A Power Supply for the T1154/R1155 (/cont)

PSU Start and Protection circuitry

Protect the heaters …

As previously said the heaters in a T1154/R1155 installation ran off 6V (nominal) DC. This becomes mandatory  as soon as a T1154 is included since the same 6V that supplies the heaters, also energises the three coils in the T/R relay. If you were to try to substitute an AC supply, the T/R relay would subsequently sound like a door-bell! … on TX and RX! So don’t try it.

I won’t go into the design of the 6V supply right now except to say that I was a bit paranoid should the series regulator fail and the heaters in the transmitter and receiver be subjected to the raw DC from the rectifier (~12V). This would undoubtedly be an expensive failure, so I automatically built in a well tried and tested ‘over-voltage’ protection circuit. It was at this point that I realised just how simple it would be to make the PSU mimic the supply on the bomber.

How it works …

The mains supply takes the place of the dynamo attached to the port inboard engine (the starboard inboard engine drove the hydraulics, in the case of the Lancaster). Momentarily closing S2 is like connecting the dynamo to the large on-board lead-acid accumulator which was invariably 24V. Current flows in the primary of the transformer. A crude rectifier and smoothing circuit then generates approximately 24V DC and relay RLA energises. When the relay closes, S2 and the 300R resistor are shorted out. However, no other supplies are ‘up’ yet since the LT and HT dynamotors were controlled by way of the System Switch on the T1154. When the System Switch is moved to ‘STBI’, +24V is applied to relay RLB via PLD pin 3 on the front of the T1154. When RLB closes, the mains supply is finally routed through to the 6V, 220V and HT power supplies. However, there is a further interlock involved with regards the HT (typically 1200V) supply, so at this point the HT supply is not running since, in the bomber, the HT dynamotor was not switched on until the System Switch was set to ‘TUNE’.

The way the protection circuit works is very simple. The level of the 6V supply is monitored by way of the Zener diode via the 120R resistor. The value of the zener (I think 6.8V) is chosen such that should the 6V supply exceed 7.5V, the potential divider formed by the inclusion of the 180R resistor at the gate of the thyristor, causes the thyristor to ‘fire’ and thus ‘steal’ current from the relay (RLA). When this occurs, the contacts RLA/1 open, the 24V supply fails and ALL the relays in the PSU de-energise and ALL the supplies drop to zero volts … thus protecting the T1154 and R1155 heaters from over-voltage.