Triacs vs. Relays

Sure. Relays are slow, but they offer a combination of low ON-state resistance and low OFF-state capacitance that no solid state device can approach. I often use them in front ends.

Cheers

Phil Hobbs

There are lots of isolated opto-SSRs and opto-Triacs

but aside from

I use tons of relays for signal switching. No semiconductor can rival a good little surface-mount DPDT relay.

(271 relays on that one)

There are SPDT RF coaxial relays that work beautifully at 40 GHz.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics

You will find lots of relays in cars and trucks. Triacs are not easy to shut off DC power.

Dan

I'm amazed to hear that. How do you maintain uniform transmission line impedance through the internals of a relay? You'd have to have relays specifically designed for RF signal paths, surely?

Don't forget about overload. A semiconductor fails soon after its rating (for your TRIAC, maybe 8-16A for 16ms), whereas a mechanical relay will carry fault current (100s A) and probably still open again. (If the contacts weld, it might not reopen on its own, but a firm jolt, or a little poking with a screwdriver if the contacts are accessible, ought to do.)

The on-off ratio of a mechanical relay is unbeatable.

The risetime (namely for a mercury-wetted reed type) is amazing, fractional nanoseconds. Semiconductors have only been able to beat that relatively recently (since the 70s?).

Contacts are ludicrously nonlinear beasts. It should be no surprise that common relays make so much trouble with RFI and transients.

I don't think anyone's ever constructed a SPICE model of a relay, to a sufficiently accurate degree that it could be recognized as such; that way lies madness (and convergence errors)!

Tim

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"Julian Barnes"  wrote in message  
news:noq270$79e$12@dont-email.me... 
> In a (very) recent project, I have successfully used a tiny triac[1] in a 
> mains voltage switching role. 40 years ago I'd have used a relay for this 
> and I can't think of any alternative in those days. Now I've used this 
> thing the size of a small pea and it doesn't even have a heatsink tab of 
> any kind for mounting yet it can switch up to 600V at 800mA. This to me 
> (having been absent from progress for extended periods) is like black 
> magic. OK, so the old mechanical relays provided secure galvanic 
> isolation which a triac can never approach I'm guessing, but aside from 
> that, is there any role for the mechanical relay in modern electronic 
> design? 
> 
> 
> [1]http://www.farnell.com/datasheets/84063.pdf

Step-recovery diodes, tunnel diodes, avalanche transistors, and spark gaps were fast in the early 1960s. TDs were available in 1961, and parts with 25 ps rise times were around by about 1964.

HP had a 12 GHz sampling oscilloscope in 1966.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics

Omron's literature also mentions water heaters, refrigerators, air conditioners, home appliances, and small electric appliances among other applications. Modern consumer grade UPSes typically use about a half dozen relays. This closeup [1] shows several relays from a tear down of an older UPS. [2]

Note.

1. formatting link
2. formatting link

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Triacs have been around for more than 40 years.

And nothing to use it on. :-)

I hate relays - they fail far too easily. But when it comes to switching low-level signals in a high-voltage environment, there's no substitute. {Switching or multiplexing neural/physiological signals when some of those signals have stimulators attached - i.e. >100V}. e.g. Supertex multiplexers are much too noisy.

If I had to make something that would still need to work after a nearby lightning strike, I would choose a relay over a triac.

Have you ever seen a dimmer switch for a house light, where the dimming knob doesn't do anything, but when you switch on the light it is just

100% on? That used to have a triac in it, now it has a melted triac in it. I think the gas in the light bulb sometimes arcs when the filament burns out and then that kills the triac due to the large current. Maybe now that filament bulbs are becoming uncommon, that falure mode will go away, but triacs are unsuitable for dimming many of the replacement bulbs so then the triac dimmers may have to go away too.

Yep. We had a lightning strike (6AM) at a substation 2 miles from our house... all underground feed to us from the substation.

Blew the dining room dimmer, so 400W came on abruptly... woke me up from the sudden light... I thought the house was on fire.

Also got two TV sets and a PC :-( ...Jim Thompson

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I have a TPDT (3 sets of contacts) relay that appears to be about 10 amps each or maybe 30 amps total. I can't read the markings but it pulls in at 15 volts and drops out around 3 volts, so it's not a 12 volt coil unless something's wrong with it.The coil resistance is 110 ohms and it weighs 7 oz.. So at 24 volts, the dissipation is about 5 watts which seems high to me, or is this normal for a 30 amp relay? Or, it could be an 18 volt relay where the dissipation is only 3 watts, but that seems like an odd number.

And they're hardly any better than they were 40 years ago. Electrically fragile, large losses and no inherent isolation.

--sp

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Other HP 12 GHz oscilloscopes?

Sylvia.

I don't think 5 watts is excessive for a larger relay like this. There was one on my furnace that was on all the time the furnace was not running. It was not only pretty warm to the touch, it hummed loudly. I'm pretty sure from the heat it make it was dissipating around 5 watts.

--

Rick C

Microwave links ?

Sampling scope nominal bandwidth applies only for periodic waveforms, the non-repeatble waveform bandwidth is much smaller. Thus a sampling scope should be quite usable for a microwave link carrying a composite video or a multiplex carrying multiple telephone circuits.

** You forgot to mention triacs cannot switch DC voltages.

OTOH triacs can switch *fast* and *often* and by using phase control regulate AC power to a load.

A 40A, 800V triac with 400A single cycle pulse rating, 2.5kVrms isolation from its mounting base and which, if over-voltaged, merely switches itself on is hardly "fragile" !!

... Phil

Oh, it can be done. The crowbar will get a bite every time you reset it, of course. Sometimes, it doesn't matter whether the switch turns off (artillery fuses have a lifetime turnon count of 1).