Fluke 875 manual Fail/Safe Circuit, Recorders and Computer Buffers, Pen Clamp

Function Description

Readout Module876A-1 4

There is a yellow ALERT ALARM jumper connected to the collector (pin 1) of the alert alarm relay driver stage of U503. The following options are available with the presence or absence of the jumper when the radiation level exceeds the threshold and triggers the alarm:

Manual Reset - The alarm will continue to be activated even after the radiation level recedes below threshold until the SAE/RESET indicator push button on the panel is depressed. This action occurs with the jumper in place.

Automatic Reset - The alarm will continue to be activated only as long as the radiation exceeds the threshold level. This action occurs with the jumper removed.

The yellow warning light acts similarly to the alarms. It lights when the alarm relay is de-energized.

Each relay contains four Form C contacts, but only two of them are accessible from the rear connector. The connection to the yellow panel warning light is from terminal 2 of the printed circuit board, which is connected to the collector (pin 14) of U503 through R526.

Fail/Safe Circuit

Input to the fail/safe circuit comes from pin 1 of U2 on the power supply board, entering the relay driver board on pin H of the relay driver printed circuit board. The input is a DC voltage that is either high (15 V) or low (0 V); if it is low, relay K3 is energized (non-alarm condition). If the signal is high, the green SAFE/RESET lamp will go off and the relay is de-energized, indicating a fault somewhere in the system. The circuit consists basically of two inverters in tandem so the voltage on the collector of Q502 is approximately the same as the input voltage, the diode CR508 preventing current flow through the solenoid if the collector is slightly higher than the 14 V at the other end of the solenoid coil.

Recorders and Computer Buffers

Provisions have been made for delivering DC output voltages to a recorder and computer for further processing. The buffers are located on the relay driver board. If a commercial device is to be connected, a signal isolator must be installed between the device and the 876A-1 Readout Module output. Inputs to both computer and recorder buffers arrive at pin R of the relay driver printed circuit board, coming from terminal 3 on the ECS printed circuit board. The computer buffer consists of and OP AMP using pins 1, 2 and 3 of U501, the output being taken off the relay driver board at pins 4 (+) and 2 (-) of P502. Resistors R532 and R533 act as voltage dividers for the output. Their values for standard usage and available options are shown in Table 4-1.

The recorder buffer consists of an OP AMP using pins 5, 6 and 7 of U501, the output being taken off the relay driver board at pins 1 (+) and 3 (-) of P502. Resistors R530 and R531 act as voltage dividers for the output. Their value for standard usage and available options are shown in Table 4-1. Both buffers are disabled during the ECS test by a switching circuit on the ECS board, so that the ECS current is not recorded. This circuit consists of Q201 and Q202 on the ECS board and is activated by a muting voltage during the ECS test.

Table 4-1. Resistance Options for Voltage Dividers

Resistor	Std.	Option 1	Option 2	Option 3
Recorder	0 – 1 V	0 – 10 mV	0 – 5 V	0 – 5V
Voltage
R530	1 kilohm	119.8 kilohms	200 ohms	200 ohms
R531	200 ohms	200 ohms	1 kilohm	1 kilohm
Computer	0 – 5 V	1 – 100 mV	0 – 50 mV	0 – 5 V
Voltage
R532	200 ohms	11.8 kilohms	23.8 kilohms	200 ohms
R533	1 kilohm	200 ohms	200 ohms	1 kilohm

Pen Clamp

4-5