Temperature control of mini fridge with peltier element

[DarkZero]

Other day i found broken fridge with those peltier elements, element was broken so i replaced it and it sure worked. But i noticed it had only connections for 2 fans, element and display and what i found weird is that there is no probe/sensor yet it keeps temperature right i double checked that it doesn't have any sensor only thing i found on the display board is PWM NTC what ever that means.

now the only thing that comes to my mind is that the thing is measuring current trough the element and it somehow knows what temperature is based on that or im terribly wrong ?

[danadak]

PWM is Pulse Width Modulation, is for driving the Peltier on and off
with a duty cycle to control its heat.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&sqi=2&ved=0ahUKEwj4-qbytYbMAhXCPB4KHaYaAEMQtwIIKjAC&url=https%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3DYmPziPfaByw&usg=AFQjCNFLKCUlP5EkApXCLbxSk_85Rk8YuA

NTC maybe Negative Temperature Coefficient, may imply there is an NTC
thermistor on Peltier plate for sensing temp.

http://www.omega.com/prodinfo/thermistor.html

Regards, Dana.

[DarkZero]

i am familiar with the terms PWM and NTC but this is writen on a display board maybe some chinglish ? and i doubt they are PWM the peltier because relay is used to switch on and off, i just cant see benefit with using PWM on a peltier since they take ages to cool anything even at full power. also i disassembled whole unit and there are no NTC thermistor or such but the temperature is bang on i checked it with thermometer.

[Rager]

Do you have pictures?
I would have guest the same as danadak.

[rafl]

I've recently partially reverse-engineered a similar display board for a wine chiller that sounds pretty much identical to what you've described.

That board uses the NTC pin to output what looks to just be a square wave (90Hz, constant 75% duty cycle). The resistance between that output and ground is what's used to determine the measured temperature, so I think your guess of a current being measured might be right, but it's the current through the negative thermal coefficient thermistor located on the cold side of the thermoelectric element, not the element itself. I haven't looked at how the current/resistance measurement is implemented. It looks like there might be an opamp and other support-circuitry on the power board in the back that's also involved.

The PWM output pin confusingly doesn't output PWM, but just a constant 5V or 0V, depending on how the temperature measurement compares to the set-point. The power board in the back does have some circuitry with a TL494 that's generating PWM output for the fans. I wonder if perhaps it's named like that because it's one of the inputs to the PWM circuitry in the back.