RE: [CAD_CAM_EDM_DRO] Re: L298 goes boom ( It's Fixed!- Yeah! )

If I may add:

The 78xx series does have thermal shutdown, but it isn't as you might
expect. It doesn't go off for, say, 10 seconds, then come back on. It
drops out for milliseconds, then comes back on, then drops out again.
Very hard problem to find.

Use thermal grease (from Radio Shack, Jameco, local PC store, etc.). It
makes a world of difference in heat conduction.

Regards,
JJ

Be Kind, Be Careful, Be Yourself

> -----Original Message-----
> From: caudlet [mailto:tom@...]
> Sent: Friday, August 09, 2002 11:00 AM
> To: CAD_CAM_EDM_DRO@yahoogroups.com
> Subject: [CAD_CAM_EDM_DRO] Re: L298 goes boom ( It's Fixed!- Yeah! )
>
>
> One of the great "gotcha's" in electronic design is the removal of
> heat from active components. All the ratings on the components are
> at lower temp ratings and have to be heavily derated for each degree
> of C above that nominal amount. Lets take the case of the poor
> 7812. It has to put out 12VDC of regulated voltage at up to 1A. It
> has 38VDC Input so that means we have to dissipate the other 26Volts
> as heat (26V * 1A = 26Watts). All of that heat is concentrated in
> the die of the chip and thermally has to get from there out to the
> mounting tab and from there out to the heatsink and from there out to
> the surrounding air. If you cannot get 14 to 16VDC @ 1 AMP from
> another source I would recommend you use a DC to DC switcher module.
> They are available from a varity of surplus dealers: Marlin P. Jones
> (www.mpja.com) or BGMicro (www.bgmicro.com) to name a couple. You
> can get multiple output units that have 12 and 5 volt regulated
> outputs and they run cool. At the very least those regulators need
> some pretty good heatsinking themselves. The 78XX series is supposed
> to have its own thermal shutdown and shorted output protection but I
> doubt you want to loose critical voltages on the middle of a cut!
>
> The amount of heat dissipated in a switching module like the l298 is
> dependent on the head voltage and the efficiencies of the internal
> devices. Hopefully they are optimized with quick turnon and turnoff
> times at the switching frequency. Turning off a semiconductor slowly
> (more than a few microseconds) leaves the device in a linear range
> (the difference between controlling a 100W light bulb with a switch
> versus a pot) and causes excessive heat losses. There are other
> factors as well. You have to trust the chip designer since you have
> little access to internal switching components. Needless to say that
> the goal is to keep the internal die cool and if there is not good
> thermal conduction between the heatsink and the device you may still
> be able to put your hand on the heatsink and fry the chip! The
> movement of air across the surface of the heatsink (BETWEEN the fins)
> will make a big difference in how much heat can get dissipated.
>
>
> --- In CAD_CAM_EDM_DRO@y..., "tauscnc" <tauscnc@h...> wrote:
> > Hey Afogassa,
> > Man sounds like your really having trouble keeping them cool. I
> would
> at the edge of the board it would be easy<snip>.
>
>
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