Re: Making my own lab power supply from a pc powers supply (atx)...


"DaveM" <masondg4499@xxxxxxxxxxxxx> wrote in message
news:NO2dnZ2dnZ2ImKnPnZ2dnXpyYN-dnZ2dRVn-yJ2dnZ0@xxxxxxxxxxxxxx
> "Daniel Morrow" <videoman@xxxxxxxxxxxx> wrote in message
> news:11fr35milno3v97@xxxxxxxxxxxxxxxxxxxxx
> >
> > "Michael Black" <et472@xxxxxxxxxxxxxxxxxxx> wrote in message
> > news:ddk0cp$f2j$1@xxxxxxxxxxxxxxxxx
> >>
> >> "Daniel Morrow" (videoman@xxxxxxxxxxxx) writes:
> >> > I made my own lab pc power supply (atx) from a spare atx power supply
> >> > according to one of the many Web Pages on the web and my 4 10 ohm 10
> > watt
> >> > 10% tolerance parallel connected resistors as a legitimate load to
> >> > allow
> > the
> >> > power supply to function gets too hot (after about 1 to 2 minutes of
> > being
> >> > on the resistors are too hot to touch). I thought the cumulative
> >> > wattage
> > of
> >> > this resistor "pack" would be sufficient to handle the power,
> >> > especially
> >> > after doing the calculations on the ohm's law calculator web page
(one
> > of
> >> > them at the very least) and the parallel resistor calculator web. Is
> >> > the
> > 10
> >> > % tolerance unbalancing the resistor load (each resistor being
> >> > different
> >> > just enough to improperly handle the current/power?)? Does anyone
have
> > any
> >> > resistor recommendations that I could get at radio shack or some
other
> > easy
> >> > resistor arrangement? I don't want to take apart the main unit of the
> > power
> >> > supply because of the electrocution/shock danger. Thanks - I thought
> >> > for
> >> > sure these parallel resistors would be able to handle the power
> > dissipation,
> >> > later!
> >> >
> >> >
> >> That's one of the errors people make. They think PC power supplies
will
> >> make great general purpose power supplies, but the reality is that for
> >> many uses they provide way too much power. You waste the power in the
> >> dummy load, because you don't have need for enough current to load the
> >> supply properly. If you did need that much current, then you'd not
> >> need the dummy load.
> >>
> >> Better to grab a transformer (and high value electrolytics) out
> >> of a piece of consumer electronic equipment, and build up a power
supply
> >> that won't need a dummy load. You can take that PC power supply, strip
> >> it
> >> (the bridge rectifier can be used in the new supply) and use the case
> >> for your new linear power supply. The hole for the AC input jack is
> >> already there, as is the power switch.
> >>
> >> Resistors in parallel do not increase their wattage handling.
> >
> > What about ohm's law and parallel resistors, p=p1+p2+p3+p4.... etc.? My
> > main
> > intention is not to argue but to learn, isn't the power dissipated over
> > each
> > resistor equally, thus 4 resistors = 4 times less the amount of
watts/heat
> > dissipated over each resistor in parallel? Interesting, what about in
> > series, would that be better? Thanks for the comment.
> >
> > And
> >> since they are in parallel, they end up being a lower resistance, so
> >> when you put that across the load, they have to pass a lot of current,
> >> and hence wattage.
> >
> > Hmmm, what if I up the resistance and use the same power of resistors in
> > parallel? Later!
> > Michael
>
> \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\
>
>
> Daniel, you're letting yourself get confused by the fact that the
resistors
> are getting very hot. Power is measured in watts, and watts are
experienced
> in the form of heat. The fact that your resistors are in parallel doesn't
> affect the power that each one is dissipating. In a parallel
configuration,
> each resistor has 5VDC across it. Since each resistor is 10 ohms, then
each
> one is dissipating E^2 / R watts, or 25 / 10 = 2.5 watts. Since there
are
> 4 resistors, they are dissipating a total of 10 watts. 10 watts is a fair
> amount of heat, and if the resistors aren't mounted to a large metal
surface
> (a heat sink) or cooled by a fan, they will get hot to the touch.

Wow, major revelation! Are you saying that these 10 watt resistors can
handle 2.5 watts even if that means they get this physically hot? In other
words - they are supposed to feel this hot and if I want them to feel cooler
I need to attach them to a heat sink with heat sink compound? Should I just
go ahead and use the power supply and ignore the heat? Should I install one
10 ohm resistor (10 watt, 10%) inside the power supply instead? I always
thought a resistor would feel cooler than it's rating when the wattage
needed is lower than the resistor's rating.

Sorry guys for all of the questions, I am relearning these basics that I
originally learned in high school and when I was in high school I had only a
brief introduction because we needed to cover so much material in such a
short time (the story of my public school life - hence mathematics as well).
Please bare with me, my goal is to get this project done with as low a heat
output as possible (call me a perfectionist). Thanks.


>
> The effect of connecting resistors in parallel or series has no effect on
> ohm's law. Each resistor will behave the same, whether it is in series
with
> other resistors, or in parallel. But, if you connect your four 10-ohm
> resistors in series, then your total resistance is now 40 ohms. E^2 / R =
> 25 / 40 or 0.625 watts TOTAL!!! Each resistor is now dissipating only
> 0.156 watts. Why?? Because you have divided the voltage across each
> resistor by 4.
> 5 / 4 = 1.25 volts.
> E^2 / R = 1.56 / 10 , or 0.156 watts in each resistor.
>
> Looking at the current load that a series connection will provide,
> I = E / R = 5 / 40, or 0.125 amps.
> That's not enough to provide a minimum load for your power supply.
>
> No matter which way you look at it, the law can't be broken.
> Cheers!!!!!
> --
> Dave M
> MasonDG44 at comcast dot net (Just subsitute the appropriate characters
in
> the address)
>
> Never take a laxative and a sleeping pill at the same time!!
>
>


.

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