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