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

I need a Voltage rail to track another rail with a fixed 5V drop between the two. ie if main rail is at 16V output at 11V, main rail drops to 15V output at 10V etc. Need to pull up to 500mA from this tracking output.

An obvious way would be a series power NPN transistor with 5V6 zener between collector and base and resistor from here to ground but I'd also like the tracked output to have some current protection in case the output gets shorted.

Is there a sneaky way to do this with standard 3 pin fixed or adjustable regulators that would provide both the fixed drop between the two and short protection?

Thanks

Reply to
The Hemulen
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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.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com
Reply to
Tim Wescott

the

ut

een

rt

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

George H.

Reply to
George Herold

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] | . | | | . | | | . ----+-------------------+-----+-------- . .

Reply to
bloggs.fredbloggs.fred

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

formatting link 

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com
Reply to
Tim Wescott

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com
Reply to
Tim Wescott

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

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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

Reply to
mroberds

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:

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

--> Vmain

                 |                   |
   |                   -
        ^ 6.25V
 - ref
             |  |     |          |
+--|I   O|----------|-----+---> Vmain-5 V
     |  |  A  |          |     |
  -----           |     |
          |     |
  |     |

== C

                 |                  [R]    |
         |                   |     |
 |                   |     |

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

Nice!

George H.

Reply to
George Herold

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

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

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com
Reply to
Tim Wescott

YIKES!

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

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

formatting link

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.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

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 | + | |____| + | | \

Reply to
mroberds

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

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
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

formatting link

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

Reply to
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

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

some

g Rail

F1

ing Rail

          === C2
          |
|
|
|

---------+

          |
          |
 |      /\_/

|      | |

     | |
  | |
        | |

--o Tracking Rail

        | |
        | |
     | |
    | |
      | |
|
    | |
    | |
|
      | |
      | |

---+ |

          |

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

Ground

--
Cheers, 
James Arthur
Reply to
dagmargoodboat

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

--
Failure does not prove something is impossible, failure simply 
indicates you are not using the right tools... 
nico@nctdevpuntnl (punt=.) 
--------------------------------------------------------------
Reply to
Nico Coesel

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