RE: [CAD_CAM_EDM_DRO] RE:heatsink flexing was: chopper max voltage

-----Original Message-----

>From: ballendo@... [mailto:ballendo@...]
>The L298 "legs" are bent in a "Z" shape to accomodate this thermal
>expansion.

That will certainly help.

>I HAVE to disagree with your statement that hanging a heatsink in the
>air will do less damage than fixing the heatsink as securely as
>possible....

Clearly there is a point where the heat sink is so heavy that it requires some
support other than the component leads.
Perhaps we are talking about different sizes of heat sink.

>... And, Flexing of the pc bd should definitely be avoided!

This will happen, even with the Z-bend in the component leads--both the PCB and
the leads are springy, and each will flex a bit to take up a share of the
strain. What the bent leads do for you is to allow the PCB to flex less and have
the leads take up the strain instead.

The expansion problem is also greatly affected by *where* the heatsink is
attached. If the transistor mounting screw is also what attaches the heat sink
to the chassis (and the PCB is also attached to a nearby point on the chassis)
there won't be a problem. On the other hand if there is 6 inches of aluminum
heatsink between the transistor and the heatsink chassis mount, then 6 inches of
steel back to the PCB attachment, there is bound to be considerable differential
expansion, which will flex (and eventually break) the transistor leads.

Another way to circumvent the problem is to use a socket for the component
leads.

But, as I mentioned above, it mostly depends on how big a heat sink we are
discussing. As a rule of thumb, I consider how long I could hold the part by its
leads between my thumb and finger with the heat sink attached. If I think I
could hold it that way until I get bored, the heatsink needs no further support.
If I think I will get a cramp in my hand first, the heatsink needs support.
-Kevin Martin