SparkFun Forums

[barrysfarm]

I'm at the tail end of a project i've been working on for a while now. Hooray!

In my device, i'm using a LM7805 voltage regulator (http://www.sparkfun.com/commerce/produc ... cts_id=107) to bring the 9v wall wart adapter down to a regulated 5v for my project. The 7805 is generating a bit of heat trying to dissipate that extra 4v (actually, i'm not sure if this is how it works... i'm pretty new to electronics). I've attached a heat sink (http://www.radioshack.com/product/index ... Id=2102857) to it and it's still a bit warm.

The device is enclosed in a wooden case, which has a fair amount of breathing room & an open back. The warmth being generated probably isn't that hot. I can touch the bottom side of the regulator with my finger. The heat sink is noticeably warm, but not too much above room temperature. I used hooked up a digital thermometer (http://www.sparkfun.com/commerce/produc ... cts_id=245) and and arduino to the hottest part and it seems to top out around 90 degrees F after a few hours.

The device is something that is running all the time. Is there anything i need to worry about? If for some reason it gets hot enough to do some damage, would the regulator stop working before a fire broke out?

I appreciate your input.

[felis]

7805 has built-in thermal shutdown circuit with trip point at 125degC. It's below autoignition temperature of anything you may possible have in your house ( for example, autoignition of acetone is 465degC ).

[barrysfarm]

7805 has built-in thermal shutdown circuit with trip point at 125degC. It's below autoignition temperature of anything you may possible have in your house ( for example, autoignition of acetone is 465degC ).

Great! That should alleviate my concerns, since i'm about 100 degrees C below the trip point.

Thanks!

[bigglez]

In my device, i'm using a LM7805 voltage regulator to bring the 9v wall wart adapter down to a regulated 5v for my project. The 7805 is generating a bit of heat trying to dissipate that extra 4v (actually, i'm not sure if this is how it works... i'm pretty new to electronics).

Almost right. The regulator handles the difference from the
load voltage (5V in your case) to the bulk voltage
(9V in your case). The power in the regulator is voltage
times current equals watts. Watts are dissipated as heat.
Failure to remove the heat will shorten the life of the IC
regulator. The regulator can work "forever" even if the
heatsink is too hot to touch by hand.

The heatsink is actually cooler than the IC package's metal
tab, the IC die (chip) inside is much hotter than the metal
tab. The maximum operating temp for the chip is about
125 deg C, and is protected by circuitry that monitors the
temp of the chip. It's fail safe!

I've attached a heat sink to it and it's still a bit warm.

In the modern world we expect electronics to run cool.
Much of it is portable and battery powered (cell phones,
iPods, etc.)

Not too long ago electronics ran very hot! A typical TV
set drew several hundred watts from the wall outlet.

Much of the thermal management skills of engineers
has been lost, I don't think schools teach heatsink design...

The device is something that is running all the time. Is there anything i need to worry about? If for some reason it gets hot enough to do some damage, would the regulator stop working before a fire broke out?

No cause for alarm. If your project stops working, or
it cycles on and off by itself you should suspect the
regulator is overheating.

The power in the regulator could be reduced by adding
a resistor. The resistor can go in series to reduce the
bulk voltage to the regulator, or in parallel to bypass the
regulator and shunt some of the load current.

The resistor value is easy to calculate but we'd need
to know the DC current in your load and also the ripple
voltage on your bulk DC input. (If you your bulk DC
is regulated to 9V already the problem is much easier
to solve).

At this point you don't need to do anything! Enjoy your
project, and make something new with your new
knowledge.

[barrysfarm]

bigglez:

Wow! I wasn't expecting an answer this detailed. Thanks for explaining how it all works. I'm no longer worried on this thing catching fire, or really overheating.