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From: email@example.com (N. Thornton)
Subject: Re: Doubling wallwart power rating
Date: 7 Oct 2002 05:19:09 -0700
NNTP-Posting-Date: 7 Oct 2002 12:19:09 GMT
"Phil Allison" wrote in message news:<firstname.lastname@example.org>...
> "N. Thornton" wrote in message
> > > > Wall warts come in plastic cases, which must be very poor heat
> > > > dissipators. If vent holes are added top and bottom, I wonder what
> > > > sort of power out increase one could get.
> > > > More TF cooling = more i flow acceptable.
> > > > More diode cooling = more i flow acceptable also.
> > > >
> > > > Anyone have an idea what sort of gains we might be looking at?
> ** I am challenging the presumption that adding holes will always do
> that. The unit may become covered up with an item of clothing for example.
> They often sit in power boards on the floor - many of mine do.
That would cause an increase in temp either way, vented or not. I
would say a wallwart should have a thermal fuse in its TF, though they
Looks at first sight like the airflow over the wart would be reduced
by the same %age either way. Is that wrong?
> > I can't offhand think of a scenario where a ventilated wall wart would
> > become dangerous whereas a non vented one would not, in the same
> > situation.
> ** A glass of juice spills on it or rain water. The sealed one can
> be easily and safey cleaned - the other cannot.
True, but looking around me practically none of the electricals here
are waterproof. Why would a wallwart need to be different?
> > Why would wall warts need to be waterproof?
> ** Safety - and small object proof too as Tom Faloon seems to agree.
> The fact is they sit on the floor so often where gravity does its worst.
Indeed, as do sockets and numerous other things. They are OK, and meet
all legal requirements.
In the UK electricals do not have to be either waterproof or tiny
object proof. They do need to be touch proof, with specific simulated
finger test probes. Small holes would easily comply.
Mains sockets are sometimes installed in the floor facing upwards,
with a flap cover that is lifted up. Such a socket is of course prone
to bits falling into it while the cover is up, but it is accepted
wiring practice for commercial premises.
Tom Falooin wrote:
>Increasing ventilation won't permit a appreciable increase in output
>(Maybe a few % at a rough guess) Find data sheets for typical
>and look at the derating curves provided by the manufacturer.
Top internal temp for both TFs and diodes is sround 200C, and
atmospheric is region of 20C. Allowing such items to sit in a case
that would reach 60C external temp at required operating current
(around 2x spec) means a bit above 60C internal temp, lets say 70C.
Theta meltdown minus theta case is then about 200-70 = 130.
For a vented case, ballpark...
Theta meltdown minus theta case is about 200-30 = 170.
Thats 31% increase in acceptable power diss.
P = I^2 R for the TF, so current increase is going to be 14.5%.
Guess I've anwered my own question then. :)
>My advice is - don't drill holes in the plastic case of a wall wart,
>it provides a safety barrier to prevent users from coming in contact
>dangerous voltages - typically 120 V or 230V depending on where you
Mmm, it would remove the insulation barrier. But that would be OK in
some regions of the wart, where the airgap maintains a satisafactory
>Adding vent holes allows provides the possibility of users touching
>high potentials, due to conductors entering the case.
The holes would be small enough to avoid that.
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