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Topic: External power regulator getting really hot (Read 1 time) previous topic - next topic

rbmk

Hi, I'm pretty new to electronics and am still learning a lot everyday. This stuff is really fun!

However I got one problem I just don't understand. I'm trying to control a ledstrip with a Arduino UNO and a ethernet shield. With the 12V directly into the Arduino, the regulator next to the connector got really hot.

Now I got a L7805CV regulator that I use, that is capable of 1.5A. As soon as I connect the Arduino and ethernet shield, the regulator heats up, just like it did with the regulator on the arduino itself. The only thing that is connected, is the Arduino.

How can I use this regulator to get 5V without overheating? Am I missing something? I probably do, but what ;) It must be possible to run this without a mighty big heatsink?

I've also checked a simple ledstrip controller from China, which got a LM78L05 regulator (100mA). That one doesn't get noticeable warm. It even sits in a small, completely closed box. I wouldn't dare doing that with my own project now.

MarkT

The ethernet chips takes quite a lot of current.  The regulator has to drop 7V from 12V down to 5V, so if the current is, say, 150mA then the regulator will dissipate over 1 watt.  A small heatsink on an external regulator is a good idea...  Alternatively use a 9V external supply or a _regulated_ 5V external supply (USB charger perhaps?).
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rbmk

Also without the ethernet shield, the regulator gets really hot. Even with a heatsink it's very uncomfortable to touch. When I connect the USB to the arduino to program, the regulator cools down quickly.
I need the 12V for the led strip and I don't want to use 2 powersupplies.

How come, for example, this little smd regulator in the ledstrip controller can give 5V without getting extremely hot? Is this because the current needed to power the controllerchip is much lower and thus the heat dissipation is less? I also noticed the controller has a little 47ohm resistor between the Vin and capacitor.

robtillaart


Can you measure with a DMM how much current it draws?

Add a second voltage regulator to bring the 12V to 7V or so. That could have a separate heatsink too.
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
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MarkT

7V isn't available in standard chip ranges, 8V is, and is closer to being midway between 12 and 5
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robtillaart


OK 8V it is  ==> 7808 like this one - http://www.parts-express.com/pe/showdetl.cfm?Partnumber=7808 -
Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)

rbmk

Tested with 3 different regulators:

L7805CV
only arduino: 140mA
with ethernet: 280mA

Can't touch after 10 secs

LM1086CT-5.0
only arduino: 170mA
with ethernet: 340mA

Can't touch after 10 secs

LM1085IT-5.0
only arduino: 180mA  - Can't touch after 15 secs
with ethernet: 370mA - Can't touch after 8 secs


The Arduino itself, running from 12V has no problems. This also uses a linear regulator also, right? Same as the ledstrip controller I mentioned.
It only heats up when the ethernet shield is connected.

retrolefty

You do realize that most all linear regulator chips have internal self-protection feature that shut the chip down if temperature or current draw exceeds their maximum design specifications? So while you may not want to touch them that doesn't mean the any damage is sure to follow. If you are concerned or don't want to live with the inefficiency of linear voltage regulators then just go with much more efficient switching voltage regulators. Their price and availability have greatly improved in recent years.

Lefty

James C4S

Multiply those currents by the voltage drop of the regulator (12v-5v=7v) to get the power dissipated by the regulator.

The rule of thumb I was taught is that if you can touch a chip for 5 seconds the temp is generally not over 125F.


The Arduino itself, running from 12V has no problems. This also uses a linear regulator also, right? Same as the ledstrip controller I mentioned.
It only heats up when the ethernet shield is connected.


The on board regulator uses the PCB as a heat sink...
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Runaway Pancake

I'd like to see a picture of the regulator with the heat-sink.
If it's just with the little finned guy then that won't get you much.  Supplement that with a chunk of scrap aluminum (a/k/a "mass") -- or another heat-sink, back-to-back like.
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Grumpy_Mike

Quote
with a chunk of scrap aluminum (a/k/a "mass")

Well the mass of a heat sink has little impact on the ability to transfer heat into the ambient, what is much more important is the surface area. The effectiveness of a heat sink is measured in degrees centigrade per watt. That is the relationship between how hot the surface gets above ambient to the amount of power you are putting into it.
http://www.thebox.myzen.co.uk/Tutorial/Power.html

Runaway Pancake

:smiley-roll:
My meaning was a considerable piece of aluminum. A 1/8-in thick, 1-in sqr of aluminum has the same mass (weight) as that same piece pounded out into a sheet of foil.  Maybe that should have been "concentrated mass".   
In most of these hobby contexts, I'd say a device bolted to a bar of aluminum runs cooler than the same with some flimsy fin-job.
Well-designed stuff doesn't have these big Vin:Vout differences anyway.
Extrusions!
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"You gotta fight -- for your right -- to party!"
Don't react - Read.
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Grumpy_Mike

Quote
A 1/8-in thick, 1-in sqr of aluminum has the same mass (weight) as that same piece pounded out into a sheet of foil.

Yes and that foil would be a much much better heat sink than as a 1 in sq.

Runaway Pancake


Yes and that foil would be a much much better heat sink than as a 1 in sq.

You cannot be serious.
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

retrolefty



Yes and that foil would be a much much better heat sink than as a 1 in sq.

You cannot be serious.


Yes. There is a reason that many heat sinks have all those funny thin fins and shapes. Exposed surface area is more important then mass, so all things equal a flat thin finned anodized black heatsink will be much more effective at dissapating heat then a square metal cube of the same mass.

Lefty

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