Don - KM4UDX
You question the parentage of my 510? Their father was Mr. Vishay and the mother was a Mouser. Sniff sniff sir! (Hahah)
Okay, as a self taught rube, who believes trial and error is the righteous path, the notion that you could predict failure by some "math" is a dark and frightening vision. And that the math cruelly suggests my heat sinks are "too small" is simply an insult on the chalkboard. As my incarcerated nephew is fond of saying...."yeah, I got too small for you right here..." hahaha.
To control the fan, after experiencing a error/trial ratio of approximately "1" , I settled on glueing these thermal switches that close at 60deg C on to the heat sink. Given the heat transfer time lag from junction to die to sink to switch to fan to cooling, well, it may be too little too late, as they say.
Given that I direct couple my heat sink to the 510, the heat sinks are at voltage. Not totally wise but marginally more efficient while at substantial risk to accidental sparkification. And the point here is that the sinks are as big as will fit in my hand bent aluminum open chassis without 100% inevitable shortification. Hahah.
So in sum, all these issues are of my own making. Who knew?
Thank you all so much for the help with the bias under 24V and the math lesson (evil business I say! Get the pitchforks and stakes!)
Last question...how do I test the value chain between TX relay and 510 using just a DVM? When in TX mode, does the gate go high? There is always voltage to the source and the drain is always at ground...so I'm guessing i can sense the toggle on the gate? If that works, then the whole TX relay and drive transistor stuff is working? Will the DVM register the delta?