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'[EE]: Power Supply Advice'
2002\09\18@111323 by Robert E. Griffith

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My device has two display options – one a VFD that sucks down the current
and the other an LCD which is a tea toddler.

I am providing 2 options on the PCB – one a LM1117-5.0 linear regulator
requiring a 9V unregulated wall wart and the other is no regulator requiring
an external regulated 5V@2A <5V@2A>  switching PS.  I will use
different size connectors for each option so that I can’t get them mixed up
and use the wrong external tranformer / PS on a particular unit.

It got me thinking, though.  What if a costumer grabs a wall transformer
they have lying around from a different product that happens to have the
same connector?  For the unit with the linear regulator on board I can place
a diode in series with the power connector to protect it against reverse
voltages.  Also, the linear reg would take the heat if a higher voltage wall
wart were plugged in.  If they plug in a voltage higher than the linear reg’
s limit, well then they will just have to buy a new unit:)

When using the external switching PS, it seems harder to prototect against
misuse.  I am thinking zener diode in parallel to the power connector would
protect against over voltage.  I see a 5.1V zener at digikey that is
available in 1W, 2W, and 5W.  It seems like it would be a show down between
the zener and the PS to see which is more buf.  I guess the zener would draw
whatever current is required to pull the voltage down to 5.1V.  If that is
more than the zener’s rating it will eventually fail.  When a zener fails,
would it short or become open?  If the zener does not fail, the unit would
still work, so the user would not know that anything is wrong but the wall
transformer would be working past its limits and probably get pretty warm.

Alos, a series diode (to protect against reverse voltage) would be a problem
for the PCB that uses an external regulated 5V PS since the diode voltage
drop would decrease the supply voltage too far below 5V.  So is there
another way to protect against a reverse polarity wall wart being plugged
in?

Thanks for considering these issues for me.

--BobG

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2002\09\18@112408 by llile

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Here are my thoughts:


One option is to use higher voltage, semi-regulated power in the external power supplies, run it through a diode bridge to guard against reverse polarity, and then re-regulate it with a small low dropout regulator on your board.  This guards against idiots with reverse polarity wall warts,and  higher voltage wall warts.  I tend to think EMI generating power transformers belong somewhere not near my board, and power supplies belong somewhere close.
What about this design:  9V in, through a fuse, with a 12V zener on the input, then a linear regulator.  If somebody plugs in a 12V wall wart either polarity, it blows the fuse.  If a wall warrt is reverse, it blows the fuse.
You could probably use the fuse/zener trick with your external regulated power supplies as well.  Even a teeny 500mW zener would probably blow a 1 amp fuse without destruction.


-- Lawrence Lile





"Robert E. Griffith" <.....bobKILLspamspam@spam@JUNGA.COM>
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09/18/02 10:10 AM
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       cc:         Subject:        [EE]: Power Supply Advice


My device has two display options ? one a VFD that sucks down the current
and the other an LCD which is a tea toddler.

I am providing 2 options on the PCB ? one a LM1117-5.0 linear regulator
requiring a 9V unregulated wall wart and the other is no regulator requiring
an external regulated 5V@2A <5V@2A>  switching PS.  I will use
different size connectors for each option so that I can't get them mixed up
and use the wrong external tranformer / PS on a particular unit.

It got me thinking, though.  What if a costumer grabs a wall transformer
they have lying around from a different product that happens to have the
same connector?  For the unit with the linear regulator on board I can place
a diode in series with the power connector to protect it against reverse
voltages.  Also, the linear reg would take the heat if a higher voltage wall
wart were plugged in.  If they plug in a voltage higher than the linear reg'
s limit, well then they will just have to buy a new unit:)

When using the external switching PS, it seems harder to prototect against
misuse.  I am thinking zener diode in parallel to the power connector would
protect against over voltage.  I see a 5.1V zener at digikey that is
available in 1W, 2W, and 5W.  It seems like it would be a show down between
the zener and the PS to see which is more buf.  I guess the zener would draw
whatever current is required to pull the voltage down to 5.1V.  If that is
more than the zener's rating it will eventually fail.  When a zener fails,
would it short or become open?  If the zener does not fail, the unit would
still work, so the user would not know that anything is wrong but the wall
transformer would be working past its limits and probably get pretty warm.

Alos, a series diode (to protect against reverse voltage) would be a problem
for the PCB that uses an external regulated 5V PS since the diode voltage
drop would decrease the supply voltage too far below 5V.  So is there
another way to protect against a reverse polarity wall wart being plugged
in?

Thanks for considering these issues for me.

--BobG

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2002\09\18@132342 by Roman Black

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llile@SALTONUSA.COM wrote:

> One option is to use higher voltage, semi-regulated power in the external
> power supplies, run it through a diode bridge to guard against reverse
> polarity, and then re-regulate it with a small low dropout regulator on
> your board.  This guards against idiots with reverse polarity wall
> warts,and  higher voltage wall warts.  I tend to think EMI generating
> power transformers belong somewhere not near my board, and power supplies
> belong somewhere close.


Rather than use 2 plugs (and two alternatives for
wall warts) and all sorts of protection components,
what about putting a bridge rectifier after one
input plug then a simple 3 transistor boost regulator
(or IC boost reg) that will produce your 5v @ 2A from
ANY input voltage from about 3v to 20v, even reversed
polarity or AC!

It will work with any plugpack and that is your
safety factor so you save the cost of a plug and
all the other safety components. And gain something
in portability. :o)
-Roman

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2002\09\18@132817 by Francisco Ares

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Robert E. Griffith wrote:

>...
>
>When using the external switching PS, it seems harder to prototect against
>misuse.  I am thinking zener diode in parallel to the power connector would
>protect against over voltage.
>
Include a series fuse, so it will burn when the zener starts conducting.

Never done that, just thinking it might work like a crowbar.

Francisco

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2002\09\18@160621 by Robert E. Griffith

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This would be a cool solution, but I am afraid of using a boost regulator
because I would like to get this board FCC class B certified and I think it
is more likely not to pass without revisions if I have a switcher on board.
I may be being paranoid out of ignorance.

--BobG

{Original Message removed}

2002\09\18@161049 by Robert E. Griffith

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I like the idea of the slightly higher semi-regulated PS but did not find
anything to fit the bill at digikey or mouser.  In the end, my customers
might have to show a little common sense:)  At least some of the scenarios
are easy to account for.  The fuse is a good idea.

--BobG

{Original Message removed}

2002\09\18@164220 by Gordon Varney

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Bob,
       Why not use an AC bridge diode with a cap. Then it would not matter
if the customer used an AC transformer, or a DC and connected it in
reverse polarity.  No reverse polarity protection needed.

With the right regulator, the customer could hook up 8Vdc - 30 Vdc or
AC and it would not matter. (did not know your current draw) Most
LDO's and good linear regulators have a thermal protection circuit,
Use Just enough heat sinking to protect your equipment and run cool
under the correct wall wart. Let the regulator go thermal if a wall
wart with to much voltage is used. or just monitor the heat sink with
a thermister.

Lawrence is correct, use a slightly higher voltage and an LDO is the
best option. Never leave it up to the customer to use common sense
(HA!! that's an oxymoron) Protect your equipment at all cost.  Fuses
are good, they are my friend. :-)

Gordon Varney
http://www.talk2it.com




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2002\09\19@005524 by Jim

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   "When using the external switching PS, it seems
    harder to prototect against misuse."

I have a somewhat technological solution to propose
as opposed to the usual set of choices including shunt
Zeners, serial diodes to protect polarity and the like -

- this solution involves the minimal amount of circuitry
which to implement a "input power sense and qualify or
screening function" and would "test" the applied power
before allowing the application circuitry to be "switched
on" under the watchful eyes and supervision of the "Power
Control Circuit".

Bascially, consider this "power sense" circuitry
would contain these functions:

1) a 5 V comparator (this would sense a proper DC power
  input in the range of between 4.9 and 5.1 volts or so)
2) a relay and driver (this actually applies power to the
  application circuit once the DC input power has been
  deemed it be safe/in range)
3) power-switch input request interface; ON and OFF (this
  would be means by which a "request" to apply power to
  the application circuit is made by the operator)
4) simple logic to orchestrate this combination of inputs and
  outputs (very simple logic - using diode OR gates, simple
  series transistor AND gates, along the lines of simple
  relay logic. It could even be 4000 sereis CMOS for wide
  DC range input and low/near zero standby current draw.)
5) It's own crude wide-range power supply (simple series diode
  to a cap/bleed resistor with a clamp-zener to prevent over-volt
  damage via application of incorrect DC input)
6) Optional alert/warning indicators (LEDs) for display to the
  operator of incorrect applied DC input power.

RF Jim

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