Using an AC Circuit to Turn on Another AC Circuit - Page 1
Forum section: electronics / projects

author: dellsam34
author_rank: Regular Contributor
timestamp: January 20, 2025, 05:26:09 am
content: I'm looking for an idiot proof solution, The problem is, everytime someone is cooking using the stove's hot plate they always forget to turn on the range fan to extract vapors and fumes resulting in a smelly house unless all the windows are opened. My initial thought in solving this issue is to build a clamp amp or amp probe around one of the stove power cord wires and use that as a trigger to turn on the range fan so that way as soon as the stove is turned on, the range fan comes on. I'm in the US, the stove runs out of 220VAC and the range fan runs out of 110VAC. I'm not really into electronic circuit design, but I'm open for pre designed circuits or a DIY ready product, I just haven't been able to find what I'm looking for using google search.

author: dellsam34
author_rank: Regular Contributor
timestamp: January 20, 2025, 05:39:52 am
content: I think I may have found the jackpot:https://www.amazon.com/CrocSee-Miniature-Normally-Monitoring-Detectable/dp/B086YT49H7/ref=pd_bxgy_d_sccl_2/141-6734972-5135907?pd_rd_w=1f43R&content-id=amzn1.sym.53b72ea0-a439-4b9d-9319-7c2ee5c88973&pf_rd_p=53b72ea0-a439-4b9d-9319-7c2ee5c88973&pf_rd_r=W2YFQTS7HT8JK766VHJW&pd_rd_wg=JowLj&pd_rd_r=95397b09-b073-4af6-b235-69526eed3398&pd_rd_i=B086YT49H7&psc=1

author: WattsThat
author_rank: Frequent Contributor
timestamp: January 20, 2025, 05:56:22 am
content: Just be aware that the output of the current sensor is “pilot duty” meaning you do not want to control a fan directly with the normally open contact as it is only rated for 2.5 amps. You’ll want to drive a relay with contacts rated for the fan amperage. Keep in mind many AC motors draw 5-10 times higher amps for a brief time at startup, this drops to the rated amps once they reach rated speed.Here’s a 30 amp rely:www.amazon.com/Tnisesm-Mounting-Connect-Terminals-HF105F-4-AC120V-8X/dp/B087G3FWVT/ref=sr_1_21

author: inse
author_rank: Super Contributor
timestamp: January 20, 2025, 06:20:18 am
content: If the hot plate has a physical switch, you could just tap there to control a relay with the right coil voltage or use a capacitive dropper.If there was a „hot“ indicator, that would be an even better place to tap into.In case the hot plate is thermostat controlled, the current detection method will cycle the fan as it does for the cooking plate.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 20, 2025, 08:33:11 am
content: Quote from: dellsam34 on January 20, 2025, 05:26:09 amFor a similar problem, I used a Solid State Relay, with a series resistor as the source 230V load (rangehood lights) was only single digit watts rated.( I was not sure how the capacitive dropper would affect that)I found a SSR with a 230V input (not common, but they do exist) - come in 10A, and mine was Ok to switch a modest load of 4 slave lamps.It should be fine with your extractor fan, also a modest load.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 20, 2025, 01:38:45 pm
content: How about some temperature sensors or a thermostat to turn the fan on if the area below is well above room temperature?

author: dellsam34
author_rank: Regular Contributor
timestamp: January 20, 2025, 08:57:48 pm
content: @WattsThat: Good tip, thanks for the heads up.@inse: That's out of the question.@NiHaoMike: That's an interesting idea, This will allow the fan to keep working few minutes later to completely extract everything. Plus everything will be tacked in the range housing, no external wires, I think I may have to go this route, do you have any recommendations?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 21, 2025, 03:32:21 am
content: This example has a relay, and a delay timer.https://www.aliexpress.com/item/1005005110922247.html

author: dellsam34
author_rank: Regular Contributor
timestamp: January 21, 2025, 07:37:57 am
content: Quote from: PCB.Wiz on January 21, 2025, 03:32:21 amI don't think I will be needing that, The relay linked above is much better.

author: Poroit
author_rank: Frequent Contributor
timestamp: January 21, 2025, 07:47:11 am
content: G'day dellsam34,Maybe you can adapt a W1209 LED Temperature Controller 12V NTC Sensor Red Display Temperature Control Range - 50-110 ° C Relay Output.Just watch the response time of the sensor.You may need to remove the metal cover (if possible) or source your own bead thermistor so that the fan does not run for to long after the stove is turned off.

author: dellsam34
author_rank: Regular Contributor
timestamp: January 21, 2025, 07:51:38 am
content: I think I found what I'm looking for, What do you guys think?https://us.amazon.com/Hipoilk-Thermostat-Temperature-Ventilators-Replacement/dp/B0DJY52TND?th=1

author: inse
author_rank: Super Contributor
timestamp: January 21, 2025, 08:36:39 am
content: Don’t know if this is going to work.If you mount it close to the cooker hood, it might cycle on and off during cooking as temperature would drop when the fan starts running.You need something to detect heat radiation

author: dellsam34
author_rank: Regular Contributor
timestamp: January 22, 2025, 08:07:09 am
content: I will see if I can extend the heat sensor to a location not affected by the fan's air flow.

author: inse
author_rank: Super Contributor
timestamp: January 22, 2025, 09:07:03 am
content: Make sure the sensor is either galvanically isolated or take care you insulate it accordingly.I still don’t regard it a promising approach.

author: Langdon
author_rank: Frequent Contributor
timestamp: January 23, 2025, 04:04:21 pm
content: AC relays do exist.I have seen 120VAC relays.They are not that common, but not that uncommon either.Old-school Air Conditioning units use a 24VAC to switch the Compressor (and possibly also the fan) on.You can just use a 24V transformer and a 24VAC relay.Or you can just use 3 diodes, a resistor (to reduce the voltage a tad), and a DC relay.You can get away with 1 diode, but 3 is better.Example circuit:Connect "AC Neutral" to the "negative" or "positive output" of a rectifier diode, and connect the other pin of the diode to one pin of the relay coil.Connect "AC line" to the "positive input" of a second diode.Connect a resistor (about 10Ω) between the unconnected pin of the second diode and the unconnected pin of the relay coil to reduce the voltage (the voltage of rectified DCreachs the AC voltage times the square root of 2). (24VAC rectifies to as high as 34VDC)Connect a third diode facing "backwards" across the relay terminals to deal with the inductive reverse voltage released by the relay coil.This could be done with just 1 diode and a resistor, but the 3 diode approach is probably safer (the diodes block half the AC) and more efficient (reverse voltage is shorted by the diode for free, instead of contributing to the power consumption of the device)

author: dellsam34
author_rank: Regular Contributor
timestamp: January 23, 2025, 06:05:57 pm
content: Quote from: Langdon on January 23, 2025, 04:04:21 pmI don't quite understand your post, I'm looking for a way to sense the current without modifying the 220V circuit for the stove or interrupting it. I have no clue how you implement what you are talking about without chopping off the 220V wires and inserting such components. A schematic may help illustrate your approach but I don't think it fullfills my need.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 24, 2025, 12:32:57 am
content: Quote from: dellsam34 on January 21, 2025, 07:51:38 amI don't see how you would use it. It appears to monitor the temperature of the air flowing around it, so where would you mount it to get a reliable switch-on as soon as the hotplate is working? And it's actually quite bulky - it'll be hard to put it somewhere out of sight, and yet still exposed to the heat from the hotplate.I think detecting hot air might be tricky anyway, because the hotplate might not generate much air movement (especially if it's the infrared type) - the greatest output will be in the form of radiant heat. The device you linked will eventually respond to radiant heat because its outer case will warm up, warming up the air inside, but it's probably not going to be as responsive as you require.As has already been said, detecting current flow in the hotplate will drive you nuts as the fan starts and stops in time with the thermostatically controlled hotplate element.If it were me, I would I would use the "hot" indicator and tap off that.Does your stove have a "hot" indicator or not?

author: dellsam34
author_rank: Regular Contributor
timestamp: January 24, 2025, 07:25:06 am
content: I don't need immediate response, The steam or fumes from cooking are not going to generate as soon as the hot plate is turned on, So a delay makes the process more efficient, Fan starts when heat is built up, and stops later after the heat and steam goes down even if the stove had already been turned off. I do have "Hot" indicator but I don't want any wires going in or out of the stove for safety purposes and I don't want any modifications to the stove. I just have to find a suitable location for the thermostat, maybe in the front control panel where the buttons are, this side is not directly exposed to the air flow and it receives a lot of heat from the top of the hot plate, If I can glue in the sensor probe with thermal paste to the inside surface of the stainless steel sheet metal.

author: SteveThackery
author_rank: Super Contributor
timestamp: January 24, 2025, 12:25:04 pm
content: Understood. Let us know how you get on.

author: zapta
author_rank: Super Contributor
timestamp: January 24, 2025, 06:10:19 pm
content: There are of the shelf master/slave switches where turning on the master device applies power to the slaves. For examplehttps://www.amazon.com/AblePower-Energy-Protector-Power-Managed/dp/B07DGLR17P

author: dellsam34
author_rank: Regular Contributor
timestamp: January 24, 2025, 09:45:42 pm
content: That's 120V only, it won't work in my case.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 24, 2025, 10:10:19 pm
content: Here's a switch I built to turn on a shop vac being used with a power tool (bandsaw) whenever the saw was in use:It works really well.The transformer came from a GFCI outlet. The incoming power just goes through the single large conductor.The SSR I used is plenty beefy enough to handle your fan as a load.Of course, to use this with your stove you'd have to connect the switch between a plug and a receptacle for the oven plug, assuming the stove isn't directly wired into a junction box. (You probably wouldn't want to cut open the stove cord, although you could do that.)Maybe this'll help?

author: inse
author_rank: Super Contributor
timestamp: January 25, 2025, 04:59:54 am
content: Quote from: dellsam34 on January 24, 2025, 07:25:06 amAnd how was this to be installed?Quote from: dellsam34 on January 20, 2025, 05:26:09 am

author: Analog Kid
author_rank: Super Contributor
timestamp: January 25, 2025, 05:22:15 am
content: My switch could be connected between the stove plug and the outlet, with a short pigtail with a plug. No modifications to anything (except maybe to the fan, which would have to somehow be plugged into the switch).

author: dellsam34
author_rank: Regular Contributor
timestamp: January 25, 2025, 07:45:14 am
content: Quote from: inse on January 25, 2025, 04:59:54 amAround the stove wiring inside the cabinet, and goes up inside the cabinet then to the right but still inside the kitchen cabinets away from the stove cavity, I cannot modify the stove or have wires dangling behind it, I will be replacing it in a year or two, so I don't have to do it all over again.

author: dellsam34
author_rank: Regular Contributor
timestamp: January 25, 2025, 07:46:31 am
content: Quote from: Analog Kid on January 24, 2025, 10:10:19 pmThis won't work for me, The stove is 220v.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 25, 2025, 06:24:05 pm
content: Quote from: dellsam34 on January 25, 2025, 07:46:31 amIt'll work:You can either power the fan from one of the legs of the 240 VAC (= 120 V), or run a separate 120 volt line in for the fan.Ackshooly, the SSR has nothing to do with the ability to use this on 240 VAC. That's up to the current transformer, which can easily handle that. You just need to make sure that the conductor going through the transformer (just a piece of insulated wire) is a big enough gauge (10 AWG?) to handle the stove current.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 25, 2025, 06:43:22 pm
content: I've redrawn the circuit for your application:

author: dellsam34
author_rank: Regular Contributor
timestamp: January 25, 2025, 11:26:33 pm
content: It is still invasive as it involves cutting wires, As you already know here in the US the 220V wires are too thick and I don't think they can be wired with spade connectors, it's a fire hazard waiting to happen, By the way the local code allows only twisted caps isolators, This is why I was initially looking for an amp sensing solution before I switched to heat sensing solution, I do really appreciate your effort to help.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 25, 2025, 11:31:03 pm
content: No cutting wires at all.Notice the plugs and sockets on the schematic? This is a totally sef-contained "plug-n-play" thing.I built mine in a square outlet box which will hold the two sockets (one for stove, the other for fan).Two plugs emerge from the box, one for the 240 for the stove, the other for the 120 for the fan.You'll probably have to go to an electrical supplier for the stove plug and socket, unless your local big-box home-improvement store happens to carry them. (There are several types.)Here's an updated schematic making the wiring clear (no ambiguous grounds shown).

author: drksy
author_rank: Regular Contributor
timestamp: January 26, 2025, 03:13:40 am
content: Quote from: Analog Kid on January 25, 2025, 11:31:03 pmOP may be confused looking at this picture since the wire going from the socket to the stove seems to be cut to passing through a transformer where you marked a star. It's probably more helpful to show how a current transformer looks, maybe with exact part numbers. The thick live wire carrying the current to the stove would simply pass through the center of a toroid, so it isn't being cut. It's similar to how AC current is sensed with a clamp meter.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 26, 2025, 03:26:26 am
content: There's a picture inthis post.No part #, because it's cannibalized from a GFCI outlet, available everywhere.The wire that passes through the current-sensing transformer is inside the box, between the plug and socket for the stove.Plug the plug into the outlet the stove is currently plugged into.Plug the stove into the switch box.Here's a picture of another type of current transformer found inside a GFCI outlet:The power line (in the schematic the line between the plug and the socket that goes to the stove) is the thick conductor through the center (there are two here, but you could just discard the other one for this type; you only need 1 conductor going through to form the primary side of the transformer).The coil of fine wire is the secondary, as shown in the schematic.Does that make sense to you?

author: Analog Kid
author_rank: Super Contributor
timestamp: January 26, 2025, 04:01:18 am
content: This shows how everything fits into a box for my shop-vac switch:Yours would be a bit tighter, since you need 2 outlets side-by-side (the 240-volt outlets are singles, not duplex), but I think you could cram everything into one of these square boxes. You might need to use a deep box.The orange cord is the incoming power. You'll need 2 cords, one for the stove (240 volts), the other for the fan (120 volts).

author: inse
author_rank: Super Contributor
timestamp: January 26, 2025, 06:19:36 am
content: Educating the people to use the hood when cooking is not an option?

author: dellsam34
author_rank: Regular Contributor
timestamp: January 28, 2025, 08:33:23 am
content: Quote from: inse on January 26, 2025, 06:19:36 amMe using the phrase "idiot proof" was not a coincidence.

author: Zero999
author_rank: Super Contributor
timestamp: January 28, 2025, 09:49:35 am
content: Quote from: Analog Kid on January 25, 2025, 11:31:03 pmA potential issue with that circuit is current transformers don't like being open circuit, so that Schottky diode might be subject to high reverse voltages. A simple fix is to put another diode in the reverse direction, directly across the transformer.The OP's cooker might also draw too much current for that tiny transformer to handle. Your saw is much lower power than a heating element. GFCI transformers arenotdesigned to handle the full current continuously.If breaking into the cooker wiring is impractical, then a split core current transformer can be used, which simply clips over the current carrying cable.EDIT: GFCI transformers arenotdesigned to handle the full current continuously

author: Langdon
author_rank: Frequent Contributor
timestamp: January 30, 2025, 11:22:34 pm
content: I have designed 2 devices that can accomplish this task.Both are simple, reliable, and consume no standby current.WARNING: One of the line wires for the stove must draw 0 current when the stove is off.This will not work on stoves that don't utilise the neutral.Determine which line does not power the clock and draws 0 current when the stove is off; this is is the line that is switched.

author: Analog Kid
author_rank: Super Contributor
timestamp: January 30, 2025, 11:40:35 pm
content: Quote from: Zero999 on January 28, 2025, 09:49:35 amI don't think any of that would be a problem using this with a stove instead of my power tools.It's true that the stove is going to draw a lot more current than my bandsaw, but as you wrote, GFCI transformers are designed to handle that kind of current continuously, so I don't see that as a problem.Keep in mind that the high current is flowing through the "primary" which is just a thick piece of wire rated at the stove's full current (I'm guessing 10 ga. wire here), while the secondary really doesn't matter; its ratings aren't going to be exceeded at all.So far as your first caution about open-circuit voltages, don't think that will be a problem either. The Schottky diode I used is rated at 40 volts, waaaaay above anything that GFCI transformer is ever going to deliver.Worth a try in any case. So what do you say, OP?

author: Zero999
author_rank: Super Contributor
timestamp: January 31, 2025, 08:56:34 am
content: Quote from: Analog Kid on January 30, 2025, 11:40:35 pmThe secondary does matter.The current flowing through the secondary is proportional to the primary current. Under normal operation in a GFCI, there are two primary windings, consisting of the phase and neutral, running through the middle of the current transformer. The currents through each primary are out of phase, so their fields cancel. Under fault conditions, the flux no longer cancels, a current is induced in the secondary, which triggers the trip mechanism that safely disconnects the power.In your circuit, there is only one primary turn and the current continuously flows through the secondary. If the current is too high, for too long, then it will overheat. The continuous secondary current rating is not specified. It might work in the original poster's application, or it might smoke.QuoteHow do you know that? Have you connected it to an oscilloscope?Isn't it easier just to add another diode (1N4148 will do) to avoid all doubt?References:https://www.google.com/search?client=firefox-b-d&q=current+transformer+open+circuit+high+voltage

author: Langdon
author_rank: Frequent Contributor
timestamp: February 05, 2025, 02:19:32 pm
content: Just use my design!I will post an improved schematic later today.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 05, 2025, 03:40:46 pm
content: 1N4148 are suitable for low-current only.They are great and cheap and very reliable, but should never be used for power applications.Use 1N400x or something.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 05, 2025, 04:17:00 pm
content: All my schematics are located here:https://www2.langdonstaab.ca/files/Automatic%20switch%20schematics/

author: Analog Kid
author_rank: Super Contributor
timestamp: February 05, 2025, 10:43:52 pm
content: Could you possibly make your schematics any more unreadable?Heck, if you could supply a readable one I could draw a real nice standard schematic from it.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 07, 2025, 08:23:45 pm
content: I'm a 16-year-old with a writing disablility.Cut me some slack!The V3 one is better.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 08, 2025, 10:41:55 pm
content: Perfect solution:https://www2.langdonstaab.ca/files/Automatic%20switch%20schematics/Automatic%20fume%20hood%20switch%20V3.pdfFor resistors labeled a and b, use some resitive device that limits the voltage suppied to the relay.A combination of a PTC thermistor and resistors should work (coils need more power to turn on)

author: Analog Kid
author_rank: Super Contributor
timestamp: February 09, 2025, 02:42:43 am
content: I see what you're doing there. Two relays, one self-activated by the oven, the other one controlling the vent fan.Couple things: why two relays? You could just use a double-pole relay in the oven circuit to control itself (latching), with the fan switched by the other set of contacts.Not sure why you have that "reset" switch which would de-energize the oven; is that some kind of safety feature?Also not sure why you have the oven relay connected to both legs of the circuit through diodes; one side would suffice to energize the relay.This would work. I still like my design better: uses no mechanical relays, which are expensive, noisy and harder to mount than my small solid-state relay.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:35:21 pm
content: I think one SPST 50A relay and one SPST 15-20A relay is probably cheaper than one 50A DPST relay.For lower, equal amperages, one relay is fine.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:37:11 pm
content: You press RESET when you turn "off" the oven.The oven still recieves power through the line that is not switched and the neutral (for the clock and stuff).RESET turns the fume hood off.(You won't remeber to turn the hoodoffbecause it is noisy)Technically, you only need 2 diodes, not five.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:45:46 pm
content: Quote from: Analog Kid on February 09, 2025, 02:42:43 amYou don't really understand how my circuit works, do you?Obviously, you can substitue a solid-state switch for the relays in my design.The relay is just a suggestion an equivalent is fine.50A solid-state switches are not so cheap either.But using a solid-state switch for the hood should be fine.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:51:16 pm
content: The GFCI transformer method proposed by Analog Kid should work fine too.Just cut the wires that go through the coils and feed a thich stove conductor through the hole.Though weird stray AC currents may sporadically turn on the fume hood for no reason (this device is perhaps a bit like the opposite of a surge protector).

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 06:57:56 pm
content: Quote from: WattsThat on January 20, 2025, 05:56:22 amIssues with this method:What if the 30 amp relay draws more than 2.5A when the coil is energised.The clock draws power when the device is off. Generally onlyoneof the wire leading to the stove has no current passsing through it when the stove is off.FUN FACT?: Over the couse of its lifetime, a microwave'sclockwill consume as much electricity as the electronics that actually cook stuff. (microwaves are usually only used sporadically, the clock runs 24/7)

author: Zero999
author_rank: Super Contributor
timestamp: February 09, 2025, 07:02:54 pm
content: Quote from: Langdon on February 07, 2025, 08:23:45 pmHe has a point. It's very difficult to understand unclear schematics. My drawing and handwriting are just as scruffy as yours, so I use CAD. My favourite is LTSpice, even if I don't simulate it. KiCAD is much better for drawing, but is bigger.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 09, 2025, 07:56:23 pm
content: Why would I spend time learning and using an annoying CAD program in order to give away my design for free?What's the incentive?I enjoy electrical engineering, not CAD.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 09, 2025, 07:56:30 pm
content: Quote from: Langdon on February 09, 2025, 06:51:16 pmNo, that won't happen.I've used my shop-vac activation switch pretty extensively and tested it with various loads (power tools).There's a lower limit to the amount of current a device has to draw before it'll turn on, so it effectively filters out any kind of spurious currents (like, say, the clock on an oven). Anything below 2-3 amps continuous current won't trip the SSR.Try it, you'll see.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 09, 2025, 07:59:16 pm
content: Quote from: Langdon on February 09, 2025, 07:56:23 pmFair question.However, I would consider the LTSpice alternative:It's freeThe schematics it produces are a lot better than your hand-drawn ones (no offense intended)It's a lot easier to learn than any CAD programIt also allows you to simulate your circuit once you've drawn it

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2025, 08:46:41 am
content: Quote from: Langdon on February 09, 2025, 07:56:23 pmBecause you'll have to if you decide to do it professionally. Otherwise, you don't have to do anything, but don't whine and play all sorts of cards: age, disability, when people tell you they can't understand your scribble.Heck, I started using a computer to help be draw schematics, long before I did it professionally. One of the reasons was because I found I couldn't read my own schematics, when I came back to them a couple of years later. It also saved me time, as it reduced the number of errors, when it came to building it.Unfortunately I did poorly at Design and Technology A-level because it all had to be hand drawn. Fortunately it wasn't a problem at college because I didn't have to draw in exams and I used CAD for assignments.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 10, 2025, 12:24:50 pm
content: Quote from: Zero999 on February 10, 2025, 08:46:41 amHeh; similarly, as a programmer, that's why I take care to generously comment my code. Not as a courtesy to anyone else--I'm a hobbyist and not a production code monkey--but because I want to be able to figure out what the hell I was thinking when I come back to that code a year or two later.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 10, 2025, 03:47:04 pm
content: Why not just use my two-diode method to power a transformer and then usethatto power a relay?Pulsating DC can power a transformer; that's how inverters and switching power supplies work(AFAIK).A standard 24V/120V transformer (like the ones used for doorbells and thermostats an stuff) should work.To a transformer or any inductive device half-wave 240VAC should be more or less the same as 120VAC full-wave.LTSpice runs on Windows and Mac; I use Ubuntu and macOS High Sierra (10.13.6).

author: Zero999
author_rank: Super Contributor
timestamp: February 10, 2025, 09:10:57 pm
content: Quote from: Langdon on February 10, 2025, 03:47:04 pmNo. It will smoke. A mains frequency transformer is totally different to a fly-back transformer used in a switch mode power supply.QuoteI use LTSpice on PCLinuxOS under WINE and it works perfectly.

author: Analog Kid
author_rank: Super Contributor
timestamp: February 10, 2025, 10:44:10 pm
content: Quote from: Zero999 on February 10, 2025, 09:10:57 pmSo you're saying that this won't work?If so, why? I don't see why having only one side of the cycle (positive in this case) would cause the transformer to take up smoking.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 10, 2025, 11:10:37 pm
content: A 120V transformer that is fed 240VAC through a diode will recieve basically half the 240VAC.240÷2=120I don't see why it would smoke.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 10, 2025, 11:12:44 pm
content: Anyway, you could always just use a 240V:24V transformer.What's the problem?

author: themadhippy
author_rank: Super Contributor
timestamp: February 10, 2025, 11:52:56 pm
content: Quotewhy waste money on diodes and transformers.just use a relay with a suitably rated AC coil

author: Analog Kid
author_rank: Super Contributor
timestamp: February 11, 2025, 12:21:15 am
content: Quote from: themadhippy on February 10, 2025, 11:52:56 pmBecause the relay would have to be energized only when there's current flowing through the stove.You can't put the relay in series with the stove.You can see that, right?That's why my idea is so much better ...

author: pgo
author_rank: Regular Contributor
timestamp: February 11, 2025, 01:16:31 am
content: Quote from: Analog Kid on February 10, 2025, 10:44:10 pmPlease don't use this circuit. The transformer will saturate and be destroyed as the average current is now non-zero.

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 08:54:30 am
content: Quote from: Langdon on February 10, 2025, 11:10:37 pmQuote from: Analog Kid on February 10, 2025, 10:44:10 pmBecause the transformer is being powered off DC, not AC.The transformer is an inductor. The reactance limits the current more than the resistance. The circuit you've shown will just put DC through the winding, so the only thing which will limit the current is the resistance, which will allow far too much current to flow and cause smoke.https://en.wikipedia.org/wiki/Electrical_reactance#Inductive_reactance

author: Analog Kid
author_rank: Super Contributor
timestamp: February 11, 2025, 09:38:54 am
content: Well, it's being powered by sort-of DC (we've had this discussion elsewhere here). Specifically, half-wave rectified AC.So is it the fact that the current never reverses polarity that would fry the transformer? Because it's not a constant DC current, but pulsating.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 11, 2025, 01:46:16 pm
content: Quote from: themadhippy on February 10, 2025, 11:52:56 pmSee my V3 design.https://www2.langdonstaab.ca/files/Automatic%20switch%20schematics/Automatic%20fume%20hood%20switch%20V3.pdfOnly needs 1-2 relays/solid-state switches (for high current, you probably need a least one relay. On DigiKey.com, there are lots of 40A SSRs, but no 50A!)

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 02:08:30 pm
content: Quote from: Analog Kid on February 11, 2025, 09:38:54 amThe fact there is significant net DC current in the transformer will cause the core to saturate, too much current to flow and overheating.Heck, even connecting a significant load to the secondary, via a single diode i.e. a half wave rectifier, can cause it to overheat.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 11, 2025, 02:13:13 pm
content: Honestly, relays are so cheap, I don't know why SSRs are ever used.https://www.digikey.com/en/products/filter/power-relays-over-2-amps/188?s=N4IgjCBcpgLGAGKoDGUBmBDANgZwKYA0IA9lANrgCsABAIIjFgBs9j4AHG8QEwLcgeYATx4iAzCNoNerGSHH9544crHLJy6e3Fz2sJfu3FYXeVUPEqxkM0u2bAdnuObHexxsBOe4l%2BIBMD5AxUCLQLtA1xEEe1E4nhseSOVYgXFw%2BQN7C3s7F0yQAF1iAAcAFygQAFUAOwBLcoB5dABZfExcAFcAJ3wQAF9iAFoeZBA0SHKerqJSChAqYoGh23H6gBMq4cQIMsrIEHZygE9S-sPOtBWgAhttps://www.digikey.com/en/products/filter/solid-state-relays-ssr/183?s=N4IgjCBcoCwMwyqAxlAZgQwDYGcCmANCAPZQDa4ArAAQCCIRYAbHQ%2BAOyuMAcXIATAAY%2B-GCJr0i-TpIG9ZcYQv584EtjCUb1RSlt06QTfUcPsT7Q9xPd%2BAOiuGAnCaeGwgk2A98wQ3-zueiKefHpaALpEAA4ALlAgAKoAdgCWsQDyaACyeBg4AK4ATnggAL5EALT8SCCokLFFBYQk5CCUIBFlFUa1qQAmCZUeEDHxkCBssQCe0aUT%2BajdQABut you can pay way more for an expensive SSR is you wish.It's your choice.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 11, 2025, 02:15:35 pm
content: This looks like an amazing solution, if you are willng to pay for it:https://www.digikey.com/en/products/detail/selec-controls-usa-inc/RSS-TA1A140ZP/21802521It even accepts AC input!

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 03:30:49 pm
content: Here's another idea.Use a 240V:6V mains transformer in reverse to sense the current.D1 & D1 clamp the voltage across the primary of the transformer to about 0.8V peak-to-peak. They need to be rated to carry the full load current.The relay must have a 24VAC coil and be capable to switching the fan.The transformer's secondary winding must be able to pass the required current for the relay coil, which is typically 50mA for a power relay. 0.05*240 = 12, so use a 12VA transformer.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 11, 2025, 03:51:53 pm
content: Quote from: Zero999 on February 11, 2025, 03:30:49 pmCool idea!But what diode can handle 50A!!!!Also, this design would be very easily triggered.0.8VAC could just as easily be induced by stray eddy currents.Get a multimeter and set it to VAC; touch one probe to the ground of an outlet, the other to a conductive object (such as yourself).It will probably measure something like 0.8VAC.Also, many diodes only have a voltage drop of about 0.5VThe stove is more than a simple resistor, it is like a bunch of resistors that can be switched on and off with a clock connected between one line and the neutral (or across both lines).My solution seems more practical; 2 normal diodes vs 2 HUGE diodes and a transformer.

author: Zero999
author_rank: Super Contributor
timestamp: February 11, 2025, 04:08:00 pm
content: Quote from: Langdon on February 11, 2025, 03:51:53 pmWho said anything about 50A? Where did you get that figure from?Anyway, there are plenty. Heck there are even diodes capable of 50kA and I forgot, that each diode only conducts half a cycle, so it can be rated for a little less.QuoteNot true.QuoteNow connect a 1k resistor between the meter probes and watch the voltage fall to near zero.QuoteOnly Schottky diodes and the symbol I used is for silicon.QuoteSo what?QuoteI don't know. I couldn't read the first two scribbles and that link to the third one wouldn't work for me.

author: inse
author_rank: Super Contributor
timestamp: February 11, 2025, 07:08:04 pm
content: You seem to get along pretty well even without the thread starter…

author: Analog Kid
author_rank: Super Contributor
timestamp: February 11, 2025, 10:05:35 pm
content: Quote from: Zero999 on February 11, 2025, 04:08:00 pmThat comes from the current drawn by the stove, which must pass through the diodes, right?After all, you yourself wroteQuote(With some possible derating since each diode only conducts on half the cycles, as you wrote.)BTW, who the hell knows where the OP is at this point?

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 09:35:18 am
content: Quote from: Analog Kid on February 11, 2025, 10:05:35 pmNo one said the stove draws 50A. 12kW sounds like a lot for a stove.Quote from: inse on February 11, 2025, 07:08:04 pmQuite often people like to discuss things as a thought experiment.In this case, I doubt measuring the power to, or consumed by the stove will work that well. The stove might have a thermostat, in which case it won't draw power whilst it's on. A temperature, humidity or steam sensor might be more reliable methods of turning the fan on and off.

author: inse
author_rank: Super Contributor
timestamp: February 12, 2025, 10:26:49 am
content: What about a low-tech approach?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 12, 2025, 02:52:37 pm
content: The stove is not just an oven, it is also a cooktop.What happens if the oven is preheating on quick preheat (full power), all the heating elements are on HIGH, and something is connected to the auxiliary outlet drawing 16A (the breaker won't trip instantly.)?

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 03:01:07 pm
content: Quote from: Langdon on February 12, 2025, 02:52:37 pmHow do you know that? Your crystal ball must be better than mine.QuoteI thought you said 50A and now you're talking about a 16A breaker?

author: Langdon
author_rank: Frequent Contributor
timestamp: February 12, 2025, 03:06:00 pm
content: Some stoves have an auxiliary 15A 120V outlet on them that you can plug stuff into.The auxiliary outlet has its own 15 amp breaker (kuo-yuh or similar).However, you can actually draw more like 16A becuse the breaker won't trip immediately.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2025, 03:10:57 pm
content: Quote from: Langdon on February 12, 2025, 03:06:00 pmYou're just making baseless assumptions.My stove is a separate unit to my oven. My stove is gas and the oven is electric. I could, if I wanted to, I could replace my gas stove with an electric stove, or change my electric oven for a gas oven.

author: Langdon
author_rank: Frequent Contributor
timestamp: February 12, 2025, 03:33:02 pm
content: I am simply suggesting a hypothetical scenario where that design would not work.It's a cool design though.