Combining the best of TRIACs and relays - Page 1
Forum section: electronics / projects

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 01:18:50 am
content: I’ve been thinking of making a microcontroller-operable mains switch*, and I want it to be good: 10A inductive loads (fridges, dehumidifiers, bar heaters, just about anything that can legally be plugged into a 10A socket), long life time, etc. Now several options present themselves:Optotriac + triac:A classic solution; I even have the example schematic from the MOC3023 datasheet running a fluorescent light at my place right now. The only trouble is that you can’t get away from that 1.5V drop with a triac, which becomes an annoying thermal (/inefficiency) issue at 10A draw.Relay:The even more classic solution. The problem with this is that I’m having trouble finding relays that are happy with 10Ainductiveloads; obviously arcing on opening the relay is an issue.So I’ve come up with the very “clever”/overkill idea of using these two approaches at once. The idea is, connect these two switching methods in parallel. When the microcontroller commands that the load should be off, the relay is opened while the triac is still conducting. Once the relay contacts are clear, the triac is switched off. The triac doesn’t get very hot because it’s only seeing 1.5V * I dissipation for a fraction of a second; the relay doesn’t arc because the triac clamps the voltage across the relay to 1.5V (until the relay contacts are well clear). A snubber for the triac and a 700V varistor would complete the overkill.Where does this scale rate on a spectrum from 0:“Absolutely pointless/bistable 10A inductive relays aren’tthathard to find” to 10:“Common practice”?* Yes, I know, don’t mess with mains, etc, etc. I’m normally the one saying this. I have an RCD, isolated differential probes, an isolation transformer, and the knowledge that none of these things (especially the isolation transformer) are magic paragons of safety, and will only debug on mains as an absolute, absolute last resort. And the final product will be in an earthed conductive case, because I think that's best practice?

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 19, 2015, 03:05:20 am
content: They're called "hybrid relays".

author: richard.cs
author_rank: Super Contributor
timestamp: May 19, 2015, 07:16:52 am
content: How about a pair of mosfets connecteddrain to drainsource to source. You need a floating supply to drive the gates but it's not really harder than the sequenced relay and triac approach. A few diodes and a very small ferrite transformer.Edited to correct pre-coffee stupidity

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 08:09:22 am
content: Quote from: NiHaoMike on May 19, 2015, 03:05:20 amGood to know I'm not completely madQuote from: richard.cs on May 19, 2015, 07:16:52 amYeah, I was waiting for eneuro to suggest that. My concerns are:-- Don't you mean source-to-source? Otherwise the Vgs of the two mosfets isn't the same because Vs isn't the same node.-- The floating supply can't be shared between multiple output lines -- which is fine, it just means that that floating supply is genuinely a per-channel thing.They're roughly similarly complex solutions, I'd probably make my choice between them based on which has the lowest on-resistance (the dual mosfet solution has twice the resistance as a single mosfet, which will already have a higher Rds that the lower breakdown voltage mosfets).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 19, 2015, 08:33:48 am
content: QuoteISTR reading a magazine project like that maybe 25 years ago.Quote from: richard.cs on May 19, 2015, 07:16:52 amThat's called a Solid State Relay (SSR) - readily available off the shelf in a range of sizes from hundreds of milliamps to tens of amps.

author: richard.cs
author_rank: Super Contributor
timestamp: May 19, 2015, 08:47:47 am
content: Yes, that should have been source to source, ooops!There are many different ways of driving the gate, I'll describe a few:1) Drive a transformer from a single I/O line (which must be low impedance and produce fast edges). Produce a single pulse of current on each transition and store charge on the mosfet gates in the on and off states. Tricky to get right and best for situations where you're switching regularly, e.g. for phase angle control.2) Resistive bleed-down on the gate and a single I/O pin which oscillates continuously at some tens of kHz when the mosfet is on. Minimal external hardware but fairly slow switching and either consumes CPU or requires the use of a timer pin.3) As above but with external oscillators. For example a quad nand gate with each gate configured as an oscillator, running only when it's second input is high.4) Separate power and switching. A single free-running oscillator (could be one transistor and a feedback winding) driving a transformer with an arbitary number of secondaries. Opto-isolated switching. Variants include putting real gate driver chips on the floating side.And yes, that's one type of solid state relay (and what most people mean by SSR), but some manufacturers make things with triacs in and also call those SSRs. It depends how much you want to make yourself, how many channels you intend to switch, and whether Ron is your single most important requirement.

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 09:09:12 am
content: I had a look through digikey. Firstly, all the SSRs above 10A I found were triac/SCR based, and required heatsinking. I think looking at the MOSFET situation reveals why this is the case (I'm sure SSRs use MOSFETs at lower amperages):I also had a look for MOSFETs under $20, VDS limit of at least 600V (which I think is marginal given I'm working at 240VAC, plus surges and inductive spikes and such). You're looking at 42 milliohm, or 4.2W dissipation per mosfet (84 milliohm, 8.4W dissipation per channel). That, too, requires heatsinking.Meanwhile, a30 A overkill relaydoesn't require heatsinking, costs $30, and has a resistance of 30 milliohm max. So this seems to simultaneously be the cheapest and the best, at least by my metrics: prioritise cost and dissipation, ignore switching delays and lifetime (to an extent). The relay even has integrated spade terminals, so I don't even need spade terminals on my bom! (Apart from all the other ones...)I wonder, does the triac protection scheme mean that I canalmostignore the "electrical lifetime" (e.g., 10k ops) on the relay datasheet, and just look at the mechanical lifetime (e.g. 1M ops)? I mean, split the difference, 100k ops is more than enough for my purposes.

author: matseng
author_rank: Frequent Contributor
timestamp: May 19, 2015, 09:13:26 am
content: Quote from: rs20 on May 19, 2015, 09:09:12 amQuestion - if 42 milliohm would generate 4.2 watts of heat in the mosfet, wouldn't the 30 milliohm also generate 3 watts of heat in the contact elements of the relay? Won't the surfaces be degraded over time by that?

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 09:21:18 am
content: Quote from: matseng on May 19, 2015, 09:13:26 amWell, you're not wrong -- I squared R still holds of course. That's distributed through the whole relay, not just the contact point (although I imagine it's mostly the contact). Maybe they have lots of contact points, maybe having solid metal there means really good natural heatsinking, maybe the contact points can take being 300 degrees C; they're not semiconductors. I mean, it's rated for 30A, so if I use it at 10A, it's dissipating 9 times less than its design load. I don't knowhowit dissipates 27W, but you'd need to ask the relay designer

author: mzzj
author_rank: Super Contributor
timestamp: May 19, 2015, 09:26:41 am
content: Quote from: rs20 on May 19, 2015, 09:09:12 amYou beat me to it. I was just going to point that mosfet solution becames easily very expensive if you want considerably lower losses than triac.With only 1 amp or so mosfet would be easy winner in efficiency but at higher currents its not so easy.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 19, 2015, 09:42:34 am
content: Another issue with a hybrid solution is that power relays are not designed to switch with zero current- they are designed to make use of the switching current to break through any oxide layers on the contacts, so using a power relay with a triac bypass could end up with the triac conducting when you wouldn't expect it to after a long time of operation.Probably wise to turn off the triac once the relay _should_ have closed.

author: nctnico
author_rank: Super Contributor
timestamp: May 19, 2015, 09:47:43 am
content: Quote from: mikeselectricstuff on May 19, 2015, 08:33:48 amAFAIK an SSR often uses a triac. The cooling requirement of the housing should provide a hint.Quote from: rs20 on May 19, 2015, 09:09:12 amTrue but if you put several cheap MOSFETs in parallel the dissipation can be reduced considerably. It is a price versus space problem.

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 10:03:27 am
content: Quote from: nctnico on May 19, 2015, 09:47:43 amI often wish I could have a parametric sort with my own formulae with digikey etc for exactly this sort of question -- things like lumens per dollar and price*resistance. Downloading the entire catalog and doing my own excel calculations get tiresome... There might be a 50c mosfet in there with only double the on resistance, but I'd never find it by hand.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 19, 2015, 10:22:07 am
content: Quote from: rs20 on May 19, 2015, 10:03:27 amMOSFET on-resistance is inversely proportional to die area, which is the major cost, so you're not going to see huge differences in this vs. price between manufacturers

author: rs20
author_rank: Super Contributor
timestamp: May 19, 2015, 10:32:53 am
content: Quote from: mikeselectricstuff on May 19, 2015, 10:22:07 amWell, that's true only to some extent. I'd have thought that prices are set by supply and demand (although I suppose the suppliers define supply), and perhaps certain manufacturers with cheap die area don't go all the way down to 40 milliohm, and so aren't represented on the first page of results. Maybe there's 60 milliohms for $6-$10, that is actually sensible unlike what I wrote before.And, at some point the die doesn't fit in a TO-220 anymore(kidding... probably [some of those IRF directfet packages areprettybig...])

author: richard.cs
author_rank: Super Contributor
timestamp: May 19, 2015, 12:01:26 pm
content: Quote from: mikeselectricstuff on May 19, 2015, 09:42:34 amAgreed, allow the relay to make under load (which for an inductive load is fairly gentle unless it saturates) but to break with the triac switched in.I'm not against relays, if the space, coil power* and lifetime suit your application then go for them. I suspect the need to make under load means you won't get the 1M operations but you'll probably exceed the 10k substantially. I notice that on the relay you've selected the spade terminals can only take 20A unless they're soldered. I'd also worry that there is no spec for anything other than a resistive load though in conjunction with a triac you can probably ignore that.*16A octal relayswith DC coils take about 1.5W but some of the larger contactors take a couple of amps at 24V. The 30A relay you've found actually looks pretty good in this respect.

author: eneuro
author_rank: Super Contributor
timestamp: May 19, 2015, 05:18:39 pm
content: Quote from: rs20 on May 19, 2015, 08:09:22 amMaybe, but probably not for inductive loads, since didn't tested them in such configurations and used those two in spot welder primary driving, but they only triger two thyrystors in anti parallel for 230VAC mainsWaiting for more powerfull 600V mosfets to switch 1kW flat-iron based reflow oven, but still will use two PC817 in anti parallel with 200k resistors to monitor mains voltage zero crossing, while I do not think it is best idea switch powerfull loads at maximum currents in voltage maximum in the case of resistive load-for those reasons decided not to mess with high inductance transformer primary switching in spot welder, but used thyrystors and those low current AC mosfets switches simply works as AC switches to block thyrystors gate current limited by 330 Ohm resistor and 1N4007 1000V diode to ensure each thyrystor gate will be trigered with positive gate voltage and current limited to ~1A

author: SeanB
author_rank: Super Contributor
timestamp: May 19, 2015, 06:06:43 pm
content: Relay plus a snubber network across the contacts will have a pretty long life. Cheap and you can add a MOV instead of the snubber.