GE VMIVME-4941 Gas turbine circuit card

If the input jumper is in position JPR8, the current to the photodiode is limited by the series resistance of R3 and R4 (about 1.15Kohms). A protection circuit consisting of Q3, R100 and R101 is included. If the current through the photodiode exceeds about 8mA, the voltage across R100 and R101 is sufficient to turn Q3 on, shunting any additional current away from the photodiode. The voltage drop across Q3 will be equal to about 2V (Vphotodiode + Vbe = 2V). The current demanded by the 1771-VHSC input circuit would be about 18mA (23V - 2V/1.15K = 18mA) which is well within the capability of this driver.

+12 to +24V Single Ended Driver

 Some European-made encoders use a circuit similar to the lower circuit in Figure C.2. The current capable of being sourced is limited only by the 22 ohm resistor in the driver output circuit (R). If a 24 volt supply is used, and this driver supplies 15mA, the output voltage would still be about 23V (15mA x 22 ohms = 0.33V, and Vce = .7V).Open collector circuits (the upper circuit in Figure C.3) require close attention so that the input voltage is sufficient to produce the necessary source current, since it is limited not only by the 1771-VHSC input resistors but also the open collector pull-up. Jumper position provides some options as shown in the table below.

Supply Voltage verses Jumper Settings

In this example, you must increase the supply voltage above +12V to make sure there is sufficient input current to overcome the additional 2K source impedance. Note that there is insufficient current with the jumper in the 12-24V position and a +12V supply While the transistor protection circuit limits the optoisolator current to a safe value, make certain that the voltage range jumper JPR9 is not in the circuit. With JPR9 in, you can exceed the 1 Watt dissipation rating on the 150 ohm resistor (R4) and cause permanent damage to the circuit.

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