Go Down

Topic: How to measure chip (or other component) temperature? (Read 1 time) previous topic - next topic

Docedison

No, I think a 7905 is different enough (data sheets attached) the input lead (1) is ground the tab/center pin/ground (2) is - supply and the third pin is the - output lead...
The device won't work properly but sorry nothing really reversed... I know I've used
Now IF you REALLY want to EXPLODE a TO-220 transistor, connect the emitter of a TIP42 to a real stiff (> 10A) power supply and then just ground the base... even works with the little TO-92 types... In my career as an engineer I've probably popped... a hundred or more..
I used that "trick" to get rid of my boss... he had a bad habit of looking over my shoulder.. I could never tell if it was something he was looking for to criticize or something to learn... But a "Slip" with a screwdriver or a meter probe always worked... even one time when After I had done for the transistor.. he handed me the negative meter probe and walked away.

Bob
--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard

SirNickity

Come to think of it, it wasn't that I used the wrong part, it's just that I swapped the ground and input pins on the PCB.  First PSU design and I didn't realize they were in a different order between the 78 and 79 parts.  Positive rail worked fine.  Negative rail exploded.  Oops!

That's a great story, BTW.  Ever worry that the boss thought of you as "the clumsy guy that's always blowing up parts?"  ;-)

JimboZA

Just for laughs I tried the following experiment....

I put a 7805 on a 12V supply and loaded it with a 10  Ohm resistor. Voltages measured at 12 and 5, and current measured as expected (497 Fluke mA to be exact). (Yes the resistor is hefty enough: 10W) So this set up is losing 7V @ 500mA for a power dissipation of 3.5W.

Without a heatsink, that 7805's temp shot to 110C in mere seconds. I turned it off then, since the datasheet said 120C tops.

I rummaged around and the smallest heatsink I could find is about the size of 2-3 Arduino Unos. Mounted the 7805 on there (no conductive goo though) and it settled just under 40C.

I now realise the importance of heatsinks, so that was a useful 1/2 hour of Saturday morning buggering around which beats shopping with Mrs Jimbo.
Roy from ITCrowd: Have you tried turning it off an on again?
I'm on LinkedIn: http://www.linkedin.com/in/jimbrownza

oric_dan

Quote
So this set up is losing 7V @ 500mA for a power dissipation of 3.5W.

Without a heatsink, that 7805's temp shot to 110C in mere seconds. I turned it off then, since the datasheet said 120C tops.

That's pretty close to what the d/s indicates to expect. If you find the lines that say
----------
package thermal data (see Note 1)
PACKAGE BOARD ?JC ?JA
POWER-FLEX (KTE) High K, JESD 51-5 3°C/W 23°C/W
TO-220 (KC/KCS) High K, JESD 51-5 3°C/W 19°C/W
NOTE 1: Maximum power dissipation is a function of TJ(max), ?JA, and TA. The maximum allowable power dissipation at any allowable ambient temperature is PD = (TJ(max) - TA)/?JA. Operating at the absolute maximum TJ of 150°C can affect reliability.
----------

and you plug in, you get 3.5W * (3°C/W + 19°C/W) = 77 °C rise --> 99 °C (including ambient).

There are various heatsinks with different capabilities, but I like a nice compact one (25.9°C/W),

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=158051&langId=-1&storeId=10001&productId=158051

I'm not quite sure how to calculate overall heatsinking of chip+heatsink, but I guess the two will
work in parallel, so (22 || 25.9) = 12 °C/W --> 42 °C rise at 3.5W. Something like that.

There are also heatsinks made specifically for your DIP16 L293,

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=2077335&langId=-1&storeId=10001&productId=2077335

Go Up