# Heatsink question

Discussion in 'Electronic Design' started by pimpom, Feb 26, 2009.

1. ### pimpomGuest

I want to couple two TO-220 transistors to their common heatsink
by mounting them on a slab of aluminium and then bolting the slab
to the heatsink.

The Al piece is 4.5mm thick and 30mm wide. From the center of the
transistor mounting screws to the point where the slab starts
mating with the heatsink is about 25mm. The section of the slab
covering the heatsink is bent 90 deg and is 25mm long, i.e., the
slab-heatsink contact area is 30x25mm. The contact area is
reasonably smooth and will have heatsink compound (thermal paste)
applied between the two surfaces.

| |
_| |
|| |
|| |
25mm|| |
Tr || |
_____ || |HS
___|____|===________|| |
4.5mm |____________________| |
|<--25mm-->| |
| |

Can anyone please calculate or give an educated estimate of the
thermal resistance added by the indirect mounting?

2. ### PeterDGuest

How thick is that 25mm flange that attaches to the main heatsink? How
thick is the main heatsink?

3. ### pimpomGuest

The heat conduit is of the same thickness throughout - 4.5mm,
including the flange that attaches to the main HS.
The section of the heatsink covered by the flange is 4.7mm
finned blackened aluminium, made by a company that's been in the
business for at least 30 years. The heatsink I'm going to use is
rated at 2 deg C/watt.

The two transistors together are expected to dissipate 10-20W
total in normal use, with occasional 30W peaks. A bit marginal,
resistance introduced by the indirect mounting. At least the HS
will be mounted outside the cabinet in free air.

4. ### PeterDGuest

Personally I would not do this... 4.5 is a bit on the thin side, but
without doing the computations I can't tell you what the actual result
would be.

Personally I think you should reconsider your design...

5. ### pimpomGuest

Thanks. That's the kind of guesstimate I was looking for and it

6. ### pimpomGuest

It may come to that. If possible, I'd rather change the HS and
flange dimensions than the mounting method.

My problem is that I live in a remote corner town of India from
where getting the stuff I need is not as simple as, say, ordering
from RS in the US. The 4.5mm Al piece is from a building
materials shop and the heatsink is to be cut from two 2-ft bars I
bought during a visit to one of the larger cities some 15 years
ago and which I've been using sparingly since.

What I can do is 1) look for a thicker flange *and* 2) use a
longer length heatsink from the bar, though using a longer HS
would require some modification of the enclosure. Or, I could add
a small fan, which will also need some modification.

7. ### PeterDGuest

Your problem is that the 'flange' (the piece you bolt on to the heat
sink and mount the transistors to) won't conduct sufficient heat to
teh main heatsink.