Mains Voltage Meter, Mutual Shutdown System

The 723 IC is powered through R11, which allows ripple filtering by C9, and voltage protection by DZ1,2. These Zeners are not required to reduce ripple; their function is purely protective, preventing the 40V Vmax spec of the 723 being exceeded should the mains voltage rise above nominal.

The current through the pass devices TR4-7 is shared equally due to the current-sharing resistors R35-45. The drop across these at full load is approx 200mV which is more than enough to swamp Vbe differences in the main transistors.

50 Amp Schottky diode D7 on the output allows supplies to be paralleled for redundancy.

The Mains Voltage Meter.

The incoming mains voltage is continuously indicated on the front panel by a 3.5 Digit Panel Meter. (DPM) This is powered from a +5V supply derived from the +12V subrail by 7805 regulator REG2.

The +5V supply is reduced to +2.5V by R30,31, and buffered by IC2-A to generate a half-rail for the DPM input.

The incoming mains is monitored by differential amplifier IC1-A, which looks at the voltage between Live and Neutral; this avoids problems with having one side of the meter connected to Earth, which is often not at quite the same potential as Neutral. The Voltage is picked off via two special BSI safety-approved 680K resistors on the voltage select PCB (SC3817) which connect to CN3. The circuitry around IC1-A (R21-24) gives unity gain, but the presence of the 680K resistors on the input gives suitable scaling of the 50Hz signal.

The signal is applied to precision full-wave rectifier IC1-B, IC2-B via DC-blocking cap C5; this circuit is biased by the +2.5V halfrail. The rectified output is smoothed by C6 and scaled by R39 and PR1 before applying it to the DPM input on CN1 Pin 8. (IN HI)

Note the DPM has a differential input, so the cold side (IN LO, Pin 7) is connected to the +2.5V halfrail.

The Mutual Shutdown System.

Equipment containing some kinds of op-amp is vulnerable to damage when only one of the +/-17V rails fails, as in this case excessive supply currents can be drawn, damaging fuse resistors on the modules.

When both 17V supplies are working normally, current flows through DZ8, OPTO1, and across to the RH PCB. OPTO1 is on so the base drive to TR22 is shunted away and the 723 works normally. Likewise, TR24 is kept on via R100, and ensures TR28 stays off.

If either supply fails there is insufficent voltage to keep DZ8 conducting, and current flow ceases in the mutual shutdown line. TR22 is turned via the special supply generated by D13, C26, and the internal node of the 723 IC3 is pulled down to ground, turning it off. Likewise, in the -17V supply, TR28 is turned via the special supply generated by D14, C25, and the internal node of IC4 is pulled down to ground, turning it off.

C27,30 delay the action of mutual shutdown sufficiently to allow the supplies to start, as they may not rise to working voltage at exactly the same rate.

Circuit Description