Track another Voltage rail with fixed 5V drop @500mA question

Are you drawing current from your (supply-5) to ground, or are you drawing current from supply to your (supply-5)?

If it's the latter, then use a negative linear regulator. If it's the former then look at the three-terminal precision shunt regulators and see if you can get creative -- some of those can be used with a transistor in the loop to give you better current handling, although you'd still have the shorted-output issue to contend with.

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You could hang a negative three pin regulator 'upside down' off the top rail, but that will only sink current. (I think)

George H.

Something cheap, the 6.25 could be 6.2V zener or precision shunt IC: Please view in a fixed-width font such as Courier.

. . . MAIN RAIL >--+-------------------+---------> Vmain . | | . | | . | - . | ^ 6.25V . | 317 - ref . | ----- | . | | | | . +--|I O|----------|-----+---> Vmain-5 V . | | A | | | . | ----- | | . | | | | . | | | | . === -------------+ === C . | | | . | [R] | . | | | . | | | . ----+-------------------+-----+-------- . .

This should work if you're pulling current and if the regulation doesn't have to be precise. The voltage at the output will be zener + baseR drop

• BE drop. R1 and R2 are there for coarse current limiting -- the current will go up to a couple of amps in the event of a short circuit, but the transistor won't be trying to flow an ocean of current at least.

Vsup o-------------------o---------------. | | | | | .-. z | | R1 A 4.3V | | 10 | '-' | | | | | | | R2 | | 100 | | ___ |/ '---|___|-----| |> | '-----o Vdropped

(created by AACircuit v1.28.6 beta 04/19/05

Great! But to make life easier, just use a TL431 instead of the zener. ...Jim Thompson

D'oh. I tried to come up with something like that and it completely evaded me. It's obvious now that I see it, of course.

Gads! Who introduced the variable-width font ?:-(

I've recently decided an additional "test" during interviews... resume in Helvetica... toss ;-) ...Jim Thompson

Main Rail o---+---------------------------------o Main Rail | +--->|--->|--->|--->|--->|---~~---o Tracking Rail D1 D2 D3 D4 D5 F1

Ground o-------------------------------------o Ground

D1-D5: 1N4001 F1: about 1 A; fast or slow blow to taste

F1 is the expensive part. :)

Matt Roberds

On Apr 11, 1:31 pm, snipped-for-privacy@gmail.com wrot e:

rail with a fixed 5V drop between the

, the 6.25 could be 6.2V zener or precision shunt IC:

--> Vmain

== C

--------------+-----+--------

Nice!

George H.

Ooooooh! That is SO accurate! Are you trying to emulate John "White Trash" Larkin for crappiest circuit "design" ?:-} ...Jim Thompson

It's not variable-width. It appears to be an "intelligent" rearrangement of linefeeds, courtesy of Pan.

YIKES!

You can stack voltage regulators, like the 1.2 and 2.5 volt supplies here:

You could for example stack a 7805 on top of a lower voltage rail and they would track.

LM1117 seems bulletproof in situations like this, where the ADJ pin could get driven strangely in current-limit situations.

I like to drive LM1117 ADJ pins from opamps, too, to make them into self-protecting power boosters.

It meets all the stated requirements of the original poster. If there are additional requirements, then the above design may not be appropriate.

For lower noise:

Main Rail o---+-------------------------------------o Main Rail | +--->|--->|--->|--->|--->|---~~---+---o Tracking Rail + | D1 D2 D3 D4 D5 F1 + | C1 === === C2 | | Ground o---+---------------------------------+--o Ground

D1-D5: 1N4001 F1: about 1 A; fast or slow blow to taste C1-C2: 100 uF. 25 V or more depending on main rail voltage

For better accuracy:

Main Rail o---+-----------------------+----------------o Main Rail | | | | | | | +----------------+ | ____ | | | | | | | | | +-----+---| U1 |---+-----+ / R5 / \ M1 | + | |____| + | | \

Crikey, another entrant to join the tag-team of Larkin/Ian/krw >:-} ...Jim Thompson

MAIN buss 5..20 Volts +---------+------++ | .-. | | | | | |10 + '-' z | 4.8V A | + |/ +----+| 5 volt drop | |> + +------------------+ .-. | | | .-. 100k | | | | '-' | |30 output LOAD | '-' | | === === GND GND (created by AACircuit v1.28.6 beta 04/19/05

Just an idea..

This is simple and if you are not looking for supper precision this should do. You can use a post regulator or something like it ofcourse and lift the common leg with a zener.

I guess it all depends on real-estate and your needs.

Jamie

Pffbt! And this from the guy who approves of hFE bias in production? Come on, it's even got damping for those pesky emitter follower mode oscillations! ;-)

Tim

some

g Rail

F1

ing Rail

---------+

|      | |

--o Tracking Rail

---+ |

/ / / / / / / / / / / / / / .----. / .----. / | |/ / | |/ / '\ /' / / '\ /' / / ____/ | | / / /___________________/ | | / / /_____ / / | |/ / / / | |/ / / /| .---' '---/ / .---' '---/ / / | ___| |/____________________| |/_____/ | '---------' '---------' | ' Main rail Tracking rail | / ____________________________________________________|/

Ground

A TCA0372 (high power, thermal and short circuit protected opamp), a shunt regulator and some resistors.