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Front Panel Operation 89

Programming Overcurrent Protection

When enabled, overcurrent protection removes the power supply output whenever it goes into CC operation. This prevents

the supply from indefinitely supplying the full programmed current to the load.

Setting The OCP Protection

To activate overcurrent protection, press . The OCP annunciator will light and power supply will continue to operate

normally until it is forced into CC operation. If that occurs, the OCP circuit will trip and the power supply will remove its

output.

Checking OCP Operation

The easiest way to check this operation at any specified current is to increase the load current beyond the programmed

current value and, if necessary, decrease the programmed voltage. This will force the power supply into the CC mode (see

Figure 5-2). When OCP trips, the Prot annunciator will light and the power supply output will drop to zero.

There is now no power supply output due to an overcurrent condition. To verify this, press and observe that the

display indicates OC.

Clearing The OCP Condition

With the OCP tripped, return to the meter mode and try to clear the condition by pressing . Nothing will appear to

happen because the reason for the condition has not been removed. Thus, as soon as the circuit is cleared, it trips again. You

can clear the OC condition by:

■ Increasing the load resistance to lower the output current below the programmed current value, or

■ By raising the programmed current to a value above that required by the load.

Clear the fault by either of the above methods. Then clear the OCP circuit by pressing . The Prot annunciator will

go off and the power supply output will be restored to normal.

If desired, you can also restore the output by disabling the OCP function (press to turn off the OCP annunciator).

This restores the output but does not clear any condition that may have caused OCP to trip.

Note Under certain conditions, the OCP circuit may fail to clear because load demand occurs before the power

supply has time to build up the required output current capacity. In such cases, disable the output (press

before clearing the OCP circuit). After OCP is cleared, enable the power supply output.

CV Mode VS. CC Mode

Once you program a voltage (VS) and a current (IS) in Figure 5-2, the power supply will try to maintain itself in either CV or

CC mode, depending on the impedance of the load (RL). If the load demands less current than Is, operation will be in CV

mode with the voltage maintained at Vs. The output current will be at some value below Is as determined by VS ÷ RL.

If the current increases beyond IS (see RL2), the supply will switch to CC mode by varying its output voltage to maintain a

constant current value of Is. As more current is demanded, the voltage decreases to maintain the increased current level. If

the load current increases to the maximum output of the power supply, the output voltage will be maintained at a near-zero

level.