the failures will be. We went from blowing a TIU channel weekly to maybe blowing one a year when we made those changes.
