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'[EE] 15 V from 7812 ?'
2003\03\07@074338 by

Is there a way to get regulated 15 V from a 7812 12 V regulator ?

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> Is there a way to get regulated 15 V from a 7812 12 V regulator ?

You would be better off with an LM317 which is designed to be used like this
BUT.

- Place a resistor R1 of 4 units resistance from output to 7805 ground
terminal.
- Place a resistor R2 of 1units resistance from 7805 ground terminal to
ground proper.

R1 will drop 12 volts.
R2 being 1/4 the size with APPROXIMATELY the same current in it will drop 3
volts.

As the 7805 passes current from Vin to its ground pin to power itself and
this will also flow through R2, the output voltage will be somewhat higher
than expected. Adjust R1 to suit.

Voltage stability will be better with an LM317 which powers itself from its
Vin to Vout drop.

I gave resistor ratios above and not actual values so you can play. Too high
and regulation gets bad. Too low and wasted current is too high.

Use an LM317 :-)
See its datasheet to get formula for resistors to use. (As above system but
Vout basic is much lower).

The diode trick that someone else mentioned will also work - it has good and
bad points relative to resistor solution.
Also could use a 2V7 zener.

Russell McMahon

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Hej James. Tack för ditt meddelande 05:32 2003-03-08 enligt nedan:

>Is there a way to get regulated 15 V from a 7812 12 V regulator ?

Put a 3V zener in series with 7812 GND leg; that will "rise the internal reference point 3 volts" so to speak.
I usually add a 100n cap parallel to the zener too.

Another idea is to use a LED (in forward mode) with suitable forward drop instead if the zener.  Green led is about 3V Uf. Then you get an "ON" indicator for free :).

Another alternative is to use a resistive divider; one R from 78xx out to 78xx GND, another from 78xx GND to real GND (and a 100 n cap in parallel), but that is more dependable of the 78xx quiscent GND current.

/Morgan

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Morgan Olsson, Kivik, Sweden

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At 08:32 PM 3/7/2003 -0800, you wrote:
>Is there a way to get regulated 15 V from a 7812 12 V regulator ?

The easiest way is to trade it for a 7815. You could also put a
zener (e.g. 2.7V or 3.3V) from the "GND" pin to ground.

Best regards,

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

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2003\03\07@094015 by
sure, just put a 3 volt zener between ground and its common pin.

James wrote:
>
> Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
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Naaah.  That is too easy. I think I remember an app note from National on
using a 3 terminal linear regulator as the heart of a switching buck
regulator.  As long as we are going to Rube Goldberg this we should do it
all the way!

-- Lawrence Lile

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03/07/2003 07:20 AM
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To:     PICLISTMITVMA.MIT.EDU
cc:
Subject:        Re: [EE] 15 V from 7812 ?

> Is there a way to get regulated 15 V from a 7812 12 V regulator ?

You would be better off with an LM317 which is designed to be used like
this
BUT.

- Place a resistor R1 of 4 units resistance from output to 7805 ground
terminal.
- Place a resistor R2 of 1units resistance from 7805 ground terminal to
ground proper.

R1 will drop 12 volts.
R2 being 1/4 the size with APPROXIMATELY the same current in it will drop
3
volts.

As the 7805 passes current from Vin to its ground pin to power itself and
this will also flow through R2, the output voltage will be somewhat higher
than expected. Adjust R1 to suit.

Voltage stability will be better with an LM317 which powers itself from
its
Vin to Vout drop.

I gave resistor ratios above and not actual values so you can play. Too
high
and regulation gets bad. Too low and wasted current is too high.

Use an LM317 :-)
See its datasheet to get formula for resistors to use. (As above system
but
Vout basic is much lower).

The diode trick that someone else mentioned will also work - it has good
and
bad points relative to resistor solution.
Also could use a 2V7 zener.

Russell McMahon

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On Fri, Mar 07, 2003 at 08:32:34PM -0800, James wrote:
> Is there a way to get regulated 15 V from a 7812 12 V regulator ?

All 78XX regulators are designed to keep their regulated voltage between the
Vout and GND (ADJ) pins. So putting a voltage divider from Vout to GND so
that the midpoint is 3V and attaching that node to the ADJ pin will do what
you want.

Note that the LM317 is little more than a standard 1.2V regulator.

BAJ

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I tried this with a 7805 with no success.
I tried various diodes, resistors & zeners. Some worked better than others
but all were poorly regulated.
Please let me know if you are successful, it would be a handy gimmick.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"

----- Original Message -----
From: "James" <asiactTM.NET.MY>
To: <PICLISTMITVMA.MIT.EDU>
Sent: Friday, March 07, 2003 11:32 PM
Subject: [EE] 15 V from 7812 ?

> Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
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At 11:10 AM 3/7/2003 -0500, you wrote:
>On Fri, Mar 07, 2003 at 08:32:34PM -0800, James wrote:
> > Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
>All 78XX regulators are designed to keep their regulated voltage between the
>Vout and GND (ADJ) pins. So putting a voltage divider from Vout to GND so
>that the midpoint is 3V and attaching that node to the ADJ pin will do what
>you want.

That's rather a bad idea because the current from the GND pin is large
(several mA) and quite variable.

It is, however, constant *enough* that the drop across a zener or LED or
diode will be fairly constant. But not a resistor.

>Note that the LM317 is little more than a standard 1.2V regulator.

The current out of the ADJ pin is only uA. In place of that, you must
draw at least a few mA from the output pin, which is often done by
the divider. So, it's quite different from a 78xx regulator. Try looking
at the schematics sometime if you want to see the differences- the LM317
is quite a bit more complex.

Best regards,

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

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James wrote:
> Is there a way to get regulated 15 V from a 7812 12 V regulator ?

Even that you can hear several suggestions for this problem, the best is to
use a 7815, since it is available at the market, and prices are cheapper
than the probably 3V zenner someone would recommend.

Use the right tool for the right job, even that for a learning experience
you can try resistor voltage dividers, diodes, zeners, etc.

This kind of voltage "adapt" with 78XX regulators is used when you need a
voltage not available directly from those kind of devices.  But for 5, 6,
9, 12, 15V, etc, nothing better than use the correct regulator.  What other
simply and easy solution you may have for less than 1 dollar?

Wagner.

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Spehro Pefhany wrote:
>
> At 08:32 PM 3/7/2003 -0800, you wrote:
> >Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
> The easiest way is to trade it for a 7815. You could also put a
> zener (e.g. 2.7V or 3.3V) from the "GND" pin to ground.

How stable is the quiescent current through the
ground pin throughout the target current range?
If it stays close to 5mA through the whole range
it *may* be possible just to use a resistor to
lift the ground pin 3v above ground and stay
within a couple % of 15v regulation.
-Roman

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> The diode trick that someone else mentioned will also work -
> it has good and bad points relative to resistor solution

> Also could use a 2V7 zener.

You need to add a resistor from the regulator o/p to give the zener
a few mA. R goes from reg o/p to ground pin and zener from ground
pin to ground. Value of R depends on how much current your zener
needs and the Vdiff between the zener and final reg o/p V.Typically
it would be a few hundred ohms to get ~ 4 to 5mA

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part 1 458 bytes content-type:text/plain; x-avg-checked=avg-ok-5AD55D81; charset=us-ascii; format=flowed (unknown type 8bit not decoded)

Yeah, drop in a 7815 in place of the 7812, simplest solution.

John

At 2003/03/07 20:32 -0800, you wrote:

>Is there a way to get regulated 15 V from a 7812 12 V regulator ?
>
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part 2 2 bytes
-
At 05:10 AM 3/8/2003 +1100, you wrote:
>Spehro Pefhany wrote:
> >
> > At 08:32 PM 3/7/2003 -0800, you wrote:
> > >Is there a way to get regulated 15 V from a 7812 12 V regulator ?
> >
> > The easiest way is to trade it for a 7815. You could also put a
> > zener (e.g. 2.7V or 3.3V) from the "GND" pin to ground.
>
>
>How stable is the quiescent current through the
>ground pin throughout the target current range?
>If it stays close to 5mA through the whole range
>it *may* be possible just to use a resistor to
>lift the ground pin 3v above ground and stay
>within a couple % of 15v regulation.

Not very. It's fairly stable with temperature, but it typically changes
by over 10% from input voltage, which means it degrades the regulation
by quite a large factor, even with only 20% of the voltage across the
resistors. It's also quite different worse-case from unit to unit.
You want to select resistors or put a pot in there, and get lousy
regulation?

There might (if you are doing analog-ish stuff) be a good reason to
use a zener (or LED) + a 7805 though- suppose you need a pseudo-ground and
a -3V / +12V supply, with +15V at high current relative to the -3V.
I used a similar circuit in a precision temperature control (actually it
was an active circuit,not a zener) which we made over 30,000 of.

Best regards,

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

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