The circuit devides into three functional blocks; the voltage regulator, the differential stage and the power stage. The Voltage Regulator This has a minimum dropout of just under 1V and a compliance up to 200V. It is designed to withstand the high dV/dt caused by plugging into an energised circuit by limiting its own inrush. D3 and D4 set the regulation voltage, turning on Q5 at about 3V, which in turn cuts off Q4 and pass transistors Q2, Q3. C2, R10 and D11-13 act to limit the inrush current when the regulator output is below regulation (when Q5 is off). D11-13 cut off Q4 once there is one diode's drop accross R10, meaning that C2 is charged at a constant rate and Q2's base (and so the regulator output) ramp linearly up to regulation in about 6mS. Q1 and friends are also a mechanism to limit inrush, cutting off Q2 when power is first applied untill C1 has charged, and again if the voltage accross R1 gets great enough to raise Q1s gate out of cut-off. The particular purpose of this arrangement was to stop Q2 from being turned on by the initial charge current into C3; it seems to just about manage this. Resistor chains are used in some places for dissapation and maximum-voltage reasons. C3 provides dominant-pole compensation to keep the regulator stable. Q6 (a Darlington) is a crowbar; it will turn on if the voltage accross R13 exceeds one Vbe, probably burning out R11, but with luck saving the rest of the circuit. The Differential Section Before the differential section propper, the reference voltage is generated. This is compared with the voltage accross the sense resistors and used to set the output current. D5 does a final round of voltage stabalisation (producing about 2V at its annode), and RV1 is trimmed to give a maimum reference voltage (RV2 fully clockwise) of 1V. S1a gives on/off action by shorting the reference voltage to ground - when it is turned on the output current is ramped up to its set point as C7 charges. [Note that it's not really neccacary to have S1a and S1b turning the circuit on and off in two different ways; zeroing the reference voltage was the method I originally wanted to use, and interupting the power-sage drive was added later as a safeguard against misbehaviour.] Q7 & Q8 and Q9 & Q10 form cascaded long-tailed pairs with Q9 & Q10 being connected common-collector (ie, not a real LTP) to give an input range to ground. These compare the reference voltage at Q9's base to the sense voltage at Q10's base. Q11 converts this to a single-ended current, and Q12 acts as a level shifter, shifting from the 3V supply to the load voltage. The freedback network consists of C8, C9, R28 and friends. The open-loop response of the differential stage is limited to something stable by C8; however, there is too much lag in the power stage to get the entire loop stable, so the network C9, R28 is used to give the differential stage local feedback at HF, but include the power stage and sense resistors in the loop at DC and LF. This seems to work quite successfully. The Power Stage This is simply a three-deep complementary Darlington. The power devices where chosen according to what I could find, and could be adjusted to taste. There are three sets of sense resistors: 0.1R, 1R and 10R. With a 1V reference voltage these give ranges of 10A, 1A and 100mA and, since there are two parallel output devices each with its own sense resistors, total full-scale outputs of 20A, 2A and 200mA. In practice I think 20A may prove unobtainable however. Negative connection is made by special "two-contact" 4mm banana sockets, in which the insertion of a plug makes a connection between two two contacts. In this device, one contact of each output socket is connected to ground, while to other is connected to the end of that range's sense resistor. In this way, when a plug is inserted into an output socket, that socket's sense resistors become referenced to ground (only one output can be connected at a time!). The contacts are labeled A & a, B & b, C & c: Plugging into the 20A output connects A to a, the 2A output connects B to b, etc. Of course, a switch could be used in stead if preferred. A problem with the wo-contact socket is that you're never sure which contact will make first - this cives rise to some contingencies that need to be taken into account. Similar problems might arise if you where to use a switch with bouncy contacts to change ranges, if a wire came astray, or under various other fault conditions. If only the node called "Ground" is connected then the node called "Current sense" could potentially look like a very high voltage with respect to "Ground". In this case, D7-D9 clamp the base of Q10 - in practice enough current can flow here to keep "Current sense" within a couple of volts of "Ground", but D15 is made a high voltage type just in case (say an output device went short or R28 went open). On the other hand, if only the node called "Current sense" is connected if could look very negative with respect to "Ground". In this case, D14 and D6 clamp the inputs - but note that this will pull the "Ground" node down and power the circuit via the current flowing out of the current sense line. D15 is used to pull the reference input (Q9's base) lower than the sense input (Q10's base) and so prevent the differential stage turning "on" and pulling full current from the load. Despite my best efforts, there is still a condition - on the instant that the "Ground" node is disconnected but when the "Current Sense" note is still attached - when a current spike can be drawn. For this reason S1b is used to remove all drive to the power stage, and I'd recomend the load is turned "off" before switching tappings. R30, C10 form a Zobel network accross the terminals to help protect against indutive loads; the feed to the regulator is taken from its centre point in a further attempt to reduce inrush. D10 is there to protect against back-EMF spikes or missconnection. Be aware that there is no safe-operating area protection for this device - just a just and a thermal cutout. The user must limit the current and voltage to safe values - the table below should be about right (from rough calculations based on the 2CS3281 datasheet): V I 5 20 10 18 20 8.7 30 5.8 40 4.4 50 3.5 60 3.4 70 3.3 80 3.2 90 3.0 100 2.8 110 2.5 120 2.2 130 2.0 140 1.7 150 1.5 160 1.2 170 1.0 180 0.7 190 0.5 200 0.3 There is no R29.