I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.

 

Hi.

Please upload you schematic diagram, and a picture of your circuit. Did you solder it yourself, or is it on a bread board?

Is it a 7808 or a 7805?

 

I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.

LM7805?
Is dropping 7 Volts at 800 mA. Yep, might get warm. It could also be oscillating, but I suspect you would know that.

 

Its simple LM7805. Output voltage comes around 4.80V when loaded. I have soldered the board myself. Will post schematic also.

 

There are several type of 7805 or equivalent
pls read http://www.petervis.com/electronics...nk/Heatsink_for_TO-220_Voltage_Regulator.html

 

Is dropping 7 Volts at 800 mA.

P=7*.8=5.6Watt
use big heat sing or low the input voltage if possible.
yes it heat at high current

 

Did you use decoupling capacitors ?

 

I have 12V input on LM7805, & output load current is aroung 600-700mA.
I have connected a large aluminium heat sink with fins on it. Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.
Can my circuit works for continuous 1 day for that? or I should go for some dc-dc.

The 7V drop x 700 mA is 5 watts. That is a big heat sink. It should also get forced air cooling from a fan.
You could use a DC/DC converter and that will reduce the heat.

You could also get a different SMPS supply to power your project (e.g. Use a cell phone charger for the 5v source) or any other DC source that is not 12 V

Finally, you can make sure that your circuit needs 5 V. Some CMOS chips can handle higher, LEDs can be powered with higher if you use appropriately sized current limiting resistors. They way, all the heat is dissipated over more components instead of one poor little 7805.

 

If you want to burn less power in the same footprint, try the muRata OKI-78SR-5 . You may not need a heatsink.

 

If you want to burn less power in the same footprint, try the muRata OKI-78SR-5 . You may not need a heatsink.

Where can I buy these. I can use them

 

Where can I buy these. I can use them

http://www.tme.eu/gb/details/oki78sr-5_1.5-w36c/dcdc-converters/murata/oki-78sr-515-w36-c/

Too expensive.

 

http://www.tme.eu/gb/details/oki78sr-5_1.5-w36c/dcdc-converters/murata/oki-78sr-515-w36-c/

Too expensive.

Depends on space and price of heat sink.

 

Too expensive.

Prices at places like Mouser and Digikey are 2-3X that!

 

Problem is that in 5 minutes of operation heat sink gets too hot, so that i cannot even hold it with hands for more than few seconds.

Being too hot for you to touch isn't a practical indication of a component being stressed.

LM7805 will operate at a junction temperature of 125C. Thermal resistance from junction to case is 5C/W, so it won't start temperature throttling until the case is 100C. Unless you're a professional chef, you'll find that temperature too hot to tolerate even briefly; but the part will be within it's specs.

 

Unless you're a professional chef, you'll find that temperature too hot to tolerate even briefly; but the part will be within it's specs.

What makes them special? Were they born with fingers made of Nomex?

 

What makes them special? Were they born with fingers made of Nomex?

No. When it comes to handling things that are hot, many of them have conditioned themselves to tolerate a higher level of pain.

 

What is the size of this "large" heatsink?

 

Hello,

Here are two articles from ESP about heatsinks.
One for the design: http://sound.westhost.com/heatsinks.htm
And one how to make then yourself: http://sound.westhost.com/articles/diy-heatsink.htm

Bertus

 

Dang....that is expensive.
I stick with 7805 and a sink

 

Its simple LM7805. Output voltage comes around 4.80V when loaded. I have soldered the board myself. Will post schematic also.

That sounds like it is loaded down more than 800 mA.