Editing PDP-8/e Power Supply Guide (section)

== How it works ==
* Relay K1, a 24 V DC relay controlled by the front-panel key switch through the interlock circuit, switches both sides of the mains to transformer T1. Thermal switch S1 opens the interlock at 90 °C ±5 °C and must be reset by hand.<ref name="mm347" /><ref name="mm48" />
* The +8 V lamp supply is a full-wave centre-tapped rectifier with no filter or regulator, fused at 2.5 A.<ref name="mm347" />
* The +15 V supply uses series pass transistor Q100, driven by emitter follower Q1 from the Q2/Q3 difference amplifier.<ref name="mm347" />
* In the −15 V supply a 723 regulator IC (VR1) drives Q300 and four parallel pass transistors Q301 to Q304, each with a 0.25 Ω 3 W emitter resistor to share the current.<ref name="mm347" />
* In the +5 V supply a 723 regulator IC (VR5) drives Q200 and the parallel pass transistors Q201 to Q206. An overvoltage circuit fires SCR Q207 if the output reaches 6.5 V, shorting the output and blowing F200.<ref name="mm347" />
* The voltage monitor negates POWER OK whenever +5 V falls below 4.3 V, +15 V below 12 V or −15 V above −13.5 V, which halts the timing generator.<ref name="mm347" />
DEC also specifies at least 2 ms hold-up at full load after loss of mains, forbids loading any output beyond 175 % of its rating, and forbids paralleling supplies.<ref name="mm48" />