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Recently my electric clothing dryer stopped working. Blew the thermal fuse. replaced. Later the heat still wasn't working and I found a lack of continuity on a couple of terminals on the timer, the heating element circuit could not be closed. There is just too much corrosion even after cleaning to get a good connection inside the timer.

Thing is this is a very basic mechanical timer/switch that costs 75$. I feel like for the same price I could run this with an ardunio instead. More fun and way cooler.

So now on to my problem. I cant seem to find a relay suitable for the load generated by the dryer. I need a relay that can handle 240 volts and from what I can see would need to handle about 4k watts? Amazon, adafruit, nothing as far as I can find. I am wondering if someone with more electrical experience can suggest an appropriate relay to use.

enter image description here

PS Notes: Looking a bit harder I found this under furnace parts on amazon. https://www.amazon.com/dp/B019132A1I?psc=1

Thanks all for your input. I suspected the reasoning behind a very simple (now I know what to call it) centrifugal switch. The heater cannot physically be a completed circuit without engaging the blower. I realize this now looking at the order of the copper bars. I don't know that is really intentional as a "safety" feature, but fair enough, you're right I don't know for sure.

I've decided to NOT do this project simply considering the risk/reward as someone pointed out. I strongly believe the risk truly isn't that great, and ill explain my reasoning below, but in any case, not a great first Arduino project. Maybe in the future if I have a spare dryer and more time to work out the safety aspects. I'm pretty sure we could, with time, make an even safer dryer that the manufacturer.

So here is why I think the danger of this is pretty exaggerated.

I still strongly believe that in the event the blower does not engage the thermostat and or thermal fuse would blow very quickly, stopping heat production.

Lets say for whatever reason the element does get stuck on. Not ideal right? but is it truly going to start a fire? I am certain lint in the exhaust is infinitely more dangerous, and ONLY when the blower is running.

The heating element itself is well sealed away from anything flammable, it is completely encased in a few layers of metal, the floor is cement and the basement wall is cement. Consider that only fresh air is pulled into the element. It is NOT full of flammable lint, the lint is no where near the element. Only when the blower is on can the heat reach anything in the exhaust section.

Having said that there are a few factors here. 1. my wife wants the dryer fixed sooner than later :O and 2. I can't deny that SOME risk at least potentially exists. 3. I'm a coder with limited electrical experience. My first project should likely not involve 240v or 4k watts :P Never the less that's to you guys I did learn a few things anyway.

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The timer has three contacts- two of which are high current and act as a permissive chain such that the blower motor is powered preferentially.

You could replace the timer with an electronic device controlling a contactor with at least three poles, rated for the maximum current the heater and motor draws together, plus some safety margin- maybe 35 or 40A. These contactors are widely available, but they do tend to create a lot of electrical noise when they switch (which can screw up your micro, but is outside of the scope of this answer).

Personally, I would not do this, given the risk-return ratio. I would certainly not use a solid-state relay, given their propensity to fail 'on'- this thing only has to ever operate a few thousand times in its life and any contactor can do that reliably. The one you linked to is not really an SSR- it's something else and is unsuitable.

I also note that your schematic shows the neutral connected to the chassis. I believe this is an old (and now considered unsafe) connection method that was grandfathered for a period of time. If the NEUTRAL becomes disconnected the USER can be electrocuted, generally considered to be a bad thing.

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    \$\begingroup\$ This is a pretty simple circuit and would be an interesting subject for a complete redesign, up to modern safety standards, so long as the designer/engineer understands the safety aspects. As you say, the neutral connection to the cabinet is not something we'd want to tolerate these days. In practical terms an arduino seems like overkill, but the OP does admit that this is for sheer coolness. The key thing to me would be to incorporate interlocks at the low level to prevent dangerous operation under fault conditions. \$\endgroup\$ – Ian Bland Oct 16 '16 at 11:26
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    \$\begingroup\$ @IanBland There are already interlocks that have passed safety requirements. It's a lot less risky to leave those in place and would be prohibitively expensive to replace them. Note for example, what appears to be a simple mechanical centrifugal switch (1M/2M) allowing the heater to be turned on only when the blower motor is physically rotating. Sure you could slap a sensor in there, but the engineering to assure safety would be non-trivial. I think (agree?) it would be an interesting class project to do an evaluation.. implementing it in a real house, not so much. \$\endgroup\$ – Spehro Pefhany Oct 16 '16 at 16:36
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    \$\begingroup\$ Valid point, but I really do not like seeing the Neutral and Earth connected like that. For a start, you'll trip an RCD instantly (I think in the USA they're called GFIs). Secondly, you can get neutral current from the rest of the installation flowing through that connnection which if sufficient in magnitude is a fault or fire risk. \$\endgroup\$ – Ian Bland Oct 16 '16 at 17:50
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    \$\begingroup\$ I really appreciate that you guys elaborated instead of going directly to "this is too dangerous". \$\endgroup\$ – Paul Wade Oct 17 '16 at 20:16
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    \$\begingroup\$ @IanBland -- neutral-chassis ties are common on older North American heavy appliances due to a wiring quirk from the 50s (6/3 NM wasn't a thing then, leading to SE cable being used for that job, which meant that only 3 wires were available, and with the controls of that day running on 120VAC, this meant that the neutral was needed too) \$\endgroup\$ – ThreePhaseEel Oct 17 '16 at 22:35
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I would certainly not replace the timer switch by an arduino or anything self-built, because in dryers, the drive motor also operates the blower fan. Which means, if by any mistake you make your drive motor isn't on while the heater is on, you burn your house down. When this happens, you want to explain your insurance company the blower manufacturer is guilty.

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  • \$\begingroup\$ I'm not sure I agree with you. While an ardunio is not a robust as this very simple timer (we are talking about 6 copper bars that are pushed together to complete different circuits). the dryer has many other components to prevent what you are talking about. Namely the thermal fuse. If the ardunio failed and some how started the heater without the blower. I expect it would blow the thermal fuse before it burned the house down. I do appreciate your advice though and will keep it in mind. \$\endgroup\$ – Paul Wade Oct 15 '16 at 21:42
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I know this idea might sound good to you and sparks your technical interest on the surface but believe me this is not the place to implement an Arduino work around. As others have responded you could potentially put yourself and family in a dangerous and life threating situation. (no electrical experiment is worth this)

Please purchase the correct parts and install them properly. There are several great online appliance parts suppliers just waiting to solve your problem. (use them!) I've used the one below with success.

www.appliancepartspros.com

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