Heatsink calculator?

[MikeHunt79]

Hi, Is there a way I can calculate or predict how much heatsink I'm going to need as I'm building my first amp.

I'm building the infamous "maplins kit" amp which uses the hitachi mosfets. I've going to be using around 50v for the rails, with 10,000uF caps.

I've got 2 modules, which I intend to bridge, to power a car sub at 4 ohms.

I'm guessing I should get around 125w from each module, which should give me around 250w to the sub.

Anyway, as this is my first amp, I'm not really sure how hot this thing is gonna get. I'm putting it all in quite a small case, (around 250mm x 200mm x 100mm) and I know I need to keep my transformer as far away from the PCB's as I can to prevent noise.

I just need to know how much heatsinking to use. I'm not really sure how many watts of heat this amp is gonna put out, but I don't want it to run too hot. I recently bought 20kg on randomly sized Alu heatsinks, would say 500g of heatsinking be enough or should I use more?

[audiofan]

Bofore looking for heatsink I would check for current and load line driving a 4 ohms load with a bridge is like diving 2 ohms load is there enough current ?

[MikeHunt79]

Oh, I totally forgot about the whole bridging thing. I know this design is ok with 4ohms and 50v rails, but I don't think it would cope with 2 ohms. I've killed amps before by running them at 2ohms, when they are only rated for 4ohms.

Maybe I run just one module for now, it should be ok for 4 ohms. I might be able to tweak the design if I figure out how to. I might be able to double up on output mosfets or something funky like that. I'm gonna get searching.

[Leolabs]

Have you visit http://sound.westhost.com/ for the article regarding calculating heatsink by their sizes???

[zlast]

THis is fantastic:

http://www.aavidthermalloy.com/technical/thermal.shtml

[jackinnj]

Originally posted by MikeHunt79
Oh, I totally forgot about the whole bridging thing. I know this design is ok with 4ohms and 50v rails, but I don't think it would cope with 2 ohms. I've killed amps before by running them at 2ohms, when they are only rated for 4ohms.

Maybe I run just one module for now, it should be ok for 4 ohms. I might be able to tweak the design if I figure out how to. I might be able to double up on output mosfets or something funky like that. I'm gonna get searching.

The formula which gives you the amount of heat dissipated by a class B amplifier is:

Pd = ((Vcc + |Vee|)^2 )/(R(Load)*2 * PI^2 )

So for a 2 ohm load with +/-50V you dissipate 250 watts per channel.

Originally posted by zlast
THis is fantastic:

http://www.aavidthermalloy.com/technical/thermal.shtml

Note that there is a correction factor to be applied to the Aavid Thermalloy calculator to correct for the length of the heat sink -- the reasons why are explained on their website.

[MikeHunt79]

Thanks for the info. 250watts of heat is a lot! I might have to have a low speed fan or two, even with a large heatsink. I might have a circuit redesign and see if I can get this thing running ok at 4 ohms instead of 2, as I don't think the components will cope with the current of 2ohms.

leolabs: I'll have a read of that heatsink article... I'm also gonna read the bi-amping one aswell.