"Fred Bloggs" wrote in message
...
:
: 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 \-----| Heatsink optional
: 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 - http://garberstreet.netfirms.com
Email -
Remove - SPAM and X to contact me



---
This email ain't infected, dude!

Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.544 / Virus Database: 338 - Release Date: 11/26/03