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2n3055 or tip3055?

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You work backwards from the maximum ambient temperature and forward from the power dissipated. From that you can determine the maximum thermal resistance you can tolerate for the heat sink which will dictate the size of the heat sink.

POWER DISSIPATED=18WATTS
max ambient temp=40'C (just in case)
What now.
I know you must be pulling the hair out of your head in frustration, but please bear with this newbie.
 
Nice looking box!
AAVID.com and other heat sink makers have data sheets. Often they show temp rise in free air and with airflow of x-feet/minute.
You need airflow. Heat sinks often face out the back of the box. In PCs they have a fan blowing on them.
You can put a temperature meter on the transistor. Do the math to get an idea of case temp verses die temp at 18 watts. Because we do not know what heat sinks you have, and do not know airflow then just measure the transistor.
 
I already have another thread running, about the airflow problem. I'm going to use 2 computer 12vdc fans, run directly from the 50V rail (with sufficient resistors of course). Oh, and those are old CPU heatsinks. The heatsinks are just where they are in the photo for demonstration purposes. They will later be mounted in the back of the box. Now it's up to me to try and make the holes for the fans. which will be quite hard, since I don't have a circle drill of that diameter.
 
POWER DISSIPATED=18WATTS
max ambient temp=40'C (just in case)
What now.
I know you must be pulling the hair out of your head in frustration, but please bear with this newbie.
And I don't have that much hair left to pull out.:D

OK. So just calculate the total thermal resistor you can have.

Assume a maximum junction temperature of 150C for safety. Then the maximum allowed thermal resistance to air would be (150-40)/18 = 6.1 C/W. From this you subtract the transistor Junction-Case thermal resistance of 1.5 C/W and another couple tenths for Case-Sink gives a maximum allowed heat sink thermal resistance of 6.1 - 1.5 - 0.2 = 4.4 C/W.

Edit: Have you ever used a nibbler tool? They work well to enlarge holes or cut irregular shape in thin metal.
 
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Are these heatsinks large enough?

Hi,

Those heat sinks look big enough for 20 watts, as long as they have free air flow through the fins. I cant tell from the pic if your cabinet has air holes, but if not you'll have to add some.
To test the heat sink, run the unit at full power for a while and measure the case temperature of the transistor. Using the data sheet, calculate the junction temperature and see if it is within reason.
 
To test the heat sink, run the unit at full power for a while and measure the case temperature of the transistor.

Only problem is, I don't even have a normal glass thermometer, and the guy I bought my FLUKE from didn't supply the temperature probes (still damn

angry about that) I'm just going to add the fans and test it by running at full current, and placing my hand over them. But it should work. I can only install the fans tomorrow.
 
Hi,

Oh sorry to hear about that. Well then if you use an eye dropper an drop a single drop of water on a horizontal part of the heat sink it should take a while for it to evaporate, not all of a sudden. If it evaporates really fast then it might be too hot. If it boils, it's too hot. If you have a fan running though it might be hard to do this test.
 
Hi,

Oh sorry to hear about that. Well then if you use an eye dropper an drop a single drop of water on a horizontal part of the heat sink it should take a while for it to evaporate, not all of a sudden. If it evaporates really fast then it might be too hot. If it boils, it's too hot. If you have a fan running though it might be hard to do this test.

An easier test (requiring zero equipment) is to lick your finger, and then touch the transistor briefly - if it sizzles, it's too hot :D

Don't do it with a dry finger though - otherwise it can burn you before you feel it.
 
I'm not sure what you are planning, but your forced air needs to flow between the fins of the heat sink
 
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I already have another thread running, about the airflow problem. I'm going to use 2 computer 12vdc fans, run directly from the 50V rail (with sufficient resistors of course). Oh, and those are old CPU heatsinks. The heatsinks are just where they are in the photo for demonstration purposes. They will later be mounted in the back of the box. Now it's up to me to try and make the holes for the fans. which will be quite hard, since I don't have a circle drill of that diameter.

I'm pretty sure that's a bad idea? For a motor you supply it with the necessary voltage, thats all.
 
I just took an old 12v chargher, ran it from the 220V mains (in south africa) and powered the fan. It works great. Has about 10mV ripple on the output. Transistors run cool.
And it makes a nice contribution to my power supply collection.
Thanks all.

Hanno.
 

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