Page 96 - Troubleshooting Analog Circuits

Apply the 5-Second Rule 83

COLLECTOR
0 EMllTERS BALLAST RESISTORS

Figure 7.3. Ballast resistors, also known as sharing resistors, are often connected to the emitters of a
number of paralleled transistors (a) to help the transistors share current and power. In an
integrated circuit (b), the ballast resistors are often integrated with adjacent emitters. (Photo
of National Semiconductor Corp's LM 138.)

came back the next day, I discovered that the TO-3 package was still quite hot-
+300 OC, which is normally recommended for only 10 seconds. When I cooled it off,
the regulator ran fine and met spec. So, the old dictum that high temperature will
necessarily degrade reliability is not always true. Still, it's a good practice to not get
your power transistors that hot, and to have a base drive that can pull the base OFF if
they do get hot.
You can also run into problems if you tighten the screws on the heat sink too tight,
or if the heat sink under the device is warped, or if it has bumps or burrs or foreign
matter on it. If you tighten the bolt too much, you'll overstress and warp the tab and
die attach. Overstress may cause the die to pop right off the tab. The insulating
washer under the power transistor can crack due to overstress or may fail after days
or weeks or months. Even if you don't have an insulating washer, overtorqueing the
bolts of plastic-packaged power transistors is one of the few ways a user can mistreat
and kill these devices. Why does the number 10 inch-pounds max, 5 typ, stick in my
head? Because that's the spec the Thermalloy man gave me for the 6/32 mounting
bolts of TO-220 packages. For any other package, make sure you have the right spec
for the torque. Don't hire a gorilla to tighten the bolts.

Apply the 5-Second Rule

Your finger is a pretty good heat detector-just be careful not to burn it with high
voltages or very hot devices. A good rule of thumb is the 5-second rule: If you can
hold your finger on a hot device for 5 seconds, the heat sink is about right, and the
case temperature is about 85 "C. If a component is hotter than that, too hot to touch,
then dot your finger with saliva and apply it to the hot object for just a fraction of a
second. If the moisture dries up quickly, the case is probably around 100 "C; if it
sizzles instantaneously, the case may be as hot as 140 "C. Alternatively, you can buy
an infrared imaging detector for a price of several thousand dollars, and you won't
bum your fingers. You will get beautiful color images on the TV screen, and contour
maps of isothermal areas. You will learn a lot from those pictures. About twice a
year, I wish I could borrow or rent one.