Crock pot modification

[Valex76x]

Hi,

I have a not-working Crock-pot with digital display:

Only the Warm function is working, Low and Hight temp is not heating up also after 3-4 hours.
Because the Warm function is working I presume the heating resistance is fine.
I would like to mod/improve the pot removing the original electronic board and adding an external thermostat controller like this one:



They are rated -50° C to 110° C (some model to 120° C).

My question is if I can power the pot resistance (a 1 cm band around the pot, 2 cm from the bottom) directly with the 220V or if need to add also a dimmer/voltage regulator like this?



To be safe I will add also at the bottom a thermal switch like this one (220V 10A 120° C):


Hope for some help

 

[kellys_eye]

Because the Warm function is working I presume the heating resistance is fine.

Is it a single resistance? Potentially a dual element?

Worst case, at this time, would be to remove the existing electronics and see if it can be repaired. Post pics.

 

[Valex76x]

Yes, it is a single filament (1 cm flat band around the perimeter). Only 2 wires are going to this resistance.
I will post some picture later today.

 

[Harald Kapp]

This discussion may be of interest to you.
We've had this topic also here on our forum.

 

[ivak245]

It is likely that the element is 220VAC, and the existing controller senses the temperature and just cycles the element on & off. You could substitute the controller you have pictured, just place the temperature sensor in the same spot as the original. Have a close look at the element and see if it has a power or voltage rating. You could also calculate the power using the element resistance, then check that the controller can handle the current.

 

[Harald Kapp]

You could also calculate the power using the element resistance,

That'll not work. Afaik electric heating elements have a positive temperature coefficient: They have a low resistance when cold, but resistance increases once the heating element has warmed up.. This means the operating current in the ON state is less than the inrush current when turnd on from the previous cold state.
The controller needs to be able to handle the inrush current only for a short period of time, the it needs to sustain the lower ON state current.

My question is if I can power the pot resistance (a 1 cm band around the pot, 2 cm from the bottom) directly with the 220V or if need to add also a dimmer/voltage regulator like this?

No need for a dimmer. A controller of the type you show should be sufficient. However, this specific model has mixed reviews on Amazon. It seems to work as advertised, but the wiring is not suitable for high loads (wires too thin).
The other controller you show seems to require an additional 12 V power supply. Probably not a setup suitable for everyday use in the kitchen.

Whatever you chose: Make sure that you observe all safety measures and keep the mains voltage away from the pot to prevent any risk of electrical shock.

 

[Valex76x]

Thank you all for the support.
From what I found on internet the resistance should be just 200W (that's why is a slow cooker) Should be not a problem for the temperature controller. The ones I posted have 10A relays and are rated 1500W.
They do not need the 12V power supply, can be connected directly to the 220V.
I don't know if I can post links of Aliexpress but just searching "Digital Temperature Controller" they can easily be found with all the specs.

I found also this that has also the timer:

 

[Valex76x]

And there are also WIFI models that can be controlled with the App, Google home and Alexa ... but I don't need a smart crock-pot

 

[kellys_eye]

Always difficult to be helpful considering we also need to take heed of personal safety and don't know of your skillsets or level of competence to task working with potentially fatal voltages.

That said, you seem to have your head screwed on the right way around and look to be approaching this with the required caution so - you have little to lose by 'giving it a go'. The methods you suggested are the right ones, your choice of device for control seems logical too. It'll all boil down (no pun intended) to the position of the thermal sensor associated with the controller but since this type of device doesn't require any real 'accuracy' I would suspect just about any solution would do something (as opposed to the current nothing).

The only other concern I'd have is with the heating element itself. If it's designed for DIRECT connection to an AC source (120/240 whatever your location demands) then you'll be ok but it MIGHT not work in that situation i.e. the controller may operate in a PID fashion and regulate/limit the supply voltage 'at all times' i.e. never sending the full voltage to it. Unlikely, but I'm not familiar with that unit anyway so err on the side of caution.

 

[Bluejets]

Because the Warm function is working I presume the heating resistance is fine.

What do you expect a thermostat of any description will do...????
Nothing changes, remaining element will simply be on 24/7 with no improvement in heat output.
It is what it is, a certain low wattage output.
These elements are wound and bonded to the porcelain or ceramic (to disperse the heat) with a similar material and then baked.
When the bonding comes away due to age, hot spot and corrosion occurs which blows the element.
No way to "repair" same.

 

[mineymoe]

That's a cool project! Let me know how it turns out. Did you manage to take a look inside yet to check out the heating element? I'm curious to see if it looks intact or if maybe the issue is elsewhere. Fingers crossed this controller plan does the trick to get it cooking again.

 

[Valex76x]

I was not expecting so many reply, great forum this one! Here are the pictures of the crock-pot wires


And here the confirmation of the heating resistance Volt/Watt


Some other picture of the original electronic board for future reference









I ordered the W3230 110/220V AC thermal controller and the 250V 10A 120° C thermal switch. I will post some update when I receive them.

 

[kellys_eye]

Fairly complex electronics for such a simple function as heating therefore the use of PID seems likely. On the plus side there's no relay (the heating element is controlled by a triac) so any high-side switching issues will be down to the triac being faulty - assuming the digital display and other switch-operated functions look to operate correctly.

The chances of a software problem causing the issues is extremely low - they either work or don't - so only a few potential areas for testing are really necessary.

You've determined that the heater works (should measure around 200 ohms) so check the thermal trip is ok (usually open-circuit until overheat situation when the contacts close but the device itself may show whether it is normally-open or normally-closed in normal use). The next would be the temperature sensor (feedback) which is probably a simple thermistor - resistance changes with temperature.

Lastly, the triac itself. This will be used to switch the heating element on/off at various points of the sine wave cycle and, as such, can cause issues if the triggering of it is affected by an internal triac gating fault. Simply replace the triac (standard part) and see if it works!

Because this is a PID controlled heater the element will 'never' see a full supply across it but your idea to use alternative devices as show are based on the bang-bang principle i.e. the application of either FULL power or NO power and the FULL power condition may just destroy the heating element.

 

[Valex76x]

What I noticed after I opened the internal pot is that if I select Low or High and I set the timer (ex 4 hours) the heating element starts be directly really hot (had to take away my finger) but after a couple of seconds it stops to heat up. If I select Warm it starts to heat up and it stays on. I have to select Low or High to stop it "before it smelt the metal pan" With the pot assembled, with the ceramic pot inside and some water, Warm was working and the water was about 60° C after half hour (measured with another external digital thermometer).

I would like to be able to see and control the temperature, that is why I went for the external temperature controller. I was also thinking about the full power when the controller power on the heating element. That is why on the first post I mentioned the dimmer/voltage regulator. On the other hand, the metallic pan where the heating element is pulled around should dissipate the heat and help to do not damage the heating element.

 

[Delta Prime]

I would like to be able to see and control the temperature, that is why I went for the external temperature controller

For safety as well good choice cuz that would be directly connected to utility mains isolation is necessary or you find out .. a slow cooker can fry a human being quickly.

And here the confirmation of the heating resistance Volt/Watt

For future reference.
The chart below you can calculate the actual resistance but it's stated before that would change with the inrush current of the heating element over time.

For kicks and giggles what you're doing has been done to the N'th degree ( pun intended).
I like your idea better!

US6573483B1 - Programmable slow-cooker appliance - Google Patents

A programmable slow-cooker appliance, in which a user sets a time and temperature for cooking a food item. A programmable controller prevents the unit from being used solely as a “keep warm” appliance, and a unique design allows cooling of the controller during cooking.

patents.google.com

 

[kellys_eye]

the heating element starts be directly really hot (had to take away my finger)

Heat is a relative concept. You're unlikely to be able to keep a finger on something that's over 60degC let alone anything higher.

I would like to be able to see and control the temperature, that is why I went for the external temperature controller.

Nothing wrong with that. I didn't know if the OEM pot allowed temperature control - seems like the digital display is for time only then?

You will, of course, just have to experiment with your external controller. The key setting will be the hysteresis as water temperature control is slow to happen and you don't want the controller going on/off like a machine gun or have overshoot/undershoot.

But whatever you use, if it's the bang-bang solution then you won't get as stable a temperature setting (over longish periods) than the PID control the OEM device gives. The 'speed controller' will, of course, require constant adjustment.

 

[Valex76x]

I removed the covering sleeve on the white wire and here you can see the thermistor on the bottom on the metal pot:


About the original display,

it shows only the counting down time, nothing else. The left menu switch is only use to select between Low, High and Warm.

On the Crock-pot FAQ website:
How do I test my slow cooker’s cooking temperature?
Using an instant read thermometer will ensure that your unit is up to temperature. Crock-Pot™ Slow Cookers reach the simmer point and stabilize on both "High" and "Low" at about 209°F.

209°F = 98.3°C

I will place the temperature controller probe as replacement of the original thermistor. Going to cover the wire with some fiberglass heat resistant wire sleeve.
The W3230 temperature controller specs are:
Temperature Control Range:-55℃-120℃
Resolution Ratio: 0.1°C(-9.9-99.9); 1°C(other range)
Color of Displaying: red/blue
Measurement accuracy: ±0.1°C
Control accuracy: 0.1°C

The setting can be changed and changing P1 should delay the ON/OFF of the relay/heating element.




About the "safety thermal switch", I found this ceramic one with the hole that I can easily fit on the bottom using one of the 2 screws.


it is also AC250V/10A
I ordered one normally close, opening at 120°C. They give an error of ±5°C and lifetime of >10.000 times. For what I'm going to use it, it should never going to be needed, I hope

I used already a similar one as main switch for a small fan on a 3D filament dryer. The dryer starts heat and at 50°C the fan turn on.

Sorry for the strange arrangement of my post, my preview looks fine (all aligned) but after saving it is all confused

 

[Delta Prime]

Sorry for the strange arrangement of my post, my preview looks fine (all aligned) but after saving it is all confused

No need to be sorry. I'm glad I'm not the only one.
But with that said I understand completely.
I think this site is haunted.

 

[Martaine2005]

People often ask me how this stuff works.... Hold on to your hat!!!

It's impossible to try and build an AI brain small enough to turn the heater on and off... So what do you build? Well a really sofistcated led clock that is read by the relays that fire the switches! Simple. Works like smoke and mirrors without the smoke!

Seems like you’ve already had a smoke of something!

 

[Delta Prime]

Seems like you’ve already had a smoke of something!

How very rude. There are rules.
Puff,puff, pass!