## Power supply dissipation

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I have some doubts deciding the power supply in a given system. The entire system is powered up using 12VDC power supply. The system has a small microcontroller unit, some relay and 12V LED panel. To decide the required capacity of the 12v supply, I simply add consumption of each unit.

I have a relay with me with a coil resistance of about 260 ohm. This gives me the current drawn as about 46 mA. The system has four relays and as such the consumption of the relay is approx 200 mA. I do not have data of LED panel but let us assume that it has a consumption of max 100 mA. The microcontroller board consumption is very minimal. It has a 7805 mounted on it and thus operates on 5V. Now say if the power supply provided is 12vdc/1A. What implications it has on 7805. (Should I worry about the dissipation here or since at output of 7805 there is not much load connected, I should be fine)

If I were to find out whether the output of 12vdc supply varies according to the requirement, what parameters should I look out at?

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The 7805 will pass whatever current the microcontroller board will use. Lets say 100ma for fun. The 7805 drops 12V to 5V. So a 7V drop. At 100ma that is 700mw. At 50ma usage, it is 350mw dissipation. Now look at the 7805 thermal data. With this information you then can decide how to mount the 7805 for cooling, if needed. There is probably not an issue. If there is, you can put some diodes in series with the 12V to the 7805 and let them drop the voltage and share the power dissipation. Sometimes this is better than putting in a heat sink. YMMV.

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"If you can touch it, it's not too hot"

Imagecraft compiler user

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But if you touch it and can read "2N2222" in the mirror, it might have been too hot. (Please do not ask me how I know this...)

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bobgardner wrote:
"If you can touch it, it's not too hot"

I've always reversed that and said, "If I don't want to hold onto it, it's too hot."

Hack a day has an article about this. Haven't watched it. Yet. http://hackaday.com/2014/03/03/h...

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