Voltage regulator heat

Ugha

A quick question about voltage regulators.

I've recently repaired a Professional Development Board by replacing the voltage regulator.

The regulator I replaced it with is the same as what was in there originally (7805a) and I
included the same style of heatsink AND used Artic Silver(the good stuff) thermal paste between
the heat sink and the regulator.

My problem is when I supply the PDB with regulated 12v the regulator gets quite warm to the
touch. Not quite painfully, but pretty close. And it happens quickly as well (less than 5
minutes or so).

I understand that its burning off 7volts of heat, but should I be concerned at all? There seems
to be no sign of thermal shutdown and no sign that it is causing any harm to the surrounding
plastic or circuitry.

Does anyone else run a PDB off 12volts? Is it very warm after just a couple minutes?

PS: The only load I'm running on it is a BS2, an SX and a LCD.

mctrivia

i have run these regulators of 12v and even 18v. The higher the voltage the less current you can get out before it heats to the point of failure. at 12v 100mA is the absolute max current you should draw. At that it will get extremely hot but with a good heat sink will not fail. You can help by placing a small 10W resister in series with the 12v in. Something small enough(usually less then 1 ohm) that at max draw the resister will have a voltage drop of 5v.

Now I use switching regulators on all my designs. No heat problems.

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My new unsecure propmod both 1x1 and full size arriving soon.

Mike Green

You have to figure the actual load current and don't forget that there are other circuits on the PDB that always draw current like the pulse generator, audio amplifier, L293D, MAX232, RTC, and the power LED. It's a few milliAmps here and a few there and pretty soon you've got 50 to 100 mA. That's not a lot until you figure in the 7V voltage drop.

To get some perspective, look at the datasheet for the 7805, particularly the temperature where it begins to shut down. They're designed to operate at a junction temperature up to 125C = 257F. There's some thermal resistance between the chip and the heatsink, but you're using a good thermal paste. The heatsink could get well over the boiling point of water before the regulator shuts down. I suspect your regulator temperature is nowhere near that. Try measuring the temperature of the heatsink.

Remember that Parallax recommends a 12V wall wart as the power source for the PDB, probably because of the L293D and its use as a motor driver. You can always run the PDB from a 9V wall wart and markedly reduce the heat produced by the regulator.

Ugha

Thanks for the help guys, it's a lot clearer to me now.

Mctrivia:
I hadn't realized that the max draw would be so low! I had assumed that I could draw the full 1.5amps. Thanks so much for the info.

Mike:
Thanks, I hadn't thought about all the other stuff on the PDB.
Your right, the temp is no where near the max rating, I was just worried that it would reduce the life of the regulator or was a sign of some problem
I might have caused when repairing the board.

I'll look around for a 9v wall wart in my giant pile-o-Smile. Since this is a pretty powerful regulator, I'm assuming an unregulated wall wart would
be acceptable as long as it's no-load peak doesn't go above the 36v that the datasheet says is max. Am I correct in this?

Weird thought/question:
If I were to use two 7805As with the V-Outs tied together to supply VDD on the PDB, would that reduce the current going through the regulators
and thus reduce the temp?

Update: I have a 1084 voltage regulator in my PDB, not a 7805a. I had both datasheets on my desktop and clicked on the wrong one
to get data for these posts. I apologize although it seems that the only major difference is the 1084 can handle 5amps with a max of
25v.

Post Edited (Ugha) : 5/15/2009 3:55:19 PM GMT

Beau Schwabe

Ugha,

Paralleling the regulators is not a good idea, it would be better to arrange them in a cascode style. Short of that you can use a switching regulator for better efficirntcies.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

mctrivia

You can parallel regulators but you have to be careful. If from 1 supply better to buy bigger regulator. If using 2 supplies use a diode on each output.

The current you can draw partially depends on input voltage. Switching regulators do not have this problem

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My new unsecure propmod both 1x1 and full size arriving soon.

Chris Savage

Please note that the 7805 is not the regulator that comes on the PDB from Parallax. The factory regulator is an LM1084.

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Chris Savage
Parallax Engineering

Philldapill

Ugha, have you tested the current draw INTO the regulator with it on the PDB, vs. it not connected to anything, i.e. just itself? If this is the same PDB that you mentioned in another thread of repairing, you could possibly still have a damaged component drawing excessive current. This COULD explain your unusually hot regulator as it would be drawing more than the normal idle current. I use a 5V regulator(uA78m05 from TI). It's in a DPAK package, which is about half the size of a normal 7805. Mine barely get warmer than room temperature at idle power consumption.

northcove

I've been using a 5V switch mode regulator from Dimension Engineering (http://www.dimensionengineering.com/DE-SW050.htm) as a replacement for a 7805 with excellent results. These regulators generate very little heat even when outputting 12V@1A with a 24V input due to their high efficiency.

mctrivia

That is a really good price. I designed a buck/boost converter in the same format. 1.8V to 5.5V input. Similar price range if I could make 100 of them.

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My new unsecure propmod both 1x1 and full size arriving soon.

Need to upload large images or movies for use in the forum. you can do so at uploader.propmodule.com for free.

$WMc%

Ugha

I would look for another faulty component on Your Pro.Dev.Board. I checked the voltage from my wallwart (from Parallax) its output is just over 16volts open circuit. I measured 14.064 volts @ the DC jack on the P.D.B. with the switch "on".

I recently ordered the "data logger" from Parallax. I ran a 96 hr. test with it and 2 T/C readers on my Pro.Dev.B along with a BS2 in the stamp bay. The voltage Reg. ( LM1084 ) "this is the OEM REG. from Parallax" NEVER got hot. Just a little over room temp., Its dissipating over 9volts buy my readings.

I'll check My boards current draw and Reply the findings to this post.

_________$WMc%__________

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