Hardware - GE 169 Instruction Manual

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GE Power Management
5 THEORY OF OPERATION

5.1 Hardware

All relay functions are controlled by an 8031 8-bit microcomputer. This IC contains internal RAM and timers, but all
firmware and display messages are stored in an external EPROM IC. A 12 key keypad and a 2 row X 24 character
display are used to enter relay setpoints and display all values and messages. A hardware block diagram is shown
in figure 5-1.
The power supply uses a dual primary / triple secondary transformer for connection to 120/240 VAC. A
24/48/125/250 VDC input switching power supply is also available as an option. Regulated +/-5 V supplies are
created for use by logic and analog ICs. An unregulated +10 V supply is used to drive the RTD selection relays and
L.E.D. indicators, and an isolated +10 V supply is used on the AC input versions to drive the output relays and read
the contact inputs. +2.5 V reference voltages are derived from temperature compensated precision voltage
reference diodes to provide stable, drift-free references for the analog circuitry. A power fail detector circuit is used
to reset the relay whenever the supply voltage goes out of the proper operational range. This hardware watchdog
circuit must be signaled regularly by a firmware-generated voltage or else the microcomputer will be reset.
Three phase CTs are used to scale the incoming current signals to the 169 relay. The currents are then rectified and
fed through fixed burdens to produce a voltage signal of 430 mV peak / FLC. This signal is then multiplexed. The
multiplexed signal is buffered and fed to an A/D converter. The digital signal is then fed to the microcomputer for
analysis. A separate ground fault CT is provided on the 169 relay to scale the input ground fault current. This
current signal is rectified and fed through a resistive burden to convert it to 1.25 V peak / secondary amps rating.
This is then fed to the same multiplexer as the phase input signals.
The temperature monitoring circuitry of the 169 relay consists of 10 RTD connections multiplexed by miniature relays
and a 4 to 10 decoder. Mechanical relays are used because of their excellent isolation, transient immunity, and
almost zero on-resistance. A stable current source feeds each of the RTDs in turn, and 128 readings are taken over
a period of one second for each RTD. This provides for stable averaging and good 50/60 Hz noise rejection. An
RTD lead compensation circuit subtracts the RTD lead resistance and then the analog RTD voltage is multiplexed
along with the phase and ground fault signals. A no sensor detector circuit indicates when no current flows in an
RTD in order to distinguish a faulty sensor from a high temperature reading.
The 8031 microcomputer interfaces with an 8155H I/O port and static RAM to drive an intelligent display module and
provide a digital output signal for a D/A converter. The analog output signal from the DAC is then converted to a
current and scaled to be 4-20 mA. The microcomputer also drives an 8255A I/O port which handles keypad inputs,
L.E.D. drivers, and external switch inputs. The data lines from the 8031 are latched before being passed to the
address lines of the EPROM and NOVRAM. NOVRAM store cycles are initiated every time control power goes out
of the recommended operating range. The output relays are controlled by the microcomputer through optoisolators
and are powered by a separate, isolated +10 V supply. A SN 75176 transceiver is used to provide an RS 422
communications interface for programmable controllers and computers for the 169 Plus.
All connections to the 169 relay are made on the I/O circuit board; transient protection and filtering are provided on
all inputs.
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