Triac Efficiency

AC arcs are usually self-extinguishing. That's why relay contacts have different AC and DC current:voltage ratings, with the DC much lower.

That doc is pretty bad.

Earth is not a triac and I've seen recent studies that the atmosphere temperature went up during the covid thing because there were less particulates in the stratosphere to radiate heat into space.

Volcanoes shoot junk into the air and cool the planet. "Volcanic winter."

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** This article from my colleague Rod Eliott is a lot better.

I particularly like the " Mosfet Arc Extinguisher " suppression circuit in fig 4.2.

.... Phil

That paper is better than the Littlefuse thing, but it doesn't explain how the passive RC (fig 3.1) can extinguish an arc. The suggested time constants are short.

The mosfet things are complex and maybe fragile.

It takes SCR plus MOSFET to switch DC; for 'off' function, you just crowbar the SCR with the MOSFET; at under 0.5V, the SCR cannot stay latched ON. That MOSFET only needs to shoulder the full load current for a millisecond or so; turnon can be slow but turnoff should be FAST so as not to burn it up.

Gate turnoff SCRs were useless, but a crowbar using a power MOS device isn't infeasible.

Not too fast, or the SCR will stay on.

Why use an SCR if the mosfet can handle the load with under 0.5 volts drop? Mosfets can get to single-digit-milliohm ON resistances these days.

SCRs have huge ON voltage drops so need big heat sinks.

torsdag den 8. juni 2023 kl. 17.48.49 UTC+2 skrev John Larkin:

once you get mains voltages single-digit-milliohm ON resistances gets a bit expensive

but you gain the advantage that you can turn off quickly in case of overload, you don't have to suffer a full half cycle

There are affordable SCR/Triacs around with high enough I2t ratings to pass on a downstream overload and trip the circuit breaker or blow a fuse without turning themselves into three terminal short circuits. Are rugged MOSFETs able to do that as cost competitive yet?

piglet

That would be 'not too prompt'; what I intended by FAST, was slew rate, not delay time. Applying high current to turn OFF implies that the voltage across the SCR starts low (circa 1.5V).

You can get SCRs with multikiloamps ratings, continuous. That's because conduction is vertical, from one face of the wafer to the other, not horizontal. That simplifies heatsinking and electrical connection.

SCRs typically drop 1.5 volts or so at rated current. It's not hard to beat that. Two or even three mosfets is a lot cheaper than an SCR and a mosfet and associated drivers and heat sinks.

I'm designing some dummy loads these days that include short-circuit programmability. I'll use a dual isolated ADC per channel to digitize voltage and current. An FPGA will model mosfet junction temperature and shut things down if the fet is in any danger.

A mosfet can be turned off to prevent meltdown. That way no tripped breaker or fuse has to be manually dealt with.

That's what I meant too. High dv/dt will re-fire an SCR.

Lateral mosfets are rare oddities. Most use vertical conduction.

Sure, use gigantic SCRs for those megawatt cases. But would you add multikiloamp mosfets to commute them off?

Rare? There's a lot of 'em in a CPU near you. The power devices with vertical drains still have source currents flowing laterally.

Maybe. Or, maybe just a capacitor-coupled SCR. Gate turnoff, while possible, of an N amp device takes about N amps...

The are only happens when the contacts open and that is when the capacitor shorts out the voltage to keep the arc from even happening. I'm talking BIG continuous arcing if this is not snubbed.

It WILL work and it DOES work.

Of course it is polarity sensitive ! This was used on a solar PV array disconnect for ground fault protection in the 1990s. It worked great.

No AC. Just DC here.

There are other ways of stopping relay arcs for AC too but involves triacs across the contacts.

boB

The R has to be small enough to discharge the capacitor in enough time for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC arc? Actually, does it?

One hint is that DC arcs are associated with inductance in the loop. That's why you can arc weld with a few car batteries, but you need a big inductor.

It prevents the arc from starting in the first place when the contact opens. As long as there was at least a few amps of current and some minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long as it was discharged when the contact opens.

DC arcs are super easy to make. Just take a power supply with over, say, 50V and with a short circuit current (Isc) of a few amps.

Short the leads together or, if needed, add some resistance to limit voltage across the power supply and pull those leads apart. You will get an arc that you can sustain until the wire disintegrates and the arc stops.

Better yet, take a solar panel (in the sun) of 100 to 450 watts and short the leads together. Then pull the leads apart. Makes very nice arcs.

Pull them apart quickly and FAR apart and minimal heat and wire damage will occur but for a relay, that won't work. Not without something to mitigate.

Now, take a newer PV system of several thousand watts and several hundred volts and do the same thing. Might not be able to even hold onto the wires in addition to being blinded by the UV.

No inductor needed if there is enough voltage. You would need at least

3 car batteries from my experience to get a sustaine arc.

An inductive energy storage arc could also benefit from a snubber of course but is easier to stop cuz it doesn't last long. Those are more like sparks and will quickly extinguish because of limited energy stored in the inductor. But enough current and voltage will have the same problem I am talking about.

We also detect DC arcs and mitigate those. Called arc fault in the PV industry.

Try drawing DC arcs. They're fun ! As long as you don't get burned or blinded.

boB

Here is someone demonstrating a small arc...