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 + 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.
snipped-for-privacy@nospam.com analyzed Watson A. Name's circuit:
Watson A.Name - Watt Sun, Dark Remover asked:
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:
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.
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 w>
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
Thanks, - Win
whill_at_picovolt-dot-com