Dwayne Reid <dwayner <at> planet.eon.net> writes:

> At 03:43 AM 11/24/2006, Peter P. wrote:
> >Maybe that won't be necessary if the heat is spread a little by 
> >adding one or a
> >few series resistors before the regulator. I like to think about the whole
> >picture. If a human must operate the box then the box cannot be over 
> >40 degC. So
> >the input to a box is limited by the size of the box eventually, before 
> > forced
> >cooling becomes necessary. E.g. a handheld size box cannot take more 
> >than about
> >3-5W input before becoming physically too hot to handle. Larger boxes scale
> >accordingly. A voltage regulator that runs 40-55 degC at normal load 
> >and ambient
> >is not 'too hot' and does not normally mandate a radiator imho.
> 
> Couple of minor points:
> 
> 1) Box already exists and is already paid for.
> 
> 2) That heat has to go *somewhere*.  If you couple the regulator to 
> the enclosure, the outside of the enclosure radiates that heat much 
> more efficiently than if either resistors inside the enclosure or a 
> separate heatsink has to radiate heat to the inside of the enclosure.

Yes but are we talking heat or temperature ? The heat will be much the same, but
the temperature of the regulator will be lower. Therefore I asked the question,
is the regulator hot enough to mandate this or not ? The OP seems to mention
about 0.4W dissipation in the TO220 (?) LM series regulator and that would
probably mandate a small clip-on radiator and likely not mandate a bolt-on or
screw arrangement to the box. ime the clip on radiator is less expensive than a
modified box, but the resistors are cheaper than both.
 
> 3) If the power dissipation is such that the enclosure would get as 
> hot as 40C or above, a linear regulator is the wrong choice.

I agree.
 
> I guess what I am saying is this: putting series resistors in front 
> of the regulator doesn't reduce the TOTAL power dissipation.  All it 
> does is move it away from the regulator.  But that heat is still 
> inside the enclosure and the enclosure is still going to heat up.

Yes, to the same temperature as with the regulator alone (although the reg will
be cooler with the resistors - that is the point).
 
> Mounting the regulator directly to the enclosure is a practical 
> method of keeping it cool at very low cost.

I do not agree about the cost. Of course I do not know your application. A
clip-on radiator will be about 10-20 cents. Two or three resistors to spread out
the heat would be 1 cent each, and would 'disappear' into the cost of a standard
circuit board assembly. A mount to the box would be more expensive than that
(including labor time etc). F.ex. with 2 33 Ohm 0.25W resistors each would
dissipate 0.12W at 60mA and the regulator dissipation should decrease to 0.16W
(from 0.4W). Thus none of the parts will be 'hot' although the total dissipation
remains the same, without drilling any holes in the (presumably presentable) box.

Peter


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