Mechanical relay driven heating element Sous Vide machine? - Page 1
Forum section: electronics / projects

author: e-pirate
author_rank: Regular Contributor
timestamp: December 17, 2024, 09:57:43 am
content: Hi folks.Recently I purchased INKBIRD 300 Sous Vide machine. After switching it on and submersing to the container with water I realized that a regular mechanical relay is used to drive the heating element. I can clearly hear relay clicking every 2-3 seconds while machine is doing its best to keep set temperature accurate. In my humble opinion it is not the best solution to drive 1KW load with a relay switching on-off every couple of seconds, I expected something like solid state relay or a triac maybe. In aquarium heaters it's a common case for relay to fail in on condition effectively cooking all the life stock. I don't think Souse Vide is any different.So, is it safe to use such a souse vide machine and I'm just paranoid?

author: moffy
author_rank: Super Contributor
timestamp: December 17, 2024, 10:16:35 am
content: The older electric elements on cooktops used to use mechanical contacts to regulate the temperature, and from my experience were quite reliable, it depends on the quality of the relays contacts, but I agree that a triac would make more sense and give better control.

author: e-pirate
author_rank: Regular Contributor
timestamp: December 17, 2024, 10:23:37 am
content: According to my experience and others posts it starts clicking only if temperature is over 70C. I've used the unit once for cooking chicken at 65C and never heard it clicking.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2024, 11:43:56 am
content: Quote from: e-pirate on December 17, 2024, 09:57:43 amWow, I would seriously consider adding another thermostat set to a slightly higher temperature, along with an alarm, or design my own with a safety relay.Quote from: e-pirate on December 17, 2024, 09:57:43 amDoes it have a thermal fuse?

author: tom66
author_rank: Super Contributor
timestamp: December 17, 2024, 12:07:28 pm
content: A relay when switching a resistive load should last around 100,000 cycles, so if the cooker gets used for 8 hours every day, I would expect only about 10 days lifespan before the relay was significantly worn. Maybe send the machine back because that seems like it is designed to fail. Even if I am off by an order of magnitude on the relay lifespan it won't last past the year.

author: e-pirate
author_rank: Regular Contributor
timestamp: December 17, 2024, 12:44:39 pm
content: QuoteI haven't took it apart yet, just got it from China. It is CE-complaint, so it should, I believe.QuoteThey give 1 year warranty, but my calculations are the same. In that scenario even if it will not cause fires, it will simply fail very quickly. I will check it with container of a bigger volume giving it more thermal mass and stability, but I don't think it will change much. It looks like a design failure. I will also ask INKBIRD about lifespan.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2024, 12:47:43 pm
content: Mechanical relay is still a pretty valid solution. Triacs have poor overvoltage handling, fail as short circuits, and have large leakage current (that doesn't matter here, but mentioning for completeness). A 20-cent (in volume) relay is a reliable solution which could last for years, while a 20-cent triac replacement would fail soon. Poor quality relay would be a problem of course, but so would a poor quality solid state replacement.It is true that failure mode for a relay is contact welding (stuck on), but the same is true for semiconductor solutions and they are much more sensitive to overload current of even short overvoltage spikes on mains.Though 3-second period is still too short for a relay, so take this as a generic comment. But you can really see relays used in industrial PID controllers which operate the relay more than once every minute.

author: e-pirate
author_rank: Regular Contributor
timestamp: December 17, 2024, 04:51:12 pm
content: I made some crude "experiments" and found, that even been submerged to container with 10l of water, the unit will start using the relay only if temperature set higher then 70.0C. Once started, it will trigger the relay approximately every 2 seconds regardless of temperature settings up to 90C (this is the most strange part). And yes, this relay drives the heating element - I've dimmed the kitchen light and can see it blinking the moment relay is been triggered.Now I'm curious whats going on there.

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2024, 06:32:34 pm
content: How do you know it uses a relay? It's possible it has a bimetalic strip or liquid/gas expansion thermostat, which doesn't have any electronics: it's just a temperature sensitive switch.

author: e-pirate
author_rank: Regular Contributor
timestamp: December 17, 2024, 06:46:40 pm
content: Quote from: Zero999 on December 17, 2024, 06:32:34 pmI think 2 sec switch time is just to fast for such kind of things. Moreover, it start to operate exactly at 70.1-70.2C - to accurate for such thing. This is just my guess.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 17, 2024, 07:29:53 pm
content: Quote from: Zero999 on December 17, 2024, 06:32:34 pmYeah. Which, by the way, has exact same limitations as relay as for lifetime. Possibly even worse because they have to be engineered with less force available. Yet, things like electric radiators switch at around T = 5 - 10 minutes (they have intentional thermal coupling from heating element to make switchingmorefrequent than from room temperature feedback alone; an injected thermal ramp to make a pulse width modulator P controller), and work easily for 40 years non-stop. Which would be around 2-4 million cycles.

author: langwadt
author_rank: Super Contributor
timestamp: December 17, 2024, 07:44:50 pm
content: Quote from: Zero999 on December 17, 2024, 06:32:34 pmfor a sous vide heater with display, <1'C setpoint accuracy, timer, and bluetooth ?

author: tooki
author_rank: Super Contributor
timestamp: December 17, 2024, 07:47:24 pm
content: Quote from: Zero999 on December 17, 2024, 06:32:34 pmBut it has digital temperature control. A bimetallic strip makes no sense in that context.Sous vide cooking requires accurate, stable temperature control. I think bimetallic strip control would struggle to maintain the temperature stability required.

author: Marco
author_rank: Super Contributor
timestamp: December 17, 2024, 09:26:26 pm
content: Maybe it's just poorly designed and there is mechanical thermal protection kicking in temporarily overriding the digital control loop?

author: Zero999
author_rank: Super Contributor
timestamp: December 17, 2024, 10:34:55 pm
content: Quote from: tooki on December 17, 2024, 07:47:24 pmIt would have been nice for the original poster to have mentioned that. It's too much to expect people here to be familiar with that model, or take the time to search for it.

author: Circlotron
author_rank: Super Contributor
timestamp: December 17, 2024, 11:02:12 pm
content: I had a Weller soldering iron and I connected the thermostat contacts inside the handle to a TO-220 size triac with the tab cut off and managed to fit it all back inside the handle. Worked well for a number of years with almost no load on the contacts. Maybe you could do something similar with the relay. Use an oversized triac for reliability..

author: e-pirate
author_rank: Regular Contributor
timestamp: December 18, 2024, 06:02:05 am
content: Quote from: Marco on December 17, 2024, 09:26:26 pmWell, that would be my guess also, but it mast be pointed that relay always start working at 70.1C. Way to accurate for something mechanical-based.