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Topic: 24VDC supply making 7805 too hot. (Read 17 times) previous topic - next topic

dc42

The switching regulator is the best solution. However, I'd like to know why you are drawing up to 200mA from the 5V supply, because this seems a little excessive given that you also have a 24v supply available. If you can't reduce that current, then a simple fix would be to connect a 47 ohm 3 watt resistor between the 24v supply and the input to the 7805. This will drop about 10 volts and take 2 watts of the power. You may also need to connect a capacitor between the input and ground pins of the 7805 to keep it stable.

Having said that, if the 7805 is still delivering 5 volts then it isn't overheating - these devices have built-in thermal protection and will reduce the output voltage (thereby reducing the current) if they start to overheat.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

djscurge

Got the V7805-1000 and it works great, provided you don't put it in backwards. :smiley-red:  The diagram I was looking at on the data sheet was apparently a top down footprint drawing and I smoked a Atmega328 and a 7404 in one shot.  Oh yeah and as the white smoke was rolling out of the 7404 I noticed the dot on the package.  Of course it would be on a Friday and absolutely no one locally stocks them.  I have a spare 128 and several 7404's but I needed the extra EEPROM.  RTFDS (thoroughly)!
"Shall I refuse my dinner because I do not fully understand the process of digestion?"  
-Oliver Heaviside

CrossRoads

@iggykopa,
Nice find. I'm gonna keep those in mind for future projects.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
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MarkT

For 4W or so perhaps the best heatsink solution is to swipe a chipset heatsink and fan from an old PC and bolt the 7805 onto that (and you might want a 7812 to power the fan(!)).  With forced air cooling even a fairly small heatsink can be effective - if you've an old computer to canibalize then its free!
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iggykoopa

Heres another drop in replacement that will save you about 4 bucks http://search.digikey.com/scripts/DkSearch/dksus.dll?WT.z_header=search_go&lang=en&site=us&keywords=811-2196-5-ND&x=11&y=21 I just got one in, all I've had a chance to do is breadboard it with an led and 9v battery, but it was putting out a steady 5.2v and seemed to be working well.

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