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Thyrectors (TVS Diode)

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This brings back memories of my current loop teletype days (driven by an 80

08). Having learned that I needed a reversed biased diode in my relay drivi ng circuit, I then encountered the relay turn off delay problem. At that ti me, I ended up with some sort of RC circuit to allow faster relay operation , yet dissipate enough of the coil induced current to prevent driver and MC U from going crazy. It was probably still half baked but it worked well eno ugh at the time.

In the article referenced, thyrectors are mentioned. I would have thought that the same problem would exist there: energy dumped from the relay coil would slow down the magnetic collapse due to current flow. The only differe nce I see is that the current flow would occur a little bit later, at the p oint of the avalanche diode breakdown.

Warren ve3wwg

Reply to
Warren
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Indeed, that's what happens. And the higher voltage drop during that breakdown is key: the fundamental inductor equation is V = L * dI/dt, so more V gives faster dI/dt. :)

One doesn't need a brand name component to do it; a bidirectional TVS is simply two zeners end to end, or you can use a rectifier and zener to get a one-sided avalanche characteristic.

Another note: the diode actually belongs at the switch, not the coil. The coil generates /most/ of the EMF, but it's not the cause. It's just doing what it's told, so to speak. The switch is what's doing the work. This distinction is not very important for relays, but becomes paramount as you go up in frequency. For SMPSs, the stray inductance of interconnects, and transformer leakage, is comparable to the winding inductance, and cannot be ignored.

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

Agreed.

The problem I see with that, is that if you put the diode across the switch (and not the coil), you then have the coil dumping into the supply circuit. In those early days, spikes in the supply were a bit of a problem for the 8008 as built. :)

I can see that SMPSs have all sorts of challenges.

Warren ve3wwg

Reply to
Warren

Well, not the logic supply. The relay supply. Also, you need a bypass at the switch, to close the loop -- otherwise it's who knows how long = how much stray inductance!

Again: the relay is just doing what it does, but the switch is the cause. Close the loop. :)

Tim

--
Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

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