Rockwell Automation T8449 Trusted TMR 24V dc Valve Monitor Module - 40 Channel - Manual (Page 13)

Trusted
TM
Module T8449
Issue 13 Apr 10 PD-T8449 13
absence of module power to create gate voltage signals to bias them on 3 (unlike
electromechanical relays for example).
The reason that the lower switches are specified to be on in the absence of control signal power is to
allow two channels to power the load should an entire slice fail. Even if an entire slice fails, the
surviving output circuits will carry the necessary control. The structure of each OFIU output is shown
below:
Figure 4 Simplified Switch Circuit Diagram
A 0.1 Ohm resistor provides a means of continuously monitoring the switch current. A signal transistor
is used to drive the gate of Switch 2. It provides Switch 2 with a negative gate voltage, to minimise it’s
on resistance, and serves to hold Switch 2 on in the event that the secondary gate control loses power.
The zener diode between the gate of Switch 2 and source is only required to protect the gate from
large voltage spikes on the drain that might capacitively couple through when Switch 1 and Switch 2
are in the off state.
The resistors in series with the gate of Switch 1 and the signal transistor serve to protect the drive logic
in the event of a malicious switch failure. The pull-up resistors define the gate voltages in the absence
of power.
3 For an unfaulted transistor.