Model 242L Operations Manual
Eberle Design Inc.
Page 3
The Model 242L operating voltage is generated by a high efficiency isolated switching power supply.
The resulting bias voltage supplied to the Input circuits is electrically isolated from both 24VDC Logic
Ground and AC Line Neutral. The VDD supply for the microcontroller and display elements results from
a post regulated 5 Vdc.
1.5.2 INPUT CIRCUIT
Resistors R21, R7, R8, R25, and R19 (R22, R9, R3, R26, and R20) provide the input bias and voltage
scaling circuit. The resulting voltage at U20.3 (U20.4) is then processed by the microcontroller U20.
Gas tube CR2 (CR3) and diode CR 14 (CR16) provides input electrical transient protection.
1.5.3 MCU CIRCUIT
The microcontroller U20 processes the input voltages to perform the voltage threshold and pulse width
filtering functions. All signal processing is performed in the digital domain and controlled by firmware
embedded in the microcontroller. This unit does not rely on analog delay, pulse, or comparator circuits
for processing the input signals.
1.5.4 OUTPUT CIRCUIT
The output driver Q7 (Q8) is isolated from both the AC Mains and the internal GND reference of the
Model 242L by opto-couplers U8 (U9). CR9 (CR6) provides electrical transient protection for the output
driver Q7 (Q8).
1.5.5 DISPLAY AND SWITCH CIRCUIT
The LED indicator DS1 (DS3) for the channel output is driven directly from the microcontroller U20. The
input TEST switch SW1 (SW3) can be used to force the input state to the active status.
1.5.6 POWER SUPPLY CIRCUIT
The main power supply is a fully isolated switching design. The cabinet 24VDC input voltage is used to
drive switching controller U18. The transformer T1 is used by controller U18 to generate the isolated
and regulated 19 VDC input bias voltage VCC that drives the input circuits. The opto-coupler U21 and
reference U25 provide the closed loop feedback to the power supply controller U18 for regulation.
A second regulator U26 generates the 5.0 Vdc internal supply voltage VDD used by the processor
circuit from the 19 VDC bias. Inductor L2, C29, C31, and C24 filter the regulator U26 output to provide
clean VDD to the processor.
1.6 MAINTENANCE
The Model 242L requires no adjustments or preventive maintenance.
1.6.1 TROUBLE ANALYSIS
The following list should be used to trouble-shoot the Model 242L installation. If the Model 242L unit
itself is suspect, see Section 1.6.2 for a complete internal testing sequence.
a. Neither channel responds to DC inputs
a. Power supply fault
The Model 242L requires a nominal 24 Vdc input voltage. The unit will operate at voltages as
low as 10.8 Vdc, however, an input voltage below this may result in the unit entering a reset
state. In this case, the unit will appear to be non-functional.
b. Channel does not detect all inputs
