GGG posted:If you build this board build in a LED light that illuminates when the TVS Shorts. While the design is meant to prevent the TVS shorting rapidly, I am sure after enough hits even the new one will short. Since it doesn't show up as a power short, like the input/output TVS, but will degrade DCS signal (which might not be detected on a small layout) a LED would be nice. It must be serviceable easily also.
Lastly, there are plenty of REV L TIU out there with out this TVS mod form MTH. Therefore your TIU is not susceptible to this exact failure. Your failure will be the Transmitter chip.
Most people do not have the issues this San Diego club has. I have repaired plenty and they have not come back. Frankly, I probably only worked on a few (less than 5), that had the TVS mod.
So the sky is not falling. But this mod may make the Rev L TIU much more robust. G
Hi there,
I thought about this too....
Doing a LED that illuminates when the TVS shorts is not easy since under normal condition there is sometimes no voltage there.
Think about the Thevnin equivalent..... when the ACT244 driver is outputting a "0" the voltage across the TVS is 0V, the same condition as when it fails.... so you would need to distinguish the two conditions from each other. The only way to do this is the TVS current:
Normal 0 TVS voltage 0V TVS current 0mA
Normal 1 TVS voltage 7V TVS current 0mA
Fail TVS voltage 0V TVS current high
So you'd have to put something in series (a current sensing resistor) which isn't so simple. First you need to make the resistor big enough that you can develop a sensing voltage you can collect with a diff amp, but that resistor itself defeats the protective action of the TVS since you're adding an RC pole at Rsense X Cj and increasing the turn on time.
I think a better way is to build something that senses the channel outputs with a high-pass filter, maybe about a 100 KHz cutoff frequency (so it only looks at the DCS signal). Then use some type of hysteresis comparator and digital latch to see if you are exceeding 7V occasionally. A good way is a 1-shot timer circuit. So everytime you see >7V... you trigger a one-shot that flashes the led for maybe 1 second. That way every-time a packet goes by, you get a LED flash. If the DCS excursion voltage is too low.... then the LED stops flashing.
I don't know about anywhere else so I can't speak to how common the TVS devices are, or how often they fail outside our club, but from what I've measured at least in my club, low DCS excursion is the root cause for almost all of our control problems. Some members have brought Rev L TIUs from home recently bought and installed and I've found the same TVS failing also.
