Those NTE output transistors have a working junction temperature rating of 150C. If the leadouts of the transistor don't sizzle a drop of water then you are way below that. If you can hold your finger firmly on the heatsink for at least 5 seconds when the amp is fully up to temperature then its fine.
Transistor will operate with a case temperature well above 100°C. But one MUST de-rate them for the actual operating case temperature.
De-rating factor = {150C - Tc} / {150C - 25C}
The first & second 150C is the maximum Tj from the datasheet
Tc is the case temperature at the interface with the heatsink (NOT the heatsink temperature)
The 25C is the ambient temperature applied by the manufacturer for the datasheet specification.
At 100°C the de-rating factor for a 150°C device is 50/125 = 0.4 (or 40%), i.e. a 100W device must not dissipate more than 40W while the Tc is @ 100°C
@ 110°C the maximum dissipation is 32W
But at these case temperatures the junction temperature is very close to the maximum allowed for reliable long term operation.
Increasing the heatsink to reduce the Tc by 40C degrees while dissipating the same power will reduce the Tj by the same differential, i.e. the Tj and the Tc are both reduced by the same amount, if the dissipation is kept the same.
take a ClassA amp where two different heatsink could be used.
The dissipation in the power device is 30W and Tjmax is 150°C
With the small heatsink the Tc is 110°C and the heatsink (1.8C/W) Ts is 80°C
If one could get inside, the Tj would be only just below maximum, maybe 148°C
With the larger heatsink (0.5C/W) but still dissipating 30W and the Ts = 40°C, then Tc and Tj will both be reduced by 40Cdegrees to 70°C and 108°C. Still very hot, but a very much extended lifetime.
The 108°C for Tj, tells me that the Rth s-a = 0.5C/W sink is still too small for the 30W dissipation
De-rating factor = {150C - Tc} / {150C - 25C}
The first & second 150C is the maximum Tj from the datasheet
Tc is the case temperature at the interface with the heatsink (NOT the heatsink temperature)
The 25C is the ambient temperature applied by the manufacturer for the datasheet specification.
At 100°C the de-rating factor for a 150°C device is 50/125 = 0.4 (or 40%), i.e. a 100W device must not dissipate more than 40W while the Tc is @ 100°C
@ 110°C the maximum dissipation is 32W
But at these case temperatures the junction temperature is very close to the maximum allowed for reliable long term operation.
Increasing the heatsink to reduce the Tc by 40C degrees while dissipating the same power will reduce the Tj by the same differential, i.e. the Tj and the Tc are both reduced by the same amount, if the dissipation is kept the same.
take a ClassA amp where two different heatsink could be used.
The dissipation in the power device is 30W and Tjmax is 150°C
With the small heatsink the Tc is 110°C and the heatsink (1.8C/W) Ts is 80°C
If one could get inside, the Tj would be only just below maximum, maybe 148°C
With the larger heatsink (0.5C/W) but still dissipating 30W and the Ts = 40°C, then Tc and Tj will both be reduced by 40Cdegrees to 70°C and 108°C. Still very hot, but a very much extended lifetime.
The 108°C for Tj, tells me that the Rth s-a = 0.5C/W sink is still too small for the 30W dissipation
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Alright so after a while of using my portable speaker i would like to tweak one of the resistor values to make it so the transistors dont get as hot. If I could drop the output power to one watt, would the heat that needed dissipated decrease, if so what resistor value should I change?
Why not to try this amp of post 1?
http://www.diyaudio.com/forums/solid-state/273020-mega-simple-class-amplifier.html
It works OK. I have designed recently. I used IRFP150N instead IRF610.
Regards
http://www.diyaudio.com/forums/solid-state/273020-mega-simple-class-amplifier.html
It works OK. I have designed recently. I used IRFP150N instead IRF610.
Regards
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R7 (diagram post #21) is the one to increase. For 1 watt rms output into 8 ohms you need to aim for about 300 milliamps total current draw from the battery. R7 may need to be as high as 390 to 560 ohms to achieve that.
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