# Heatsink type for these BJTs

#### edbarx

What heatsink rating is required for a power dissipation (heat) of 1.5W? The transistors are BJTs and are used for the VAS and input stage current source.

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#### HAYK

Do I suppose they are TO92?

#### edbarx

Definitely, no. These transistor are of the TO126 type and can be bolted to a heatsink.

#### ronovar

Let's calculate requered headsink. TO126 must be max 50C when it max dissipates minus 25C ambient temperature and that is 25C difference....

25C divided by 1.5W TDP gives heatsink requerement of 17K/W.

Let's say you have SK104-25,4 that have thermal resistance of 15K/W.

Temperature on TO-126 will be 15K/W x 1,5W TDP + 25C ambient temperature = 47,5 C wich is fine.

So multiple heatsink thermal resistance with max device dissipation and don't go below 25C...in above example 15K/W x 1,5W TDP gives 22,5C and we are fine because we are under 25C temp rise.

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#### Jeffh01

You can do a rough calculation on what you need. Let’s assume the inside temperature of your amp is 30C. You want to ensure that these devices don’t run too hot, they will fail at 150C and there is always some thermal resistance between the junction and the case, the thermal compound and the heatsink itself.
If we assume you have a heatsink that is rated at 20C/W that means that for each watt of power dissipated the heatsink temp will rise by 20C. So I’m your case assuming internal temp of 30 and a temp rise of 1.5 x 20 = 30, you would get a heatsink temp of 60C. The transistor junction would be hotter due to thermal resistance.
You can get the heatsink ratings from the likes of Farnell and RS. You may find similar on eBay and Ali, but they are not so good with the tech specs.

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#### Jeffh01

Heh, looks like we cross posted similar answers!

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#### edbarx

Thanks for replying and for posting a worked example.

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