Crock Pot modification

[cjbrown80]

Hey all,
I just registered. It seems I've become a bit lazy lately and I'm basically trying to cheat by seeing if anyone here can help me with this... instead of figuring it out on my own.

I'm not sure if any of you are familiar with crock pots (slow cookers), but it seems in the last decade or so, every major manufacturer has decided to raise the minimum temperature these devices can cook at unacceptably high (I'm assuming because some not-so-bright individual ate some undercooked food, got sick and/or complained). My previous unit died a while back and I got this current one as a gift about a year ago. In that time I have ruined enough meat to feed a small village from overcooking, due to the low temperature setting being ~250F... when it should be ~160-180F. I've already disassembled the unit and taken some pictures of the board. Just wondering if anyone has tried a project like this or would at least be willing to take a guess at which component needs replaced to lower the cook temps. Tomorrow I plan on contacting the manufacturer to see if I can get hold of a schematic.

 

[davenn]

hi there
welcome to the forums

I would expect the main temp control to be done by a thermostat mounted on the part of the cooker that gets hot ( not shown in your diagrams)
It would be an initial assumption that this thermostat would be you 250deg minimum setting and would control the heating element for that temp.

you may need a thermostat with a lower temp rating

just my thoughts

Dave

 

[cjbrown80]

I saw somewhere else online an explanation that these units are manufactured as cheap as possible, and that they might not actually have any sort of thermostat, they just put out heat at a predetermined current.

Just a wild guess here, but would one of those larger resistors be the culprit? Also, might you be able to identify the yellow component in the lower image? I'm not even sure what it is.

 

[davenn]

the yellow "thing" is a capacitor

I really suggest you look hard for a thermostat they are small often circular with a couple of lugs that wires are pushed onto with spade terminals or soldered onto.
The only thing that may have mains voltage in it may be the transistor/FET in the top right of pic 1. But I would have a pretty good guess that this is just for overall timing switching for how long the cooker is ON rather than temp control

Dave

EDIT ... where do the black and white wires go to ( off board)
where does the pink wire go to?

 

[cjbrown80]

Odd, it doesn't look like the capacitors I've replaced before (on audio and computer equipment), just a different style I guess.

I'm going to (hopefully) get a schematic from Crock Pot tomorrow. Until then, here is another picture of the wiring and you were correct! Upon further inspection, under some shrink tube, I found what appears to be a thermostat sensor attached to the bottom of the pan.

It is connected here.

Thanks for your help Dave!

 

[davenn]

Odd, it doesn't look like the capacitors I've replaced before (on audio and computer equipment), just a different style I guess.

They come in all shapes, sizes and construction types
the yellow one is likely to be a polypropylene or polyester type
note the second line on it... 0.047 uF <---- dead giveaway for a cap
also note to the right of that the X2, this indicates a mains (AC) rated cap

see on the circuit board 2 other types of capacitor ...
2 green ones they are mylar also known as "greencaps"
and to the left of the IC there's a small disc ceramic

cheers
Dave

 

[cjbrown80]

Aaah... microfarads right? Memory is a funny thing. Didn't realize all those were caps, I'm used to the cylindrical style, like the one to the right of Switch 1. I've only ever changed out blown ones with the exact same kind.

So basically what I'm trying to do is figure out if I can (by changing out one of these components to one with a different rating) impair this crock pot to cook at lower temps. It is functioning as intended by it's creator at the moment... too damn hot. Would I need to change out the thermostat as well for this to work?

 

[davenn]

without knowing the functions of the circuit, its pretty impossible to
workout what to change.
I cannot see a part number on that IC, can you ?
for all I know it could be a microprocessor, say a PIC IC, that the temperature sensor
feeds into and it then controls the on and off times of that transistor ( the one in the
top right of pic 1)

I dont know where in the world you are ?? what is your mains voltage ?


Dave

 

[cjbrown80]

No luck on the schematic from Crock Pot. Apparently they don't share that information with consumers. The woman I spoke to explained that the unit reaches the same max temp for Low or High setting, the difference being the time it takes to reach that temp.

I'm in Pittsburgh, PA, USA, so our mains voltage would be ~120.

The IC is a Holmes HE100B - R0834.

Again, thank you for your time Dave.

 

[(*steve*)]

No luck on the schematic from Crock Pot. Apparently they don't share that information with consumers.

Not unexpected. For lots of reasons. Safety is one, but reducing their sales is another)

The woman I spoke to explained that the unit reaches the same max temp for Low or High setting, the difference being the time it takes to reach that temp.

That's actually useful (and mimics the behaviour I have seen in a similar style of unit I have)

My guess is that the device you have found is a thermistor, and it forms a simple voltage divider somewhere in the circuit which then drives a comparator with hysteresis which in turn turns the heating element on or off. I would probably guess that the difference between high and low power is one of:

1) placing a rectifier in series with the element (old school)

2) phase switching to reduce the power on each half cycle

3) low frequency switching (say 1 sec on, one sec off, or even a few mains cycles on, a few mains cycles off)

The only thing here of interest to you is the way the sensor operates.

Trace the circuit back and see if the sensor forms a voltage divider. Some measurements of the resistance of the sensor at room temperature would also be helpful.

If my conjecture is right, changing the other resistor in the voltage divider will allow you to change the temperature the pot reaches.

If you do change it, please remember the food safety issues for food under about 60C

Testing would be best done using the pot filled (to its minimum depth) with water.

edit: A picture of the other side of that board would be useful. I suspect the chip is a microcontroller, however it could be a special purpose chip designed for controlling this sort of equipment. Neither is likely to make it easy to modify.

edit2: oh, and check the sensor is not a diode. They could be using the forward voltage characteristics to detect the temperature.

 

[davenn]

edit2: oh, and check the sensor is not a diode. They could be using the forward voltage characteristics to detect the temperature.

I had that thought too looks like a 1N4148 or similar glass diode

Dave

 

[cjbrown80]

After a close look at this unit again, it appears that the thermistor connects to the IC at both ends, one of which seems to hit one side of one of the green caps.

 

[cjbrown80]

I can't tell if it's actually a diode, but it's very possible. Sure looks like one, it's just too small to see.

 

[cjbrown80]

 

[cjbrown80]

 

[(*steve*)]

Yeah, that's not really helpful is it.

Have you tried measuring the resistance of the sensor in both directions?

 

[cjbrown80]

I believe it is a diode. The resistance one way was overloading and the other way was minimal. Is that right? Again, I'm a novice with this stuff.

 

[(*steve*)]

I believe it is a diode. The resistance one way was overloading and the other way was minimal. Is that right? Again, I'm a novice with this stuff.

That pretty much seals it.

You'd need to know a lot more about the IC to be able to do much.

If there's a voltage divider in the circuit anywhere you could try changing it slightly, but you'd be operating in the dark.

you could also try placing the diode in better thermal contact with the base of the pot. That might help, but it's hard to say by how much.

 

[KrisBlueNZ]

It would be useful to know the voltage across it, at different temperatures. You can measure this at the board, so you can leave the device in contact with the pan. You'll need to do this with the unit powered, and parts of the board are live, so I suggest soldering some wires onto the board and clipping the multimeter onto them, before you power it up, so you don't have to go anywhere near the board while it's powered up.

If it's a diode, the modification might be as simple as adding a low-value trimpot in series with it. But measuring the voltage across it, when it's hot and when it's cold, will tell us a lot about it.

 

[cjbrown80]

Thanks Kris. I'll try that later tonight and let you know.