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 [edited for brevity]
wart --+---+-----+
| |
| |
400 \ / 2SC2334 or TIP31
ohm / |/ NPN power TO-220
WW \-----| Heatsink optional
pot / |\
| E\
| |
| |
+----+----+---- + output
to load
- ----------------- -
Hmm, a complementary-darlington emitter-follower transistor would
be less sensitive to changes in load current:
: +6.0V, from wall wart
: --+--------+------+
: | | |
: | 220 | Q1 2sa1011
: | | |/E pnp TIP32
: | +----| etc
: | | |\
: 1.0k |/ |
: pot ---| Q2 |
: | |\E npn |
: | | |
: 7.5k +------+----- + output
: | to load
: - --+--------------------- -
Here the Q2 voltage drop is much more predictable, because it has a
relatively steady current set by Q1's base-emitter resistor, which
has about 0.55 to 0.75V across it despite output load changes. The
Q2 current is about 3 - 5mA and we expect a Vbe drop of about 700mV.
The lower end of the pot sits at about 5.3 volts.
The most serious problem with this circuit is its complete reliance
on a possibly-uncertain input voltage; the output will track any
changes in input.
analyzed Watson A. Name's circuit:
You almost had it- put the Vbe multiplier inside the feedback
loop and buffer like so:
6V ---+------+-------+ [edited for brevity]
| | |
| 22 c
| | |/ Q1
| +-----| TIP31
| | |\
== | e---+----+---- 5V
| c Q2 | |
| \| / |
| |------- \ |
| /| / ===
| e | 1000U
| | | |
GND --+------+-----------+----+---- GND
Watson A.Name - Watt Sun, Dark Remover asked:
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.
No, it's an ordinary regulator, using Q2's Vbe (roughly 0.7V) as a
voltage reference. The pot as shown would be a poor choice because
valid adjustments would only use a small part of the pot's range.
No doubt nospam knew that and was just simplifying his drawing by
using one part instead of three. This should work; the top of the
pot has about 0.82V when the regulator is calibrated for 5V out:
| | ---+---- 5V
| | |
=== | ,-2k2--+
| c Q2 | |
| \| 100R |
| |---- pot |
| /| | ===
| e 330 1000U
| | | |
GND --+------+---------+------+---- GND
This circuit has several problems. The tempco of Vbe is bad, but
the uncertain current through Q2 an even worse problem. This is
because the voltage difference between the +6V input and Q1's base
is small (and may vary widely), and Q1's base current is unknown.
If Q2's current changes by a factor of 10, its Vbe will change by
about 60mV, or nearly 10% of its value. As a result Q2 makes a
poor voltage reference in this circuit, even if manually adjusted.
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.
A big cap is fine, maybe even necessary in that location, because a
small output cap could make the feedback loop unstable. We discuss
this issue in AoE and call it brute-force compensation.
The current won't go backwards for several reasons. The wall wort
probably doesn't draw output terminal current when the AC is off.
If it wanted to, Q1's Veb breakdown would certainly be above 5V so
no current would flow anyway.
Jim Thompson wrote...
Try this...
+ From wall [edited for brevity]
wart --+---+-----+
| |
20 Q1 |
| / 2SC2334 or TIP31
| |/ NPN power TO-220
o------| Heatsink optional
| |\
| E\
| |
| o----- + output
| | to load
__|___/ 2.2k
/ /\ |
/ \-------o
/ \ |
-------- 2.7k
| TL430 |
| |
- ------o---------o------ -
You may need a compensation cap between cathode and control pin.
This is a much better approach, using TL430's 2.75V reference with
a built-in error amplifier. The more popular TL431 can be used with
its 2.50V ref if the feedback-sensing resistors are the same value.
But I'm not happy with the output emitter follower in the circuits
above, because the low Vin - Vout difference leads to such widely-
varying currents through the control element. Perhaps my circuit
can be combined with Jim's circuit:
: +6.0V, from wall wart
: --+------+------+
: | | |
: | 220 | Q1 2sa1011
: | | |/E pnp TIP32
: 750 +----| etc
: | | |\
: | |/ |
: +----| Q2 |
: | |\E npn |
: | | |
: | +---+--+---- + output
| | to load
__|___/ 2.7k
/ /\ |
/ \--------o
/ \ |
-------- 2.7k
| TL431 |
| |
- ----o----------o------- -
Thanks,
- Win
whill_at_picovolt-dot-com