Help diagnosing this voltage regulator over heating?

[artvandalai]

This hotplate stirrer stirs but the hot plate doesn't heat up, when i plug in the power cord the motor autospins even without turning the potentiometer, and this voltage regulator is over heating. The potentiometers are fine as I replaced them new and I can hear the relay clicking when I turn the knob for the heating plate. I replaced the voltage regulator but its still overheating. I switched the microcontroller from a working units to this pcb and the motor doesn't auto spin anymore when power is plugged in but the voltage regulator is still over heating. Any help diagnosing this voltage regulator over heating and motor autospinning?   

[Gyro]

What leads you to believe that the regulator is overheating? Comparing the board in your other stirrer, in one of your other threads on this, they both seem to show the same discoloration of the phenolic board. This tends to happen at quite low temperatures (~100'C) over time.

Regulators can get quite hot in normal operation. What sort of temperature are you seeing? Is it going into thermal shutdown or is it just hot to the touch?


It would be helpful if you could keep everything in one stirrer thread (rather than separate pot, triac, regulator, motor etc.) so people can get context.

[artvandalai]

It gets really hot to the touch and I can smell a slight burning smell from this part.

[Gyro]

It might still be ok. The hottest temperature that your finger can just tolerate is only 60'C. It's not burning you then it's under 100'C. The slight smell [Ed: from the board] could be down to you having just soldered in a replacement. 100'C would still be acceptable temperature. They may be relying on the regulator's (presumably a 78L05) current limit and thermal shutdown to protect against other circuit faults (shorts). Has your replacement relay got the same coil resistance as the original? That could increase current consumption. A 68C05 micro isn't going to pull excessive current without getting hot.

If everything is working correctly, the pragmatic solution would be to substitute a TO220 pkg part. That would have better heat dissipation to the air (the TO92 is effectively just dumping heat to the board). It could be that the manufacturer sailed a bit close to the wind on the rating but quite possibly. There would be a corresponding increase in current limit on the regulator though.

[artvandalai]

It might still be ok. The hottest temperature that your finger can just tolerate is only 60'C. It's not burning you then it's under 100'C. The slight smell [Ed: from the board] could be down to you having just soldered in a replacement. 100'C would still be acceptable temperature. They may be relying on the regulator's (presumably a 78L05) current limit and thermal shutdown to protect against other circuit faults (shorts). Has your replacement relay got the same coil resistance as the original? That could increase current consumption. A 68C05 micro isn't going to pull excessive current without getting hot.

If everything is working correctly, the pragmatic solution would be to substitute a TO220 pkg part. That would have better heat dissipation to the air (the TO92 is effectively just dumping heat to the board). It could be that the manufacturer sailed a bit close to the wind on the rating but quite possibly. There would be a corresponding increase in current limit on the regulator though.

On this unit I didn't replace the relay. I have multiple of these hotplate stirrers and this one is a different one from the other ones. These are alright to work with as a novice to start getting into and learn electronics repair and get used to all the tools. The TO220 is working fine on this unit, i switched the pcb where the potentiometers are with another working units to test if its the relay or TO220 and everything works. When i switch back to the faulty pcb the motor starts auto spinning when plugged in and the hotplate doesnt heat up.