Inside three different STC-1000 style thermostats

If it's a buck regulator that definitely means the thermistor is referenced to mains. The buck then rectifier sounds like them fitting it into an existing standard design.

Big Clive

I don't remember where I read this - possibly an article about "hoverboards" when those started showing up everywhere - but apparently there's a bit of a culture of cloning and copying among engineers at these Chinese manufacturers. They don't really think of it in terms of intellectual property theft; it sounded like it was more like "Ooh, that's neat, I bet I can build one of those!" Apparently that's just acceptable there.

Circuitmike

The second thermistor input is for the temperature differential controller model. I have one on my solar hot water system.

Paul Malloy

I just took apart a similar unit I got around 5 years ago. The circuitry is a bit different; it uses a buck regulator instead of an isolated power supply. The current from the buck regulator smoothing capacitor then goes through a bridge rectifier, which is very odd. Would you like pics of this board?

John Hiesey

Thanks Big Clive. Shoot. I've put one on my wish list and now I have to wait until I have a use for it. Too fun and funny. Interesting stuff indeed!

Dave Frederick

I used toothpicks in the holes the small pegs went in onder to get enough cycles, so ghetto but it kept the food cold!

Anton

I actually did that many years ago with a failed fridge thermostat, put it on an external timer that ran it for 15 mins every 2 hours. (Wouldn't work on a modern frost-free or similar of course)

Anton

The thermostat on our 40 year old fridge was crap. So I put one of those 240v boxes in the back by the compressor. I don't trust the relays so got a Kosher UK RS components 10A mains relay wired the output. 10A because compressor peak start current was high until the PTC thermistor warmed up. Fridge is now 4C with 1 degree hysteresis and 3 min delay. I never trusted relays in the box. Also fridge uses less electricity now. Happy camper. Will get another for the chest freezer soon.

Nuts 'n' Proud

The more upmarket units have other interesting features including damage limitation if the sensor fails by allowing a programmable compressor duty cycle until the thermistor is replaced or the stock is moved to a working case.

Big Clive

If the thermistor does pick up mains they can simply average over 100ms for 50/60Hz rejection. A little bit of AC content can improve DC measurements' linearity and resolution by dithering.

Mike Page

The ability to control hysteresis sounds like a great feature. It's probably the reason all the temperatures are specified to 0.1 degrees. For some applications I can imagine 1 degree being too coarse. My home thermostat is about 0.5.

Mike Page

It was an accidental pun.

Big Clive

Did BigClive make a pun at the end of the video or did I just mishear him say "interesting stuff relay interesting to look at"

I've been using the AC version to control temperature in my aquarium for almost a decade now, I've had a relay get stuck maybe a couple of times but was able to reset it by just changing temps to have the unit turn it on and off again. I also thought it should be considered a consumable and initially got two of them, the second one has been in its box since. It's even still using the original sensor it came with. Might be an interesting comparison, to see how much these things evolved/devolved from 9 years ago. There's an open-source alternative firmware available on GitHub allowing you to have more complex program/cycles, but it's targeted mostly at home beer brewing so I never tried it.

Theodor Ciobanu

My suspicion is the mains circuit was added as an afterthought to the original DC design, meaning they had to shoehorn it into the form factor. Those tiny gaps and lack of mains fuses are truly scary, but unfortunately not uncommon these days. I definitely agree with Clive's suggestion to use an external 12V DC supply. Cheaping out on the A/C version isn't worth the building burning down.

Mark Warburton

I've used these for many years to turn chest freezers into "eco-fridges" running on solar power with a 12V or 24V Sinewave 1200W inverter. Set the temperature range to 3deg cutoff and 7deg cut in to run as a fridge. Typically 250Wh/day for a 150 litre capacity chest 'fridge' at ambient 35 deg C daytime temps.

I wonder if the Chinese manufacturers (I assume they're Chinese, dunno) get tired of the other Chinese manufacturers copying their copies.

Nani Isobel

I use the same type with the brand name of ink-bird. It is a AC mains in the US so only 110vac. I am using it to control a small mini fridge for my Soda maker. Keeping my chill plate heat exchanger at 37F. And this one is in F and has only one relay. And you can set it up for heating or cooling. I know it has a thermal differential setting so the compressor doesn't short cycle. I have it set to 7F So far it has been very stable for the last 2+ years.

Charles Bruckner