Decided to have a quick trial of a lower power version of the simple regulator, taking note of some of Dave Dilatush's comments. I decided to try a nominal 12 volt in, 5 volt out 5 mA unit as suggested by Roman and Dave. I don't see too much point to such a low power limited drop design EXCEPT for battery powered applications where long battery life is desirable. In such cases I suspect that a "proper" controller would be desirable due to the extra efficiency probably achieved by the better attention to proper design which can be given by using such an IC. Still interesting though. I also tried the circuit at 50 ma output and ran Vin over 5 to 30 volts. . The circuit did NOT scale simply and easily down to a very low power low voltage design. With some playing (and design :-) ) it will work but certain care needs to be taken. Used BC337 & BC327 transistors as required. 1. Dave notes the low zener current is undesirable. I decided to try eliminating the zener. I instead divided the output voltage down to 0.6v and used this to turn on the "turn off" transistor, I used a diode to drive these two resistors so that I could also add a hold off capacitor at this point. ____________________Vout__________ | _ V Diode -- |----------- R | ----- R 1 === C \| R | Q /|--------- | | V R | | R2 | | R | | | | ----------------------------------- Vout rises above Vdesign. R1/R2 divide Vout-Vdiode so (Vout-0.6) * R1/ (R1+R2) = 0.6v to turn on transistor Q. Capacitor C retains Vout when Vout falls with time constant approx = C * R1 This works! It eliminates the zener and replaces it with the transistor Vbe - arguably out of the frying pan and into the fire. C gives the ability to increase turn off time and therefore circuit "hysteresis" / ripple voltage which as Dave notes is unusually derived in this circuit. I used C = 0.1 uF, R1 = 100K, R2 = 470K, Q = BC337. 2. Ripple voltage is affected by Cout. Making this large will reduce ripple and the circuit may not oscillated unless ripple is introduced by other means eg C in a above diagram. A more formal hysteresis method will, as dave noted, make the designers life easier but it is certainly possible to make this work well. here. 3. The "speedup" capacitor used by Dave in the two transistor circuit makes a substantial difference here. (across drive resistor to high side transistor base). I again used a 0.1 uF here. 4. I used a medium sized toroid of "forgotten" inductance. I also tried a small 2 mH potted inductor rated at 30 mA as suggested by Roman. This worked but produced substantially inferior efficiencies (as expected) 5 Swapping from a 1N4148 catch diode to a 40 volt 1A Schottky (used because to hand) produced a very significant improvement in efficiency. A BYV28 was worse than the Schottky but better than the 1N4148. I didn't end up with a good enough efficiency to be worth writing up in detail. Will play some more in due course although it may be a while.Typically was getting 65% odd at 20 volts at 50 mA. Best efficiencies were BELOW those achieved using the FET circuit yesterday. Turning the circuit into a linear regulator was not hard :-) - eg use largish Cout, no or very small speedup capacitor or feedback capacitor. However, making a design which oscillated across the full range was not hard. The circuit is certainly "pickier" when scaling than some more standard designs but it will be possible to obtain a good result for any desired operating conditions. More anon but probably not soon. regards Russell McMahon . -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics