Help needed with datasheet figure

[generic_username]

Hi,

I'm using a heat sink for an TO220 Mosfet and I want to know how much power I can dissipate with good conscience.

I used this heat sink RA-T2X-38E (1.5")
with an Air velocity of 200  cubic foot/minute

https://www.ohmite.com/assets/docs/sink_f_r.pdf

and I'm struggeling to interpret the curve, anybody familiar with this kind of figure?

kind regards

[DL2XY]

These are two diagramms in one.
Did you notice the arrows? They are pointing to the relating absisses.

[generic_username]

Thx for the reply

I noticed it after posting, but that would mean the thermal resistance of the heat sink is increasing with higher air flow, which seems counter intuitive.
The manufactor states 3.9 °C/W for natural convection I read that this would be around 50 feet/min

[BillyO]

Keep in mind also the method and technique you use to mount the device to the heatsink (amount/type of thermal compound/thermal tape/adhesive, insulator or no).  It can be quite a lot of fun (read difficult) getting it just right by taking into account all the parameters and doing it by calculation alone.  You might consider designing in a huge margin for error or do a prototype stage to do some case testing.

[generic_username]

I finally got it -.- sometimes it takes a while

This will just be a rough estimate for an old prototype if I can use for something with more power.
Will test this of course but it seems I can use it as is without a need for rework

thx for the replies

[Doctorandus_P]

In addition to interpreting the graph.

this is not correct:

with an Air velocity of 200  cubic foot/minute

"Velocity" is in distance / time, while what you are stating is Volume / time. I think it's called "flow" in English. And that makes sense. A fan displaces a certain amount of air per time unit, while a heatsink does not care if a wind is blowing outside, it only cares about the air speed close to it's surface.  There is a big difference between a fan blowing air directly at the heatsink (may include a funnel) or a fan blowing the same amount of air just past the heatsink.

Because small differences here will have a huge impact on performance it is very difficult to model or calculate it accurately, and you will have to do real measurements with your setup.

[Nominal Animal]

Here is the original chart split into the two separate ones, with the 1.5" heat sink size curve shown in red.

Note that in the first one, the air velocity is not 'cubic feet per minute', but the actual velocity, 'feet per minute' at which the air flows over the heatsink.  It depends on what kind of obstacles and turbulence you have within the enclosure, and cannot be simply derived from stated flow rate (cubic feet per minute).

[generic_username]

thank you for pointing that out with the air velocity.