[SOLVED] Question about temperature of amplifier output transisters

Hello, I built a push-pull amp with transisters that have a Ic Max rating of 3A. They are operating at 1.1ARMS or less. They get pretty hot, but I was wondering if I should spend the money on heatsinks or just drill some holes in the top of the enclosure. They are the TIP31 and TIP42 that you get at radio shack.

George

Actually it would be at about 1/2 that current because it is push-pull.

"pretty hot", If you can hold your finger on the case for 2 seconds it will be about 55 deg C. The absolute temperature of the die( little bit of silicon inside the package ) should not exceed 150 deg C. Does the case boil of a drop of water? (> 100 deg C).
How much power are they taking?, sounds like .55A X ??V = ?? watts, without a heat sink I would have thought they are only good for 2 W max. - Vsupply < 4V!!
Temperature calculations. Thermal resistance = degree/watt, die to transistor case = .5, piece of 50mm square ali = 10, so total = 10.5 degrees/watt. so if watts = 15, then temperature drop = 10.5 X 15 = 157.5 degrees so if ambient temp = 20 deg C then die temp = 20 + 157.5 = 177.5, too hot for transistor. repeat with 100mm square ali, degrees/watt = 2.5, so total = 3 (2.5 + .5), 15 watts = 3 X 15 = 75 deg C. If ambient temp = 20, die temperature = 75 + 20 = 95 deg C transistor OK.
Look up the figures and work it out.
Frank

The supply is 12VDC so I figured the max RMS voltage you'd get would be around 4.5VRMS. So it works out to be about 4W. If you're right about the 2W then I need heatsinks. I'll try to figure out from your data how to calculate the size of the heat sink. I haven't had the need to do that type of thing since I was in school a long time ago, and I just looked for the book but I can't find it. Maybe I can get one at the library. The rating on the data sheet for the TIP31 was 40W for Tc = 25 degrees C and 2W for Ta = 25 degrees C. I know that Ta is ambient temperature and Tc is the case temperature. I guess you were referring to ambient temperature above. Is that correct?

"50mm sqaure ali", does that mean 50 sqare millimeters of metal? Does it mean aluminum?

Its aluminium but its 50mm X 50mm per transistor I would have thought this would have been enough for 4W.
Frank

Keep in mind that it is the power dissipated by the transistors, not the output power, that affects the temperature rise. For a class B amplifier, the maximum theoretical effiiciency will be 78.5%. This assumes that the supply voltage is exactly equal to the peak output voltage. With a class A push-pull amplifier it will be 50%. With a class AB, the number will be in between, and will depend on how close to class B you are operating. So, with a Class B amplifier putting out 4 Watts, the minimum possible dissipation dissipation would be 4/.785 = 5.1W. Class B produces unacceptably high distortion, so all practical (analog) audio amplifiers operate Class A or AB.

Heatsink thermal grease is used to fill the microscopic spaces between a transistor and a heatsink.
A real heatsink with fins is used not a simple piece of flat metal.

A push-pull amplifier with a supply that is only 12VDC has an output of only about 1.2W into 8 ohms, 2.2W into 4 ohms or 3.8W into 2 ohms.
When two amplifiers are bridged and have a 12V supply the power output is 4.2W into 8 ohms, 7.7W into 4 ohms or 6.8W into 2 ohms.