I've finished laying out the board for my new bench power supply and I am looking for the panel mounted stuff.
It has current limiting, adjustable voltage from 25V down to 1.25, can supply 3A of current and the ripple is yet a mystery. One of the coolest things about is a comparator that selects the input voltage of the main voltage regulator. This way the regulator doesn't overheat when loaded with low voltages and high currents.
It switches a relay between the transformer's taps, giving 30V or 15V input. It triggers at ~12V and has ~200mV of hysteresis. I hope the filtering capacitor can maintain the voltage high enough for the switch to occur without problems. Also I hope I don't do an idiot mistake and make the thing to switch to the higher voltage tap when it is on low voltage mode.
It uses a low side current limiter since it is floating ground, so that isn't a problem. I simulated the response of the operational amplifier and ideally it only allows a very short surge of 8A in short-circuit, in the order of micro-sceonds. As an extra security feature, the voltage regulator also offers a over-current protection circuit.
The transformer is a 24V center tapped transformer (12V-0V-12V). The 12V regulator serves as reference for the comparator and the current limiter. I hope it is stable enough. Obviously, the peak current surge on the 12V rail will happen when the relay switches. I have added a 10uF capacitor next to it just in case. (I don't want the voltage to fall too low and make the comparator a weird relay oscillator).
The current shunt resistor is relatively small, both in resistance and wattage rating, as well in size. It is 0.1 ohms. This means that every amp makes the voltage to drop 100mV across the resistor.
By the way, does anyone know if those cheap panel meters can measure a voltage between two nodes that aren't connected directly to ground? Also, I've heard these panel meters need a completely separate power supply. If this is true, I will just add a 9V battery . So far I have some connectors for the digital panel mount voltmeters. One is set to the 199.9V range (I loose a decimal point, so it sucks!), the other one, which measures current, is set to 1.999V, since the drop on the resistor goes higher than 199mV.
This supply isn't too hackable. I might add a digital interface of sort the next time I need one of these. You know, the classical FT232R with opto-isolators, a PIC microcontroller reading from the ADC, some DAC's to set the voltage and current and an LCD readout. Maybe even some sort of timer to have the thing on for a certain amount of time and even a clock to turn it on at a certain time of the day. And I think I could add an EEPROM to store pre-defined configurations, as well as a handy auxiliary output with an LM317 and a rotary switch to toggle between 1.8V, 2.5V, 3.3V, 5V, 9V and 12V.
I think the price for the parts I don't have is around $35 to $50. I might use the opportunity to order some test leads (banana to crocodile, funky names) and extra parts to have them at my bench just when I need them.
Ivan