The output current is controlled by a small DC voltage that can be supplied by either an on-board source (Local Control mode) or by an external source (External Control mode).
There are four items associated with setting the output current:
- I SET three-pin header
- CURRENT SET pot
- Current Setpoint input (pin 4 of J2)
- Output Current Set Monitor output (pin 6 of J2)
Locate the I SET three-pin header on the upper PC board (see Figure 1). The desired control mode is chosen by pushing the provided shunt over two pins.
Local Control
Figure 1
To place the unit into Local Control mode, push the shunt over the two pins marked LCL. In this mode, a regulated internal +5 V source drives the CURRENT SET pot.
External Control
To place the unit into External Control mode, push the shunt over the two pins marked EXT. In this mode, the external supply connected to the Current Setpoint input (pin 4 of J2, see Figure 2) drives the CURRENT SET pot. That means the output current is controlled by both the amplitude of the external voltage (+20 V maximum) and the position of the CURRENT SET pot.
Note that the CURRENT SET pot is active regardless of the control mode. Zero current corresponds with the pot in its fully counterclockwise position.
Output Current Measurement
Figure 2
Whenever the PCO-6131/PCO-6141 ENABLE input is high, the unit is enabled and an internal loop current flows.
The voltage on pin 6 of J2 (Output Current Set Monitor) is proportional to the loop current. For the PCO-6131, the scale is 10 mV per A into 50 Ω. For the PCO-6141, the scale is 20 mV per A into 50 Ω.
It is a high-impedance output intended to be measured with a digital multimeter or oscilloscope. It is NOT part of the output pulse circuit, so it cannot be used with an oscilloscope to view output pulses. Instead, its purpose is to allow you to measure and adjust the output current before bringing the GATE input high and thus turning on the output.
To see the actual output pulse with an oscilloscope, purchase the appropriate Current Monitor accessory. For the PCO-6131, it’s the PCA-9155, which has a scale factor of 1 mV per A into 50 Ω. For the PCO-6141, it’s the PCA-9150, which has a scale factor of 5 mV per A into 50 Ω.
