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Installation - 3
35
OVP Considerations
CAUTION: Disabling the overvoltage protection circuit may cause excessive output voltages, which
can damage the phone under test.
The dc source is shipped from the factory with its overvoltage protection (OVP) circuit enabled. This
built-in overvoltage protection function is not programmable; it is set to automatically trip when the
output voltage measured at the sense lead terminals exceeds the programmed voltage by two volts.
Having the overvoltage and the output voltage sensing at the same point provides a more effective
method of load protection than if the overvoltage is sensed only at the output terminals of the dc source.
To disable the OVP circuit, use either the front panel VOLT PROT command located in the OV menu, or
the VOLTage:PROTection:STATe SCPI command as explained in chapter 8.
The built-in overvoltage protection circuit reduces the number of nuisance overvoltage shutdown events
since it trips only when the sense lead voltage exceeds the programmed voltage by two volts. In situations
such as where the external remote sense leads are shorted, the OVP circuit will shut down the unit if the
voltage measured at the output terminals exceeds the programmed voltage by three volts. Lastly, the OVP
circuit will shut the unit down if the voltage at the output terminals exceeds 18 volts for any reason, such
as when remote sensing around an excessive load lead resistance.
The OVP circuit contains a crowbar SCR, which effectively shorts the output of the dc source whenever
the OVP trips. However, if an external current source such as a battery is connected across the output and
the OVP is inadvertently triggered, the SCR will continuously sink a large current from the battery,
possibly damaging the dc source. To avoid this, you can either disable the OVP circuit or you can
connect an external protection diode in series with the output of the dc source. Connect the anode of the
diode to the + output terminal.
The OVP circuit's SCR crowbar has also been designed to discharge capacitances up to a specific limit,
which is 50,000 µF. If your load capacitance approaches this limit, it is recommended that you do not
intentionally trip the OVP and discharge the capacitance through the SCR as part of your normal testing
procedure, as this may lead to long-term failure of some components.
Programmable Voltage Protection
In addition to the automatic overvoltage protection circuit, the dc source includes programmable voltage
protection for output 1. This feature lets you limit the maximum allowable output voltage that can be
programmed either from the front panel or over the GPIB. This feature is useful in situations where
accidentally programming higher output voltages within the operating range of the dc source can
permanently damage the phone under test.
For example, suppose that a phone under test, which requires the output voltage to be adjusted up to 6 V,
can be damaged if the output voltage exceeds 9 volts. You can set the programmable voltage limit to 6
volts using either the front panel VOLT:PROT command in the OV menu, or the VOLTage:PROTection
SCPI command as explained in chapter 8. If an attempt is then made to program the output voltage to a
value greater than 6 volts, the unit goes into voltage protection mode and turns its output off.
NOTE: The VOLT:PROT front panel and SCPI commands do not program the tracking OVP
circuit, which automatically tracks the output voltage and trips when the output voltage
exceeds the programmed voltage by two volts.