Dropping 1V from a Regulated 6V Wall Wart

"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com... : : Fred Bloggs wrote: : >

: > Watson A.Name - Watt Sun, Dark Remover wrote: : >

: >> 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 \> pot / |\ : >> | E\ | | : >> | | : >> +----+-----+ : >> | | : >> +------> + 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. : >>

: >>

: >

: > 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 : >

: : 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

It's getting bigger. Soon he'll need a 2nd breadboard to build and test this thing. ;-)

Bill @ GarberStreet Enterprizez };-) Web Site -

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Reply to
Bill Garber
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True- I am thinking in terms of what Radio Shack has in stock- don't forget a big jumbo LED in there somewhere to indicate power applied:-)-and by their prices -this would be a $10 circuit-LOL.

Reply to
Fred Bloggs

Fred Bloggs wrote: [snip]

This one will make the output tempco essentially that of the zener which is not bad for the 5.1V's. Ripple rejection is lacking due to the low overhead -at 14dB, but your 6V is regulated anyway.

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

6V >---+--------+----------+-------------+ | | | | | / / | | 22 51 | | / / | | \ \ c | | | |/ | +----------|-----------| TIP31 | | | |\ | | | e----+--> 5V | c | | | \| | pnp | | npn |--+-----|-------- c e---+ | /| | | \ / | | e | | ---- | === | | | 1N4148 | | | | | +----|>|----+ | | | | | | | | | | | | | | | / _/ / === | | 1K /^ 5.1v 1.2K 1000U | | / - / | | | \ | \ | | | | | | | GND>--+--------+----+-----+-----------+------+--> GND
Reply to
Fred Bloggs

Ah! So, you mean R2 = 5*R1 or 5.R1 in K.A. notation.

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Mike
Reply to
Active8

That's a diskette. A floppy disk has no hub. The old joke originated way back then.

I (oops! can't read again) went to google's S.E.D. and searched on Magnet lifting floppy. It returned

soc.sexuality.spanking

as a related site (?!)

"You see, Timmy (usee2me), when Mommy and Daddy..."

"You're weird, Mom! I don't like spankings."

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Mike
Reply to
Active8

I've tried to do this on numerous occasions. If you need any amount of current at all, the big lump of stuff makes it too unweildy, even if you do bite the bullet and put in a complex circuit that works.

Now, for regulated warts, I crack the case and shunt a resistor inside the wall wart. You can add a switch if you have multiple applications. If you need both voltages at once, it's still easier to use two warts than to have this lump of regulator stuff hanging about.

I'm a little confused by your requirements. If you even considered a diode, you can't be too concerned about the actual voltage...but you're concerned about the .2V variability. Sounds like there's something in there that's important but not disclosed??? mike

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Reply to
mike

In article , snipped-for-privacy@example.invalid mentioned...

Well, the output of the wall wart is already regulated to 6V at 200 mA. But 1V at 200 mA is going to require a 5 ohm resistor. And the zener will have to dissipate a lot of wasted power. And the wall wart may not like being run at its max and overheat. Using the 1 amp rectifier in series with the output dropped about .6 to .8V, depending on current, which isn't all that bad a regulation. But the circuit I drew above gives a voltage closer to 5V over a wider range of load currents. I suppose I could've used a LDO 5V regulator, but then the output's already regulated.

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Reply to
Watson A.Name - "Watt Sun, Dar

In article , snipped-for-privacy@austininstruments.com mentioned...

Thanks, but I think someone snipped the part where I said that I was trying to get this to fit across the pins of the RF modulator, and that the wall wart was already regulated. I guess I could do it if it was surface mount parts, but I'm not experienced with those. What kind of opamp works down to only 6V? A LM358? In order to put out 5V at the emitter, it would have to swing the base to 5.7 or more volts at 200 mA, so the opamp would have to swing close to the positive rail. Maybe a better idea would be to use a PNP for the emitter follower.

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Watson A.Name - "Watt Sun, Dar

In article , snipped-for-privacy@earthlink.net mentioned...

Man, what did I start? Buncha pedants.

BTW, I scrapped a couple old half gig hard drives, and took out the magnets. Those things are powerfull little devils! Made out of Nicodemus or some other exotic sounding materials. ;-) But I held the magnet to the platter and it literally won't even attract the plated coating on the aluminum. I guess it's so thin that it's like it's not even there!

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Reply to
Watson A.Name - "Watt Sun, Dar

In article , snipped-for-privacy@earthlink.net mentioned...

I think the emitter to base voltage will be something like .6 to .75V, and this has to be subtracted from the 1V diff, leaving something like .4 to .25V. This very low voltage is what you have to work with when coming up with the resistor value to limit the current to the zener.

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Watson A.Name - "Watt Sun, Dar

In article , snipped-for-privacy@nospam.com mentioned...

I'll toss one together, but what value should I use for the Pot? 10k? Lower?

This looks a lot like the current limiter circuit, 'cept for the pot. I'm wondering if it's such a good idea to have such a large cap on the output. When the wall wart is powered off, all that current tries to go backwards thru the pass transistor.

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Reply to
Watson A.Name - "Watt Sun, Dar

can't find that spec anywhere.

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Mike
Reply to
Active8

Nyuk, nyuk, nyuk.

LOL i love it! Neodymium or Neodium (either a misspell or a TM). Good one, man.

Nostrildumbass. I love playing with woids.

Bone swar, moan sewer.

Yup. I read an article about IBM developing a new process where it's so excruciatingly absoposifriggintutely thin that it doubles the density. They should've been out by now. Another article/AD (EDN or something) described a process to photoetch(?) the R/W heads, too. I think it was a matrix of heads.

BTW. I said I was "thinking" series pass reg b4. I think I've never seen one drop 6V to 5V, either. I figgered since you already had a trans and a hulking ww pot and were thinking zener...

Now you're talking about size and RF modulator which you never mentioned. What gives, Watson?

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Mike
Reply to
Active8

Rough guess...

---+------------------+ | | | 10 | | ___ |/ +---|___|---+----| | |>

| | - | 5.6 500mW ^ +--------- | -------------+----------------

I kinda like JT's deal with the programmable zener which ain't much different.

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Mike
Reply to
Active8

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Yes, but the 5V output isn't at low currents, as you've already stated.

The LDO will make that problem go away.
Reply to
John Fields

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Closer guess:

Ib ~ Ie/ß = 200mA/100 = 2mA
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Reply to
John Fields

In article , snipped-for-privacy@austininstruments.com mentioned...

Yeah, I was thinking something really low like that.

But these approaches have been to establish a new regulated voltage, dependent on the value of the zener. There's nothing wrong with this, but it would be simpler to just use a LDO 5V regulator as someone suggested.

I figured that since the 6V input was already well regulated, that it would be easy to just drop a single volt. My subject: line kind of said that. And if I use a 1N4002 inline with the input, it gives me

5.4 to 5.2V or so, over most of the current range, which is not all that bad. And I did say I wanted to keep it simple. It's not that I'm unappreciative of all the input others have given, it's been a good learning experience seeing these neat little circuits. I've saved many of them to disk, to experiment with later. So I thank all who contributed.
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Watson A.Name - "Watt Sun, Dar

Ok, I incorrectly guessed Beta (Chaos Master told me how to get special characters and it doesn't work here) of 200 and set Iz at

40mA (a waste) to get the 500mW zener to operate at about half it's rating. I know, I only needed 10mA if Ib really were 10mA.
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Mike
Reply to
Active8

I did this a while back. Replaced the 7806 in the wall wart with a 7805. Wade

Reply to
Wade Hassler

I'll trade a pot for a zener and R anyday, but I haven't looked at what this thing will do over the full current range. It's so close to what you started with, I thought it would be a snap to check out.

I'm just glad to find an electronics post or 2 lately. Lots of long OT stuff I'm guilty! I feed it, but it's fun.

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Mike
Reply to
Active8

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