lloydi12345
Member
Okay, so I have this circuit of microcontroller, transistors to control lamps, and power regulator. The input voltage is ~17Vac and when rectified it is ~24Vdc. I am feeding the dc input to 7812 and the output is connected to 7805's input. The output of 7805 then is fed to the microcontroller. There are 20 transistors controlled by 15mA. So the consumption (without including the IC) is roughly 300mA just for the transistors. The loads' supply are connected directly to 24Vdc. The circuit is fully functional on the 1st day but screwed up on the 3rd day.
(Correct me if I am wrong)Doing my approximate calculations, I came up with this:
7812:
(24-12)(0.3) = 3.6 Watts of heat
7805:
(12-5)(0.3) = 2.1 Watts of heat
As you can see the regulators obviously gets hot so I used two big heat sinks. It takes one hour and a half to make the sinks untouchable. I turned the whole circuit ON for 2 hours then turned it off for 5 mins. The sinks cooled down just like brand new. I did the same process and so on for 3 times and everything went fine. So that's a total of 8 hours on and 20 mins. off.
Then the other day I tried 4 hours and turned it off for 5 mins. and repeated the same process once again. After every 4 hours it really got too hot but the circuit is still in a good condition.
Just to check the limit of the regulators, I tried everything ON the next day for 8 hours straight without any turn off time. I took some random glances on the circuit and I noticed the problem around 6-8 hours. The circuit resets. I knew it resets because I made a routine that could only be executed once every-time the circuit is switched. The circuit resets randomly. Sometimes it resets after a minute, two, three, or more, there's no really definite time.
The question is which do you think is broken? 7812? 7805? both? How could I check the regulators if they are broken using multi-tester only?
Thank you for your time reading.
(Correct me if I am wrong)Doing my approximate calculations, I came up with this:
7812:
(24-12)(0.3) = 3.6 Watts of heat
7805:
(12-5)(0.3) = 2.1 Watts of heat
As you can see the regulators obviously gets hot so I used two big heat sinks. It takes one hour and a half to make the sinks untouchable. I turned the whole circuit ON for 2 hours then turned it off for 5 mins. The sinks cooled down just like brand new. I did the same process and so on for 3 times and everything went fine. So that's a total of 8 hours on and 20 mins. off.
Then the other day I tried 4 hours and turned it off for 5 mins. and repeated the same process once again. After every 4 hours it really got too hot but the circuit is still in a good condition.
Just to check the limit of the regulators, I tried everything ON the next day for 8 hours straight without any turn off time. I took some random glances on the circuit and I noticed the problem around 6-8 hours. The circuit resets. I knew it resets because I made a routine that could only be executed once every-time the circuit is switched. The circuit resets randomly. Sometimes it resets after a minute, two, three, or more, there's no really definite time.
The question is which do you think is broken? 7812? 7805? both? How could I check the regulators if they are broken using multi-tester only?
Thank you for your time reading.
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