PRODUCT SAFETY SOLUTIONS
© 2017 Educated Design & Development, Inc. All rights reserved. Rev C
Page 15 of 19
THEORY OF OPERATION: (continued)
120-Volt Automatic Disconnect
A power relay, a DPST type, is used to disconnect the front panel 120 Volt outlet when the rear panel
Voltage selector is set to 230 Volt operation. If 230 Volts is inadvertently applied to the LT-952HC while the
Voltage selector is set to 115, the line fuse is designed to open which will cause the relay contacts to open.
Meter Measurement Circuit
The Meter Measurement Circuit contains the electronics to convert low-level leakage currents in the
microamp range into numbers that can be displayed on the front panel display. The Meter Measurement
Circuit contains the Meter Reference, Meter Connection, Body Circuit, Meter Range & Scaling, RMS to DC
Converter, and Analog to Digital Converter Blocks.
Meter Reference
The meter reference circuit consists of the SP3T front panel Meter Reference switch. This switch selects
between line, neutral, and ground references and sends the selected reference to the Meter Connection
Circuit.
Meter Connection
The Meter Connection Circuit receives inputs from the front panel banana jacks, meter reference switch,
and ground switching circuit. The front panel Meter Connection switch, a DP3T type, is used to select
between the various inputs. The three possible meter connections are D.U.T. ground to reference, surface
to reference, and surface-to-surface. The selected connection is then sent to the Body Circuit.
Body Circuit
The Body Circuit used is that shown in IEC 60990, Figure 4.
Meter Range & Scaling
The Voltage across the Body Circuit, which is proportional to the leakage current, is sent to the Meter
Range & Scaling block. This output is then sent to a X1 or X10 attenuator, which is controlled by the front
panel Range switch. The output of the Range switch is connected to the RMS – DC converter.
RMS to DC Converter
This section is composed of a single monolithic integrated circuit that computes the true RMS value of the
input waveform. The output of this circuit is a DC Voltage that is directly proportional to the RMS value of
the input.
Analog to Digital Converter
This section converts the output of the RMS to DC converter to a visible display on the LCD panel. This
circuit also drives the LCD display directly.
TROUBLESHOOTING:
This chapter contains answers to the most frequent questions or problems that may be encountered during
the use of the LT-952HC. Please read this section in its entirety before contacting E.D.&D. for assistance.