So here it is !
As I said I bet it's never been posted here before !
So last evening afirend dropped this thing here... his wife's wax heater !!! He cleaned it as best he could but it's still very horrible looking, tempted to wear gloves to work on this thing !
But it's just wax, it's not going to kill me...
Anyway, the announced symptoms / complaint are : " The temperature control is erractic, acting weirdly ".
So... I opened up the thing to check the pot, see if it was dirty, or the wiper not making a reliable contact... but it tested just fine ! It's wired a simple 2 wire variable resistor, not a pot. I measured the resistance with my DMM while rotating the shaft from end to end.... resistance varies smoothly from zero up to 20K or so. Never saw any hint of an intermittent wiper connection.... seems perfectly fine to me.
So then I realized the plastic bracket that hold the pot in place, is broken, in a million bits! It's beyond repair...
So maybe it's just that ! The pot not being held in place firmly, caused the weirdness ! I bet it's all there is to it...
However I can't be sure of that unless I test it... but how to test that the temperature regulation behaves properly with the input from the pot ?
I mean... the TIME CONSTANTS involved are wayyyy more than I have patience, or time really, for !
I need to find a way to have an instant feedback so that I can see how the temperature controller is reacting, in real time, as I turn the pot !
But that's not fun.... I mean, there is a circuit board in there, some electronics ! So of course I could not help but have a look at to see how it's put together...
First, the overall construction of the device :
There is the pot, a red light, power switch. Inside, there are bunch of white sleeved wires that go from the PCB to the "bowl" (whatever you call it...) where you put the wax to be melted.
all these wires look a bit intimidating, confusing... but looking closer it's not that mysterious anymore. I can clearly see two wires that must be the heating element. Then another two wires that go to something that's tightly coupled to the bottom of the bowl... so me assume it's a thermistor for the thermal regulation. Then another 2 wires going to another mysterious component, this time mounted on the metal bracket that hold the bowl inside the plastic enclosure. I thought maybe that one is a safety thermal cut-out switch, that cut the mains.
Then I looked at the PCB, and it seeed to confirm the above : what I thought might be the thermistor.. goes to the analogy part of the board, hence most likely involved in the thermal regulation. Then what I thought was a safety thermal switch... is indeed : it's wired in series with one of the mains wire that goes to the board.
Then the red light : it's wired to the analogy part of the board, not the mains side... so it's probably indicating when the heater element is being energized, rather than a dumb "the device is plugged in" indicator.
I guess this is bad news for my testing.... it means I can't turn the pot and see say, the TRIAC being driven more or less, to vary the average voltage across the heating element.... nope, it's probably just turning the heater on and off for long periods, and relying on the thermal mass to do the averaging.... so I am out of luck.. I will still have to wait for the thermal mass of the bowl to react, before I can decided if the heater is replying properly to pot inputs.... oh dear... what a waste of my time....
Anyway.
So what do have on the board ? This is an electronics forum I heard !
- Power supply : looks like we got a fuse soldered directly to the board, then probably what looks like a full bridge rectifier made form 4 discrete diodes, then followed by a film cap + zener regulation.
- Heating : looks like it's driven by a TRIAC, type BT137-600E, itself driven by a DIP8 opticoupler, a MOC3061. Looked it up, it's a photo triac with zero crossing circuitry in it.
- Regulation : I see a DIP8 LM358 op-amp, surrounded by 5 precision resistors.
- Visual inspection : can't see any suspicious solder joint, no components that have overheated or are cracked or whatever. However what I noticed, see below, is that there are a couple resistor lead up in the air, that are THIS close, 1 or 1.5mm away, from touching the metal tab of the TRIAC !!!
That's it for now.... I can now power it up and measure things, give it a test drive, see what it does... so some TE will be involved ! Yes it IS on topic !
Hopefully I can fix this thing within an hour and clear the bench and resume my normal lab activities...