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Dropping 1V from a Regulated 6V Wall Wart

I needed to drop 1V from a 6VDC 200 mA regulated wall wart, so I tried a 3 amp rectifier, but it varied by more than .2V over a range of loads. So I tried this: (view with courier font)

  • From wall wart >--+ | +---+------+ | | | |
400 \ / 2SC2334 or TIP31 ohm / |/ NPN power TO-220 WW \ + output to load

- >--------------> - From wall wart

This has some advantages and disadvantages. It's simple and cheap, and keeps the output at 5V within a tenth of a volt over a current range. But it has a minimum current below which it loses regulation and the output starts to go up to 6V, because the transistor is not conducting and the current is being supplied thru the ww pot. This circuit is sometimes used in the bias circuit for the output transistors in high powered amplifiers. Also Win Hill showed us here how to use a similar circuit to maintain the voltage steady for a current regulator circuit used on four AA cell rechargeable batteries.

I'm thinking about putting a 5.1V zener on the output so that if the voltage climbs above that, it just shunts the excess current. Oh, yeah, I set the pot to various values to see what the output voltage was with various loads. The two resistances were 120 ohms for the upper and 280 ohms for the lower. I suppose the 400 ohms total could be raised to a higher value, but the transistor needs enough base current to do its job. There's only 1V available minus the .6V E-B voltage, so even at 400 ohms, that's not a lot of current.

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Reply to
Watson A.Name - "Watt Sun, Dar
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Or just hang a dummy load on the output.. 1K0 or a bit higher (

Reply to
Spehro Pefhany

Why not a 5volt zener and a resistor?....Just a thought...

Reply to
Ross Mac

Try this...

  • From wall wart >--+ | +---+------+ | | \ | / | 20 \ / 2SC2334 or TIP31 ohm | |/ NPN power TO-220 o------| Heatsink optional | |\ | E\ | | | | | | | | | | | o-----> + | | output | / to load | \ | / 2.2K __|___/ | / /\ | / \--------o / \ | -------- | | TL430 / | \ 2.7K | / | | | | | |

- >--o----------o------> -

You may need a compensation cap between cathode and control pin.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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Reply to
Jim Thompson

...

6VDC>------+-----------------+ | | [R1] | | | +-----|+\ C | | >------B | +--| / E | | | | +----+---------+ | |+ | [R2] [C1] [RL] | | | GND>-------+-------+---------+

R1/R2 = 5

C1 = Whatever you need to keep the thing from oscillating; 100µF is probably a good guess.

--
John Fields
Reply to
John Fields 

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R2/R1 = 5
Reply to
John Fields

I read in sci.electronics.design that John Fields wrote (in ) about 'Dropping 1V from a Regulated 6V Wall Wart', on Mon, 29 Dec 2003:

Not always. (;-)

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Reply to
John Woodgate 

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I'm sure there's a joke in there somewhere... :-)

I meant that for 6V in and 5V into Rl,  R2 = 5R1.
Reply to
John Fields

I read in sci.electronics.design that John Fields wrote (in ) about 'Dropping 1V from a Regulated 6V Wall Wart', on Mon, 29 Dec 2003:

OK, let's go on in the same vein. R2 is not always 5R1; it might be 5R6 or even 47K. (;-)

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Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk 
Interested in professional sound reinforcement and distribution? Then go to 
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PLEASE do NOT copy news posts to me by E-MAIL!
Reply to
John Woodgate

I'm thinking 4.3V Zener and a series pass transistor.

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Best Regards,
Mike
Reply to
Active8

Wrong way.

Best regards, Spehro Pefhany

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Reply to
Spehro Pefhany

Why?

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Best Regards,
Mike
Reply to
Active8

NEver mind. I tried to cancel this but Gravity thinks I didn't post this.

Still at TARFU level, I guess.

OK 5.6V zener, solly.

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Best Regards,
Mike
Reply to
Active8

But Mr. Wizard, this *package* says 502 and this one's on psychedelics.

Why won't this magnet pick up this floppy disk?

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Best Regards,
Mike
Reply to
Active8

OK, it arteried: "In order to assure a voltage of 5VDC across Rl the resistance of R2 should be five times the resistance of R1." ?^)

--
John Fields
Reply to
John Fields

You almost had it- put the Vbe multiplier inside the feedback loop and buffer like so:

Please view in a fixed-width font such as Courier.

6V >---+------+---------+ | | | | / | | 22 | | / | | \ c | | |/ | +-------| TIP31 | | |\ | | e----+--> 5V | | | === | +----+ | c | | | \| / | | |-----> \ | | /| / === | e \ 1000U | | | | | | | | | | | | GND>--+------+---------+----+--> GND
Reply to
Fred Bloggs

[snip]

See also "4V-Regulator.pdf" on the S.E.D/Schematics Page of my website, for a similar application.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson 

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It will, Timmy, you just have to know how to do it.

For a clue, check out "Magnet lifting floppy" on a.b.s.e.
Reply to
John Fields

Then you can work a common 5.1V zener into the equation like so: Use a 2N3906 for the pnp for Vbe,max=6V rating.

Please view in a fixed-width font such as Courier.

6V >---+--------+----------+-------------+ | | | | | / / | | 22 51 | | / / | | \ \ c | | | |/ | +----------|-----------| TIP31 | | | |\ | | | e----+--> 5V | c | | | \| | pnp | | npn |--+-----|-------- c e---+ | /| | | \ / | | e | | ---- | === | | | 180 | | | | | +----/\/\---+ | | | | | | | | | | | | | | | / _/ / === | | 1K /^ 5.1v 1.2K 1000U | | / - / | | | \ | \ | | | | | | | GND>--+--------+----+-----+-----------+------+--> GND
Reply to
Fred Bloggs

"Fred Bloggs" schreef in bericht news: snipped-for-privacy@nospam.com...

[snip]

Now you're getting a bit too enthusiastic ;-) With all these parts, I'd prefer a LM2931 for 27 cents.

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Reply to
Frank Bemelman

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