I would have thought so, too. But I have only checked out the US companies I buy parts from and no other sources. I suppose I should check RS; I'll do that later. Anyway, the point is, you win. I didn't know you couid get small cheap relays like the ones you linked to capable of switching 15A @ 230V so once again you've disabused me of my ignorance, for which I thank you.
tirsdag den 6. juni 2023 kl. 05.03.52 UTC+2 skrev John Larkin:
it takes forever and the bill is infinite? ;)
Not a matter of winning, but sharing things that we know.
Relays can be cool, for power and signal switching.
No semiconductor can approach the relay switch figure of merit Ron*Coff, which has the units of time.
Oh, don't get technical. I hate it when people get technical.
Actually, you don't often need an infinite heat sink. The thermal spreading resistance eventually dominates.
Yeah, I'd like to see how they calculate the fees on late payment. Let's see... 2% of infinity is infinity, so now you owe 2 * infinity which is infinity. Huh, the outstanding balance didn't change.
Not only that, but latching relays draw infinitesimally small power. Right, John?
Do they charge by the square foot, you think? Hey, I wonder what the factory floor space is like. I'll bet it's pretty big!
I have seen and used snubber across relays. To extinguish DC arcs across the contacts. Sometimes you have to increase contact life.
I love triacs ! Haven't designed one into a shipping product for about 9 years but they have their place up to a few hundred Hz. Mainly like them for their automatic zero crossing turn-off behavior into resitive power resistors.
Turning off current through a 1kW load resistor through say, a FET at non zero-crossing really can bring out the inductive behavior in a circuit. Also handy to find those MOC3xxx parts (or 4x ?) that automatically turn on at zero crossing and reduce EMI from the sharp turn on. Well, sharp enough edge anyway
boB
We've used latching relays in low-level analog acquisition systems because they have very low thermally-driven offset voltages, namely no thermocouple effects from coil heating.
They work vertically. As they make it, it extends upward toward infinity. I guess they have to make sure it is well grounded to prevent harm from lightning. Hmmm... I suppose it would need to be constructed north of the tropic of Cancer or south of the tropic of Capricorn to prevent it from colliding with something in the solar system, like the sun.
I was trying to help someone with an idea of saving power on commercial lighting using SCRs. I asked why SCRs rather than triacs and he said something about SCRs working much better. He was a weird businessman though. He had someone else working on this for him, but he claimed they never made it work, so he never paid them. He was intensely concerned about the idea being stolen, but he didn't mind if it was never developed.
I'm willing to bet anyone who talks to him for more than 10 minutes would realize he was just bad news, so he never found anyone to help him with it.
Wouldn't it be trivial to use a very low end MCU to control SCR/triac timing? It could be connected to monitor both current and voltage and pick the appropriate point for turn on.
Triacs are only useful up to a certain current. For higher power AC switching back to back SCRs are used. The issue is triacs have trouble commutating off during the very brief zero crossing time - the bigger the triac the more loose charge carriers there are to recombine during that short time. An SCR has up to the next half cycle to recover.
piglet
Do you know why a snubber extinguishes a DC arc?
Oh. Changing the subject, eh? You Rascal!
Yes. We put snubbers around our large snubber resistors in our 500HP inverters. They were 500W wire-wound and had some inductance.
Yes. Of course... An R-C-D snubber.
The C has been discharged by the R when the relay opens. Then, the C, an electrolytic typically, (large-ish C) charges up instead of the contacts taking the arc. The C is a short across the contacts when the contacts open.
Then, the R discharges the Cap so it is ready for the next opening.
boB
So the snubber prevents the arc but doesn't extinguish one. The R-C-D will be polarity sensitive. And if the contacts cycle before the large-ish C discharges, the snubber won't work.
What sort of RC values are good?
Yeah, that's how we get net global warming despite our radiative sink for heat (outer space) having no end anywhere in sight, and even (in its distant parts) admirably low temperature. Our polluted atmosphere has lessened the coupling for outgoing heat.
What an interesting question! I have only ever used RC snubbers on contacts switching AC or as contact wetters for very low level DC contacts. In all these years it has never occurred to me that a snubber could extinguish a DC arc. The fact you allude to it happening makes me wonder if something devilish to do with arc's negative resistance is involved?
piglet
All very well for switching DC currents, but how to prolong contact life when the current is AC?
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