SparkFun Forums

[tozian]

I got a 7805 regulator from SFE. I have a 10uf capacitor on the input (which is nine volts) and a 100uf capacitor on the output. I then use that to power a Raspberry Pi. The regulator gets up to 176 degrees Fahrenheit. Is that a safe temperature?

[skimask]

No.
http://dlnmh9ip6v2uc.cloudfront.net/dat ... TO-220.pdf

[markaren1]

Sounds like you need to add several square inches of heatsink to get the temp down. You should be able to hold components comfortably (just as a rough guide).

[trueRF]

A heatsink would make it better, but even at 176F, it's 80C which is within the bounds of your device. Which has a max of 150C. For it to be running that hot I would check your circuit and make sure you don't have something shorted out. If you have an ammeter check the current in. Also check your voltage out. If you have lots of current in and an incorrect voltage output your chip is fried and needs to be replaced.

Good luck.

[waltr]

That 7805 is disappating 2Watts of heat (Vin-Vout) * Current.
So, yes it will get that hot without a heat sink and yes they can run hot enough to instantly burn your finger if touched.

[Mee_n_Mac]

I would agree that the 80C isn't too hot ... now. But I have to ask if the the Pi will be used outdoors (or indoors in an un-AC environment) and/or draw more current when doing more processing. What happens if this is in Tucson, AZ on a 120F (49C) day ? The case will be some 25+C hotter as will be the PN junctions. Now they'd still be below the 125C I use as a max but you'd have to wonder if a heatsink, even a cheapie, wouldn't help a lot. Even halving the thermal resistance, case->ambient, would be a large benefit. IIRC the junction->case resistance of a TO-220 package is small, perhaps 3 C, so halving the resistance, case->ambient, would about halve the junction -> ambient air temperature differential.

The OP didn't say whether his/her Pi was an A (1.5W) or B (3.5W) model. I'd have estimated about a 1 W load from the case temp so it could be either a working A or a loafing B.

[tozian]

The OP didn't say whether his/her Pi was an A (1.5W) or B (3.5W) model. I'd have estimated about a 1 W load from the case temp so it could be either a working A or a loafing B.

B! It's a B!

[waltr]

Ok, 3.5W at 5V is 700mA so the 7805 is dissipating 2.8Watts of heat.

[techtony]

There are Heat Sinks specifically designed to fit the 7805s. I have virtually the same power supply set up on my Arduino uno, but it does not get that hot. Of course, I also do not have it hooked up to 9V adapter for more than 20 minutes. I would assume it would get hot over an extended period of time, so a heatsink would be wise.

[Mee_n_Mac]

The OP didn't say whether his/her Pi was an A (1.5W) or B (3.5W) model. I'd have estimated about a 1 W load from the case temp so it could be either a working A or a loafing B.

B! It's a B!

So let me repeat myself ...

I would agree that the 80C isn't too hot ... now. But I have to ask if the the Pi will be used outdoors (or indoors in an un-AC environment) and/or draw more current when doing more processing.

Right now, given a 55 - 60C rise over (an assumed) ambient temp and no heatsink, I'll say your B Pi is loafing about, not working hard and not consuming much power. I'd guess ~1W and a bit less (80%) in the 7805 and so that's about 200 - 250 mA current draw. But with more peripherals used and/or more processing, that draw could easily double or even triple. That means the power dissipated in the 7805 will also double or triple. And that means the 60C rise would need to be 120 or 180 C in order for that to happen. Your 7805 might survive a doubling but not a tripling in current drawn through it in it's present form.

Before you smoke it, add a (?better?) heatsink and/or some more convective cooling (a fan).