Model SR715 Model SR720 LCR Meters

OPERATIONEXTERNAL BIASThe SR715/720 has rear panel connections to allow an external bias voltage ofup to +40VDC to be applied across the DUT. This supply must be floating (neitherside connected to ground) and must be current limited to less than 250 mA. Alinear supply is recommended (vs. switching supply). The supply should be wellfiltered, to remove ripple from the bias voltage. Connections to the bias supply aremade on the rear panel with two banana jacks. The red jack is the positive side ofthe supply and the black, the negative. There is an internal diode within the unit toprevent a negative voltage from being applied. The bias supply lines are fusedwith a 250 mA fuse on the rear panel, next to the banana jacks. If the SR715/720doesn't yield stable readings when the external bias switched is on, check thisfuse. If the bias supply is connected backwards, there is a good chance that thebias supply fuse will blow. Provisions need to be made to discharge the biasvoltage from the capacitor under test before removing it from the fixture. Ingeneral, external switches or discharging resistors need to be provided with thebias supply to ensure safe operation.To apply an external bias voltage, ensure that the bias supply is connectedcorrectly. Then verify that the capacitor is installed with the correct polarity in thetest fixture (positive to the right). Next press the [External] bias key to apply thebias voltage. After a second or two, the reading should be stable. After themeasurement is complete, discharge the DUT before removing it from the fixture.For occasional use (non production) with small capacitors (