There are two PCBs: one on the bottom which deals with power related matters and one attached to the top lid which is the user interface (UI). The PCBs are connected with a 3 wire ribbon cable.
The power board comprises a fuse, mains relay, relay drive circuit and a low voltage (1.2V) power supply and backup battery (cell) for the user interface board.
The UI board comprises an application specific Chip-on-Board (COB) IC, a custom LCD mated to the PCB by a zebra strip and contact pads for rubber buttons. It's connected to the power PCB by a 3 wire ribbon cable. Two wires for power (1.2V) and one (labeled 'D' on the PCB) for relay control. I found if this was left floating the relay was on and if I drive it to 0V the relay was switched off.
There isn't much that can be done with the UI board and the application I have in mind will require a separate MCU to control the relay. So I unsoldered the ribbon cable from it.
I am hoping to use the low voltage power supply to power my application. So I took a shot at reverse engineering the schematics of the power board:
So it seems there are two DC power rails: a 24V rail set by zener diode DZ1 (a 24V 1N4749A) which together with Q1 is used to energize the relay coil. A second power rail is derived from the 24V rail and is set by DZ2 and provides 1.2V for the UI PCB and the 1.2V backup battery.
The relay is controlled by line D. It's interesting to note that the Q1 (a SS9014 NPN) is a high gain transistor, so it doesn't take much base current to cause the relay to activate. If D is left to float then the small current that flows through R4 (about 73µA) is enough to activate the relay. If D is driven to 0V then that current is directed away from Q1 causing the relay to deactivate.
D2 is the usual fly-back diode which absorbs the voltage spike due to the sudden interruption of current through the relay coil. Diode D1 protects Q1 (and the driving logic) from excessive current if D line is high.
The purpose of D3 (I guess) is to prevent the 1.2V battery from draining through EC2 etc when the device is unplugged.
One important observation about the 1.2V low voltage rail: the 0V is not ground. It will vary between 0V and +24V relative to the ground/neutral line. Therefore you must optoisolate it if interfacing to external circuitry. The following is a scope trace from the 1.2V line on the ribbon cable. Connecting this to external (grounded) logic devices is likely to damage it.
So my plan is to use this device with the UI PCB removed and a Lite-On LTV-817S opto-isolator with the output connected to the D and 0V line of the ribbon cable. The relay will be on by default. Passing a current through the isolator's input LED will cause the relay to switch off. More on this another time...
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