IS Heat Sink Required?

"Nick

** The power or heat dissipation has to be estimated.

** Just multiply the current by the voltage ACROSS the LM7815.

ie Current x ( Input volts - 15 )

If the answer is less than 0.6 watts - you can forget the heatsink.

Otherwise use the " calibrated finger test ".

If YOU go "ouch !! " then it needs one.

......... Phil

It depends on the dissipation, ambient temperature, airflow ( e.g forced - using a fan - or not ) and package style.

More info needed. For starters it isn't the *current* but the *power* dissipated in the above example that matters. Is it TO-92 ( 7815L ) or TO-220 ( standard 7815 ) ? What's the input voltage ? Etc.........

Graham

Hi,

The input voltage is 21 VDC, the package of regulator is TO-220.

Thanks and regards..

Dissipation is therefore 6V x 45mA which is 270 mW.

I happen to know that TO-220 @ 25C ambient in free air is good for 2W so basically you don't need a heatsink.

May I recommend the old 'Motorola' AN1040.

Graham

Hi,

As for the Formula ie Current x ( Input volts - 15 ) = 0.045 x

21 = 0.945 Watts < (6 Watts). I think i can avoid the heat sink.

thanks Guys..

Graham gave you the specific answer, but in the hope that you might want to understand the principle involved:

The regulator dissipates power P = (voltage drop) * (current). The voltage drop is the input voltage minus the output voltage, which is 21V - 15V in your case. So it dissipates 6V * 45mA = 270mW.

That power is dissipated through the "thermal resistance" of the regulator case. If you look at the datasheet, there is a rating for theta-ja, the thermal resistance from junction to air (that's without any additional heat sink).

Thermals can be modeled just like electric circuits: thermal resistance is like resistance, power is like current, and temperature drop is like voltage drop. So using Ohm's Law (V = I * R), and substituting in the the thermal equivalents (T = P * theta), you can calculate the amount of temperature drop between the regulator chip and the air, for a given power.

Looking on the National Semiconductor datasheet for an LM7815T, I see theta-ja for the TO220 case is typically 50 C/W (that's degrees Celsius per watt). So, we can calculate T = P * theta = 270mW * 50 C/W = 13.5C. In other words, the chip will be running at 13.5 degrees above ambient, or about 40C. I also see that the max operating temp is 150C, so you are quite safe.

...Or, you could just look at the graphs in the datasheet, where it shows a chart of how much power the chip can safely dissipate, with various kinds of heat sinks and at various ambient temps.

"Nick"

** Your math is crap.

You failed the subtract the 15 !!!

PLUS the safe figure I gave was 0.6 watts !!!

** Do you cross busy streets with your eyes shut ?

.......... Phil

x

Something like your advice?

How did you arrive at that? Most TO-220 cases can safely dissipate 2W (without a sink) in normal ambient conditions since most of them have a

50-60C/W Tja.

Here's to hoping that you do.

"Anthony Fremont" = One Pig Ignorant Asshole

** My advice is for you to go f*ck yourself - Anthony.

Now that is damn good advice.

** Lots of experience.

Something congenital f****it like you does not have.

** Absolute brainless bullshit !!

Max junction temp for the LM78series is 125C, above which thermal shutdown occurs.

The local ambient may well be up to 65C when the device is mounted inside some piece of gear.

That makes the max allowable dissipation no more than 1 watt.

Even 1 watt would result in the case temp being circa 120C - far too bloody hot for any nearby electro or other semi.

The figure of 0.6 watts dissipation is reasonably safe in most apps.

** What sort of pathological pleasure you get form being THE most obnoxious prick you can is mystery to me.

Lets just hope a painful, fatal disease gets you real soon.

I'll be looking forward to it.

......... Phil

"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...

Hey, he didn't say for how long ;)

shutdown

Even 0.6 watts makes it feel pretty hot, for a calibrated finger test. When I can touch it for more than 5 seconds, I assume it's okay. I'd think that 2 watt you have instant blisters.

Then there's a diffence if you mount it vertical or flat. When mounted flat on a PCB you have in fact a heatsink (copper plane), although you won't find it on the bill of material.

--
Thanks, Frank.
(remove 'q' and '.invalid' when replying by email)

0.045

have a

shutdown

National says 150C and nobody (that I can find) says at what temperature shutdown occurs.

inside

I sure didn't see anything like that being mentioned before now.

Well duh, after you start putting arbitrary constraints on things. I believe that most consider ambient to be 25C.

Again, who said anything about these kinds of limitations?

So, in the end, you just "conjure up" an arbitrary number after all?

It's all just for you phil. ;-) BTW, I like it when you get so excited that you misspell. Speaking of being the most obnoxious prick possible, pot...kettle...black and all that.

LMAO

"Frank Bemelman"

** The "advanced finger test" ( AFT) involves a preliminary, judicious lick of the test finger - followed by a " fast acoustic analysis " ( FAA ) of any resulting sizzle and possible immediate cessation of test.

........ Phil

"Anthony Fremont" = Congenital, Pig Ignorant Scum

-------------------------------------------------------------

** The max Tj spec is *125 C* in the National data book - FUCKHEAD !!

Your LACK of any experience with the part is BLINDINGLY obvious.

** Your LACK of any experience with electronics is BLINDINGLY obvious.

FUCKWIT.

** Your LACK of any experience with electronics is BLINDINGLY obvious.

FUCKWIT.

** Your LACK of any experience with electronics is BLINDINGLY obvious.

FUCKWIT.

** Your LACK of any experience with electronics is BLINDINGLY obvious.

FUCKHEAD !!!!

** Your LACK of any experience with electronics is BLINDINGLY obvious.

YOU SMUG POSTURING PIG.

** Anthony Fremont is no more than a useless, publicly masturbating, autistic pile of shit.

Lets just hope a very painful, fatal disease gets him soon.

I'll be looking forward to it immensely.

........... Phil

Please don't top post.

temperature

FUCKHEAD

is the first match in Google. It clearly demonstrates your inability to read and comprehend information. To reiterate, the max Tj is 150C.

obvious.

Yes, resort to name calling when you can't make any other kind of argument. For all we know, the part may have a fan blowing on it. We do know that the regulator will only be supplying 45mA. It's probably safe to assume that no parts are emitting that kind of heat in the circuit.

obvious.

obvious.

You're repeating yourself phil. Perhaps a witty invocation of Godwin's Law would allow you a graceful exit from your repetitive fit.

too

obvious.

apps.

obvious.

Did I touch a nerve?

Why do you continually call me autistic? I have no trouble communicating. Perhaps it's just your interpretation of how the world tunes you out.

"Anthony Fremont" = Congenital, Pig Ignorant Scum -------------------------------------------------------------

** My National Linear Data data book CLEARLY says 125 C for the T package.

Plus ALL the electrical specs are given with Tj in the range from 0 to 125C .

** Go f*ck yourself - you sickening, autistic PUKE. ** Because that is exactly what you are - PUKE.

....... Phil

Ehhh- forgetting about the bias currents, say 4mA so that additional

4x21V=80mW for a total of 350mW typical.

Actually I hadn't but I don't like to bring in the subtler stuff for beginners straight off.

Graham

package.

Good for you phil, the datasheet I provided CLEARLY says 150C for the T and K packages. As if any of this matters anyhow. All I ever wanted to know is how you "calculated" .6W. We now know that you WAG'd it, so that answers that.

125C

So what, many times the individual specs are quoted over a narrow range, it's nothing new in datasheet land.

masturbating,

I'm sure you have everyone convinced.

"Anthony Fremont" = Congenital, Pig Ignorant, LYING Scum ----------------------------------------------------------------------

** It is the correct OPERATING temp spec for the part.

Thermal shutdown soon intervenes if that temp is exceeded.

Something I have verified with a K type thermocouple many times.

Bet a vile, autistic PUKE like you does not even own one.

Bet a vile, autistic PUKE like you does not ever designed anything.

** But it is NOT an OPERATING temp. ** Which I supplied to you in some detail earlier.

I should have given you a bullet in the head instead.

** If your "WAG'd" means using long experience and common sense to determine the safe dissipation figure to advise the OP as opposed to some wacky theoretical one - that is just what I did. ** Proves the point - FUCKHEAD !!!

** Because that is exactly what you are - you EVIL PUKE.

Plus a criminal, a liar and a net stalker.

Another typical Septic Tank - full of shit.

FUCK OFF !!!!!!!!!

........ Phil