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'[PIC] the easiest way to connect 2 power supply to'
2010\02\12@194551 by Dario Greggio

face picon face
Basically I have one AC-DC module and one DC-DC module, and I have to
power a PIC24+ENC+ some leds & 1 LCD and some sensors, it all @3.3V
I need this to be a "full-OR" i.e. the circuit should work when any of
them is available.
I thought as the very easy method of the Diodes, i.e. use 5V modules,
place 2 diodes in series per each branch, and obtain 3.6V.
I know it's not "clean" but I would not want to add other (complex)
circuitry)...

Any suggestions appreciated!
TIA

--

Ciao, Dario
--
Cyberdyne

2010\02\12@200308 by ivp

face picon face
> place 2 diodes in series per each branch, and obtain 3.6V

Dario, can you guarantee the voltage drop across the diodes as the
load varies won't affect the circuit ? For example,

1N4001 0.7V @ 10mA, 0.75 @ 100mA, 0.9V @ 1A
BAT46 0.25V @ 1mA, 0.4V @ 10mA, 0.75V @ 100mA

diode-ORing them followed by an LDO 3.3V regulator perhaps

wbr

2010\02\13@051828 by Dario Greggio

face picon face
ivp ha scritto:
> Dario, can you guarantee the voltage drop across the diodes as the
> load varies won't affect the circuit ? For example,
>
> 1N4001 0.7V @ 10mA, 0.75 @ 100mA, 0.9V @ 1A
> BAT46 0.25V @ 1mA, 0.4V @ 10mA, 0.75V @ 100mA
>
> diode-ORing them followed by an LDO 3.3V regulator perhaps

Thank you:
yes a LDO would be better, have considered it.

Actually, the slight variations in the load (which should actually
always be in the range 250-350mA), should allow for a 1N4007 diode.


--

Ciao, Dario
--
Cyberdyne

2010\02\13@094800 by Spehro Pefhany

picon face
At 05:18 AM 2/13/2010, you wrote:
>ivp ha scritto:
> > Dario, can you guarantee the voltage drop across the diodes as the
> > load varies won't affect the circuit ? For example,
> >
> > 1N4001 0.7V @ 10mA, 0.75 @ 100mA, 0.9V @ 1A
> > BAT46 0.25V @ 1mA, 0.4V @ 10mA, 0.75V @ 100mA
> >
> > diode-ORing them followed by an LDO 3.3V regulator perhaps
>
>Thank you:
>yes a LDO would be better, have considered it.
>
>Actually, the slight variations in the load (which should actually
>always be in the range 250-350mA), should allow for a 1N4007 diode.

I would suggest this is not prudent-- you *could* get flaky Ethernet
performance if the analog circuitry internal to the ENC does not have a
well-regulated supply. IIRC, only the digital circuitry has an internal
regulator.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2010\02\13@095915 by Dario Greggio

face picon face
Spehro Pefhany ha scritto:
> I would suggest this is not prudent-- you *could* get flaky Ethernet
> performance if the analog circuitry internal to the ENC does not have a
> well-regulated supply. IIRC, only the digital circuitry has an internal

Hmm, I see, this is a point indeed...
Do you have any suggestion then?


thanks :)

--

Ciao, Dario
--
Cyberdyne

2010\02\13@100625 by Spehro Pefhany

picon face
At 09:59 AM 2/13/2010, you wrote:
>Spehro Pefhany ha scritto:
> > I would suggest this is not prudent-- you *could* get flaky Ethernet
> > performance if the analog circuitry internal to the ENC does not have a
> > well-regulated supply. IIRC, only the digital circuitry has an internal
>
>Hmm, I see, this is a point indeed...
>Do you have any suggestion then?


The LDO is the way to go, maybe with a dual Schottky diode (eg. BAT160)
so only two parts total plus maybe a cap or two.

Keep in mind that if both power sources are present, and both are (nominally)
5V,  either one or both may be supplying current.


>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2010\02\13@102208 by Dario Greggio

face picon face
Spehro Pefhany ha scritto:
> The LDO is the way to go, maybe with a dual Schottky diode (eg. BAT160)
> so only two parts total plus maybe a cap or two.
>
> Keep in mind that if both power sources are present, and both are (nominally)
> 5V,  either one or both may be supplying current.

Yes, but .. ah ok: you mean:
1) Dual Schottky
2) LDO
3) cap (already there anyway)

thanks again
--

Ciao, Dario
--
Cyberdyne

2010\02\13@105132 by Spehro Pefhany

picon face
At 10:21 AM 2/13/2010, you wrote:
>Spehro Pefhany ha scritto:
> > The LDO is the way to go, maybe with a dual Schottky diode (eg. BAT160)
> > so only two parts total plus maybe a cap or two.
> >
> > Keep in mind that if both power sources are present, and both are
> (nominally)
> > 5V,  either one or both may be supplying current.
>
>Yes, but .. ah ok: you mean:
>1) Dual Schottky
>2) LDO

Might be better to have a cap right at the LDO input.

>3) cap (already there anyway)

Make sure what you have meets the LDO output capacitance requirements.

It's pretty easy to make most LDOs oscillate if the capacitor is too small,
too large, or too good quality (low ESR), or in really bad shape (high ESR).

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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