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'[EE]: Power supply reference level?'
2003\05\03@122015 by Picdude

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On Saturday 03 May 2003 07:09, Olin Lathrop scribbled:
> You are unlikely to fry a PC supply by drawing too much current from it.
> Reversing the current direction of an output may hurt it, but that
> shouldn't be possible in your scheme.  You may need a minimum load on the
> +5V output.

Cool.  For now it will always have a load, which is the programmer, and just the two power led's on that consume around 40mA, which is significant enough from a 120mA supply.

THanks,
-Neil.

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2003\05\03@122420 by Olin Lathrop

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>> You are unlikely to fry a PC supply by drawing too much current from
>> it. Reversing the current direction of an output may hurt it, but that
>> shouldn't be possible in your scheme.  You may need a minimum load on
>> the +5V output.
>
> Cool.  For now it will always have a load, which is the programmer, and
> just the two power led's on that consume around 40mA, which is
> significant enough from a 120mA supply.

No, I meant on the ***+5V*** output.  Some PC supplies shut down all
outputs when the +5V draw is below a threshold.


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2003\05\03@123558 by Picdude

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Ahhh.... gotcha now.  I haven't found this to be the case, but I'll keep tabs on this.  I could add a small load if I notice it.  But then again, some PS's need a load to maintain regulation, so perhaps I should add the load anyway.

Cheers,
-Neil.



On Saturday 03 May 2003 11:24, Olin Lathrop scribbled:
{Quote hidden}

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2003\05\03@165124 by Mike Singer

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> > > You are unlikely to fry a PC supply by drawing too
> > > much current from it. Reversing the current direction
> > > of an output may hurt it, but that shouldn't be
> > > possible in your scheme.  You may need a minimum load
> > > on the +5V output.
> >
> > Cool.  For now it will always have a load, which is the
> > programmer, and just the two power led's on that consume
> > around 40mA, which is significant enough from a 120mA
> > supply.
>
> No, I meant on the ***+5V*** output.  Some PC supplies shut
> down all outputs when the +5V draw is below a threshold.


Ground must be connected to ground, PICDUDE, period.
Better load +5v with 50 watt or more. This will rise up
12v to slightly higher value needed for high level
programming (HVP).


  Good Luck.
(You definitely need it when experimenting with PSU)

  Mike.




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2003\05\03@194433 by David Harris

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Hi Picdude-

I have to disagree with Mike, its ok to connect like you propose AS LONG
AS the power supply and PIC circuit are otherwise isolated.

I have done this myself.  I used a 12V car tail-light as a 5V ballast.
I also transferred the fan from the 12V supply to the 5V supply, which
made it much less noisy as I don't think heat will be a problem at 120
mA ;-)

David


Mike Singer wrote:

{Quote hidden}

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2003\05\03@195922 by Herbert Graf

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> Hi Picdude-
>
> I have to disagree with Mike, its ok to connect like you propose AS LONG
> AS the power supply and PIC circuit are otherwise isolated.
>
> I have done this myself.  I used a 12V car tail-light as a 5V ballast.
>  I also transferred the fan from the 12V supply to the 5V supply, which
> made it much less noisy as I don't think heat will be a problem at 120
> mA ;-)

       I'd have to agree, many power supplies deal with the "no load" issue, and
the ones that don't do not need a load anywhere near 50W to be usable. Only
way to know for sure on a particular supply is to try it out. I regularly
use an old AT power supply as my "breadboard" power supply. TTYL

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2003\05\04@034434 by Picdude

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Mike,

Care to explain *why* ground must be connected to ground?  So far it works w/o probs, btw.

Also how did you get the 50W figure for the +5V load?  The PS is rated for 4.8A from the +5V output, so that's 24W max, which needs to include my apps.  And if I had to use that much load anyway, I might as well find another power source ... such as tapping 16.6V off the laptop power-supply or something.

Cheers,
-Neil.


On Saturday 03 May 2003 15:50, Mike Singer scribbled:
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2003\05\04@035303 by Picdude
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On Saturday 03 May 2003 18:45, David Harris scribbled:
> Hi Picdude-
>
> I have to disagree with Mike, its ok to connect like you propose AS LONG
> AS the power supply and PIC circuit are otherwise isolated.

I'm assuming the PS is isolated from anything else, and this is why I mentioned the 2-prong AC plug.  However, I thought about this earlier for ICSP, and to be sure, I ended up using the 17V diff (between the +12 and -5 lines), and running that to a 7805 to feed the PIC circuit.  This way, I'm sure that the programmer and the PIC ckt have the same ground.  But on the downside, it doesn't permit me much power capacity left for my ckt.

Cheers,
-Neil.

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2003\05\04@035717 by Picdude

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On Saturday 03 May 2003 18:58, Herbert Graf scribbled:
> > Hi Picdude-
> >
> > I have to disagree with Mike, its ok to connect like you propose AS LONG
> > AS the power supply and PIC circuit are otherwise isolated.
> >
> > I have done this myself.  I used a 12V car tail-light as a 5V ballast.
> >  I also transferred the fan from the 12V supply to the 5V supply, which
> > made it much less noisy as I don't think heat will be a problem at 120
> > mA ;-)
>
>         I'd have to agree, many power supplies deal with the "no load"
> issue, and the ones that don't do not need a load anywhere near 50W to be
> usable. Only way to know for sure on a particular supply is to try it out.
> I regularly use an old AT power supply as my "breadboard" power supply.
> TTYL


I've tested the voltage (17V diff I'm using) with and without a 5V load and no difference, leading me to conclude that the load is not required.  I used 2 x 1-ohm 25W resistor in series, so 2.5A draw.

Cheers,
-Neil.

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2003\05\05@070033 by Mike Singer

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Neil wrote:

> Care to explain *why* ground must be connected to ground?

  Proper grounding is EE grounds. (Does this phrase sound
correctly in English?) Actually it should be said as:
"Ground must be _properly_ connected to ground"
I can't explain it, it's too long story. Maybe Russell will
provide good links.

> So far it works w/o probs, btw.

Personally, sometimes I cross streets on red light, it works
w/o probs also, btw.

> Also how did you get the 50W figure for the +5V load?
> The PS is rated for 4.8A from the +5V output, so that's
> 24W max, which needs to include my apps.
> And if I had to use that much load anyway, I might as
> well find another power source ... such as tapping 16.6V
> off the laptop power-supply or something.

It's my fault. I thought you had usual 150W PC PSU.
I meant that after you loaded +5V with significant load,
you would see that 12v output would go to slightly higher
value then 12V.(Guess why?) And you can use this voltage
for high voltage level programming.

  Mike.

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