Help with a poorly looking component identification plea

[Techie_hobby1]

A friend of the family has handed me a food warmer that has failed on her.
I appreciate that some people will alert me that it is probably cheaply made, but she has extremely limited mobility, and it is perfect for keeping her food nice and warm, once her carers have left.

It has 2 power options (either 12V DC cigarette lighter or AC 240v IEC C7 socket).
Basically, it powers 2 x PTC elements which are attached to a metal plate.
A (removable)metal deep tray is then placed on-top of the small plate which keeps the food nice and warm.

I thought at first it was a poorly looking metal film resistor but now I think it could be some type of fuse protection device, but the black outer casing has melted.
It is definitely cheaply made with very little in the way of components, with no PCB insight.
Just a couple of PTC elements (wired in series), a 4.7kΩ resistor, 1n4007 diode, SPST push to make switch, RS-9700B thermal protector

On a final note though.. I am going to be looking into improving its limited design though, so that I can control the temperature…was thinking possibly a potentiometer (but not sure what size I would need)
Therefore what is the best way to measure this requirement please

[sam_sam_sam]

Can you post some pictures of the hole setup to get idea how this setup works and see it actual components

[truclacicr]

How can you power a 12V element from a 240VAC source with only those components???

[Techie_hobby1]

truclacicr it doesn't power 12v..it can either be powered by a 12v supply (the DC jack) or 240ac EIC C7 plug. Apologies I thought I had mentioned that clearly in my original post, but now not sure I put it clear enough.

It seems like it is something you can take in a car with you and power from your car battery via the 12v cigarette lighter plug…or it can be powered by the EIC C7 plug (but NEVER both together.
sam_sam_sam you should be able to work out from this image. Tried my hardest to squeeze everything in without the image going too small.
Note: the second PTC element is just in view (top right corner), and the SPST switch is top centre.
many thanks

[sam_sam_sam]

These devices come in different voltages flavors but you are correct that you can not run both voltage at the same time
The first thing you need to do is look at the data plate on each heating module and you can test each heating module at the voltage that it says on it and see if it works correctly at the voltage stated on it then you need to look for thermal switches and see if you have a connection with a multi meter and make sure that you have a good connection they are a switch but controlled by temperature at room temperature it should have a connection

It also looks like it has also a thermo switch that permanently disconnects at high temperature to prevent fires if this temperature is reached I would check this as well it the small cinder device that has electrical wires on each end you should have a connection on this device if bad only replace it with the same temperature rating and only crimp in the new one do not try to solder it because you might open it permanently now if you have to solder there is a way to do take a wet wash towel and rap it around the body but leave wire connections exposed so that you can solder the wires to it

Also check all wire connections and switches as normal and make sure that you have good connections

I hope you can get this device working correctly again

[stj]

nothing is melted,
the black thing after a diode is a resistor in heatshink sleeving driving the led.
as it's 12v the resistor is probably between 1k and 2k2 if they arent overdriving it.

[stj]

i just realised what i'm looking at.
this design is questionable as fuck.
they are driving the 12v PTC's directly from the mains and relying on how they regulate to keep it all together!
that 12v jack has the potential to get spicy!
btw, at 12v the current is taxing that switch - check the switch isnt burned inside.
and do something to improve the insulation if it's not earthed.

[sam_sam_sam]

STJ

I agree with you I personally do not like this design if I own this device I would make a decision about if I am going to run it at 12 / 24 volts dc or at 230 ac and remove the components that are not needed for the voltage that you are not going to be using because if this device does not have an internal lock system that prevents you running two different voltage levels it is a very poor design and very unsafe device to use

[Techie_hobby1]

sam_sam_sam
In regard to the data plate on the heating modules, there is no signs of any information. I thought that maybe if I took the module out from inside the aluminium extrusion the part number might be stamped on that, but even after pulling that out I still see no part number/voltage etc.
The actual module that fits inside that extrusion is about 35mm x 14mm x 2mm (approx. 3.5cm x 1.4cm x 0.2cm), and has 2 identical colour leads coming out of it... I understand that these do not hold any polarity and work similar to resistors.
So my question to you sam_sam_sam is.. how can I work out the voltage.

The thermal switch I believe is the BW9700 (RS-9700-B a plastic encased switch) which was only fixed to the underside of the larger metal plate with nothing more than aluminium foil tape. Surely their is a better (SAFER) way of fixing these vital components,

stj
I am not convinced that is a resistor after the diode, because if you look at the red lead that is trailing out from the LED, it splits into 2 further paths (1 goes via that black component in question, then via the diode and into one side of the EIC C7 connector - and the other path goes into a resistor (4.7k ohm) then into the positive DC pin.

sam_sam_sam and stj
This will be run via the 240v supply only, since it is being operated at my friends home and used to keep her food warm once her carers have made her dinner.

I agree that the DC port should and WILL be removed to ensure it cannot be operated at the same time.

What else would you remove, and has anyone any thoughts on how i can create a temperature control without adding masses of extra components and without completely starting this circuit from scratch.

[stj]

meter the resistance of the black covered part.
incidentally, the way these work is they are a type of resistor that rises in value as they heat up.
so at some point they stop getting hotter and only draw enough current to maintain a set point.
the same concept is used in hotglue guns.

[sam_sam_sam]

sam_sam_sam
What else would you remove, and has anyone any thoughts on how i can create a temperature control without adding masses of extra components and without completely starting this circuit from scratch.

I would agree with STJ what type heating element these devices have and he is right that once it gets to the temperature that it is designed for it just maintains this temperature now if you want a lower temperature than what it is currently designed you need to see if there is a version that is operating at this temperature if not then you might need a thermostat to control the temperature

[petehall347]

it will be a thermal fuse and that's why it is crimped rather than soldered

[truclacicr]

truclacicr it doesn't power 12v..it can either be powered by a 12v supply (the DC jack) or 240ac EIC C7 plug.

You don't understand what I'm saying. If the element is rated for 12V, then how can it be powered from 240V using only a 1A diode and a resistor?

[Techie_hobby1]

stj

meter the resistance of the black covered part.

I am picking up around 2.2M ohms resistance across the little black component and then I applied my hot air station nozzle at a constant 100degs at approx 1cm away from the component and it dropped down to about 1.7M ohms
I am not sure this supports your theory with it being a resistor, since its value drops with applied heat.
I have zoomed in with my scope at maximum zoom and switched from my LED ring light over to my AmScope illuminator. Hopefully the image comes across a little clearer...
(Note WARNING.. I have kept the image to its large scale for closer examination)

Await your views

[CapLeaker]

Almost looks like a BBQ'd turd. lol

[Techie_hobby1]

CapLeaker 😄 yeah I thought exactly the same thing when I looked at it through my scope 😄 unfortunately that is not the answer I am looking for though.

[sam_sam_sam]

They make these type of heater elements in different flavors depending on what you need

I have seen them from 12 volts dc to 240 ac even some that are 110 volt ones

It would not surprise me if it has two of these heaters one 12 volts dc and one was 240 ac if this is the case great if not then this where if you are going to use it on 240 volts ac I made the comment to remove the 12 volt components and have it only operate on 240 volts ac

[truclacicr]

They make these type of heater elements in different flavors depending on what you need

I have seen them from 12 volts dc to 240 ac even some that are 110 volt ones

It would not surprise me if it has two of these heaters one 12 volts dc and one was 240 ac if this is the case great if not then this where if you are going to use it on 240 volts ac I made the comment to remove the 12 volt components and have it only operate on 240 volts ac

The following statements do not appear to be consistent with your hypothesis:

"Basically, it powers 2 x PTC elements which are attached to a metal plate."

"Just a couple of PTC elements (wired in series) ..."

It would make sense if the two elements were rated for 120V. Then the appliance could be used in 120V locales by rewiring the elements in parallel.

[CapLeaker]

CapLeaker 😄 yeah I thought exactly the same thing when I looked at it through my scope 😄 unfortunately that is not the answer I am looking for though.

IMHO it think it is a burnt resistor. You got AC, a single rectifier diode, then a current limiting resistor. Something has to limit current in this circuit.

[stj]

i did a schematic, it's 2 circuits,
the ptc with black wires is 12v,
the ptc with red wires is 230v
both power through the switch and drive the led.
i would check if the 1n4007 is shorted, the resistor that is toasted afterwards is driving the led so between 220k and 470k is what it really needs to be to light the led with minimal heat/current.
and test the led too.

here is the fun part:
if you connect power to either socket, the power will be present on the other connector limited only by the resistance of the cold ptc's in series - which is probably fuckall!