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how much heat can a 78L05 disipate ?

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Thunderchild

New Member
i'm using a 78l05 to powrr a pic that will run a couple of leds up to about 32 mA, what wattage in heat can a 78L05 dissipate as i'm running this between 11 and 16 volts and just burnt the reg out i think, i can't find this information ion the datasheet. I'm thinking of putting a resistor in series with it to help it cope
 

bountyhunter

Well-Known Member
Depends on the package. Can you give the package code like TO-92 or similar?

By estimate, your app will have about 0.4W of power disspation max. A small TO-92 package is about 180C/W, so the die temp would be maybe 100C. That package is the small plastic "transistor" type on three legs (through hole). If it is a smaller surface mount package, they typically handle less power unless they are special power packages with a solder down metal tab..

But, even if you overheat it, the part should not die.
 
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Hero999

Banned
It's on the datasheet, see note 5 on page 5.
https://www.electro-tech-online.com/custompdfs/2009/08/LM78L05.pdf

Z Package = TO-92 Plastic Package
Z Package: θJC = 60 °C/W

You're dissipating 352mW so the temperature rise is just over .352×60 = 21°C

If the maximum ambient temperature is 30°C the die temperature will be 51°C which is well with specification.

EDIT:
Are you're powering a couple of LEDs 32mA each?

Double the temperature rise, ΔT = 42°C, T = 72°C
 
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audioguru

Well-Known Member
Most Helpful Member
The datasheet for the 78L05 shows that it has an internal thermal protection circuit that prevents it from over-heating. If it becomes hot then it shuts-down then cools and starts working again. If you let it thermal cyle many times over and over then it might fracture due to the thermal stress.
 

Thunderchild

New Member
ah yes but this is not made by national semiconductors, infact the pinout is reversed (I discovered this after burning a couple out and back tracking to my RS order history and the specific part and its specific data sheet). 78l05 is a part number under which many manufacturers produce, a bit lime the NE555 more comonly known as the 555 and often also LM555, they all do the same job and you can't tell them apart but different companies make them

I'm using a tricolour led that will take 25 mA per colour and am running them on 16ish mA
 
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Hero999

Banned
If it becomes hot then it shuts-down then cools and starts working again. If you let it thermal cyle many times over and over then it might fracture due to the thermal stress.
In my experience, regulators don't turn back on until the load is reduced significantly so thermal cycling is highly unlikely.
 

MikeMl

Well-Known Member
Most Helpful Member
Buy a TO220-style LM7805. Bolt it to a heatsink. Temporarily substitute it for the regulator in your circuit. If that fixes your circuit, you will have evidence that your off-brand '05s are not up to your task.

Next try removing the LM7805 from the heatsink and try your circuit again. Feel the tab on the TO220; that will give you an idea of how much you are asking the regulator to dissipate in your application.
 

bountyhunter

Well-Known Member
Z Package = TO-92 Plastic Package
Z Package: θJC = 60 °C/W
That is technically wrong, since defining "junction-to-case" thermal resistance for a plastic TO-92 makes no sense.. A plastic package like the TO-92 really has no valid Theta J-C spec, because there is no part of the device which acts like a "case" designed for heat transfer which can be attached to a heatsink. A TO-3 or TO-220 or TO-263 all have a metal tab so you can define a value for theta J-C which means something.

I remember having this endless argument with the idots there the whole time I worked there. Believe it or not, they print "theta J-C" numbers on plastic packages wher it is actually undefined just because customers whine they need a value to plug into a spread sheet or a spice model. They don't care that the spec is meaningless, they just need a number. I never did it on any documents I wrote, but other idiots did. Theta J-C only is valid if the package has a surface which is designed to transfer heat.

A TO-92 standing up in free air has a theta J-A of about 180C/W. I guarantee it. The larger TO-220 device standing in free air is about 65C/W.
 
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bountyhunter

Well-Known Member
In my experience, regulators don't turn back on until the load is reduced significantly so thermal cycling is highly unlikely.
Usually, they cycle in and out of thermal shutdown a bit then establish an equilibrium where the output voltage (and load current) is reduced to a point that the power dissipation keeps the die temp right at the shutdown threshold.
 
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bountyhunter

Well-Known Member
ah yes but this is not made by national semiconductors, infact the pinout is reversed (I discovered this after burning a couple out and back tracking to my RS order history and the specific part and its specific data sheet). 78l05 is a part number under which many manufacturers produce,
If it has an industry standard number, it should have the same pinout.
 

Hero999

Banned
A TO-92 standing up in free air has a theta J-A of about 180C/W. I guarantee it.
It's nice that figure gets a mention on the datasheet.:rolleyes:

Why was the reason for omitting it or have I missed something?

You can also solder it down to a PCB with a 2 square inch copper area under it and it will drop the thermal resistance from about 65C/W to maybe 30C/W.
Does that also help with the TO-92 package?

Usually, they cycle in and out of thermal shutdown a bit then establish an equilibrium where the output voltage (and load current) is reduced to a point that the power dissipation keeps the die temp right at the shutdown threshold.
What's the likelihood of that damaging the IC?

Does that happen when the IC is short circuited? Not in my experience but I could be wrong.
 
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Thunderchild

New Member
If it has an industry standard number, it should have the same pinout.

nope exact reversed pinout as compared to the national one and all others i believe, seems that this is what RS has bvecome but at 5p a piece I'm not going to argue, The reg was overheating due to my shorting the pic i was powering, the pic now has a resistance of 4 ohms over the supply pins although it is still recognised by the pickit2 programmer, naturally i have replaced the pic and its all fine
 

Mr RB

Well-Known Member
All the 78L05 have a funky pinout compared to a 7805, but they are always the same pinout which is used for all regulators in the TO-92 package.

And little heatsinks for TO-92 are common and cheap. I've got a ton pulled from old TV set neck boards.
 

Hero999

Banned
Usually, they cycle in and out of thermal shutdown a bit then establish an equilibrium where the output voltage (and load current) is reduced to a point that the power dissipation keeps the die temp right at the shutdown threshold.
Do you test for thermal cycling? Is there a general limit before the IC is killed?
 

Thunderchild

New Member
All the 78L05 have a funky pinout compared to a 7805, but they are always the same pinout which is used for all regulators in the TO-92 package.

And little heatsinks for TO-92 are common and cheap. I've got a ton pulled from old TV set neck boards.

well if you use the flat side as a reference these are reversed the pins are in the same order but back to front !
 

bountyhunter

Well-Known Member
Originally Posted by bountyhunter View Post
A TO-92 standing up in free air has a theta J-A of about 180C/W. I guarantee it.
It's nice that figure gets a mention on the datasheet. Why was the reason for omitting it or have I missed something?

basically, because the person who wrote it was an idiot who did not understand what theta J-C means, which is thermal resistance junction-to-case. There is no defined heat conducting surface on the case to attach to a heatsink.



Originally Posted by bountyhunter View Post
You can also solder it down to a PCB with a 2 square inch copper area under it and it will drop the thermal resistance from about 65C/W to maybe 30C/W.

Does that also help with the TO-92 package?
No and No. How do you solder a plastic case to a PC board? And if you do glue the plastic down to copper or aluminum, the thermal rsistance of plastic is so high that the effect of the heatsink is negligible.


Originally Posted by bountyhunter View Post
Usually, they cycle in and out of thermal shutdown a bit then establish an equilibrium where the output voltage (and load current) is reduced to a point that the power dissipation keeps the die temp right at the shutdown threshold.

What's the likelihood of that damaging the IC?

Zero, for a properly made part because the thermal shutdown will save it. If it dies, it means the shutdown was not working and that happens fairly often



Originally Posted by bountyhunter View Post
Usually, they cycle in and out of thermal shutdown a bit then establish an equilibrium where the output voltage (and load current) is reduced to a point that the power dissipation keeps the die temp right at the shutdown threshold.

Does that happen when the IC is short circuited? Not in my experience but I could be wrong.
Yes, in most cases with small packages, the power dissipation in short circuit is enough to cause the thermal limiter to cycle on and off. In many cases, the package and die get thermally saturated and establish a balanced equilibrium where the current limiter is holding power dissipation steady right near thermal shutdown. The cycling behavior may stop, but it's not always obvious whether the curr limit or temp limiter is driving.
 
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bountyhunter

Well-Known Member
All the 78L05 have a funky pinout compared to a 7805, but they are always the same pinout which is used for all regulators in the TO-92 package.
I thought the pinout on the positive regs was always I-G-O from left to right looking at the flat face with writing on it and the legs pointing down (Input - Ground - Output).

I might be in error, that's just what I remember. The ground is nearly always center because it is the lead frame internal connection.
 
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