How Hot Can The LM3875 Get - do I need vent holes?

[ransom peek]

Here is my finished LM3875. There is an aluminun plate that covers the top, and a wood enclosure the whole thing slips into.

I am trying to figure out if I need vent holes.

The heatsink fins are facing the side instead of up as would be better of course. Heat sink compund was used. 27.5 volts at each rail. 330 VA toroid.

I have only tested for heat with the amp at about 75% of volume, for a few minutes with the aluminum top loosely on to keep the heat in for the test.

Chips get hot, but easy to touch and keep a finger on. Heat sink gets warm, not hot. Toroid stays cool.

With wood suuronding the aluminum and running at full volume and at extended duration will be a better test.

QUESTION: How hot can it get comfortabley - the chips too hot to touch still ok? If the heat sink gets hot but not too hot to hold is the temp ok?

Have others had to add vent holes, and if so hoy many? Any pics?

[pinkmouse]

The cooler you keep the chips, the more reliable they will be. A good basic rule is that if you can't keep your fingers on the heatsinks more more than a couple of seconds, they are too hot.

Personally, I would put vent holes in, above and below the heatsinks so the air can move past the heatsinks as it warms.

[poobah]

P. Mouse is right on the money... every 11 C increase in temp cuts the life in half.

I can't see everything so clearly, but if you can manage a row of 8 1/2" (13mm) holes through the top, bottom, AND the fins, that would make a world of difference. I don't know how long (deep) the fins are though.

On your next amp (or this one) use heat sinks that have vertical fins... you'll still need the ventillation top and bottom. Horizontal fins, unless they have a fan are really crippled... about 10 - 20% as effective as vertical fins.

[ransom peek]

Maybe best way is to put 3 half inch diameter holes in the heatsink itself - vertically, so the draft created will suck up the hot air through the fin area. And the same holes on the housing directly above and below these. Sacrifice a bit of fit area but get a nice draft flow upward to vent.

Maybe a few additional holes in the top and bottom housing in the same general area.

[ransom peek]

Is it correct that this is a bigger heatsink than would normally be needed (so the loss of heat sink mass from the added holes will not be a problem), but because of the poor orientation of the fins, even with some holes for venting, it may just be adequate?

[poobah]

Heat sink mass helps... mass to heat is the same as capacitance to current... it will absorb heat (current) temporarily and therefore limit the temperature (voltage) rise.

The resistance to ambient (a resistor bleeding/burning the energy) in a heat sink is about fin area and the cross sections of metal form heat source to ambient. Mass will store heat but it won't shed it... a low resistance (big fins and thick webs) from sink to air will. Vertical is SO important.

I think some holes that get a vertical flow going are a great idea... even if you lose 10% of the fin area.

I think he could do more than 3 holes easy 6... 8.

[Dominick22]

I like those gold plated fuses/fuse holders you have there...Can you tell me where I can find them?

Dominick

[frank1]

It is not good practice to put heatsinks inside an enclosure. The whole box and everything inside will heat up.
The fins should be in free air.
If the box itself is the heatsink, then thats a different matter but you then cannot sleeve it in wood....
Frank

[ransom peek]

Originally posted by Dominick22
I like those gold plated fuses/fuse holders you have there...Can you tell me where I can find them?

Dominick

They are not gold plated fuse holders, just looks that way from the picture...

[ransom peek]

Originally posted by frank1
It is not good practice to put heatsinks inside an enclosure. The whole box and everything inside will heat up.
The fins should be in free air.
If the box itself is the heatsink, then thats a different matter but you then cannot sleeve it in wood....
Frank

Instead of the horizontal fins with vertical holes, how about this puppy on the top or side? It is 6 inches long. See picture...

What is the best orientation for this heat sink -
1. flat base down, fins up
2. flat base vertical, fins horizontal
3. falt base vertical, fins vertical

Notice the fins have a T notch on ends that will interrupt air flow.

Easiest way to mount and best fit is #1 above, on top of amp. But the air flow???