Circuit Descriptions
WARNING: Carefully read the warnings in the Safety section before working on either
the main or display/keyboard PCBs.
Power Supplies
The mains transformer, bridge rectifier and reservoir capacitors provide approximately +10V and -
12V when the instrument is operating. IC17 regulator provides +5V for the microcontroller IC10
and the display. IC11 and IC12 provide ±5V for the analogue circuitry. Power for the isolated side
of the RS232 interface is provided by the host computer.
Required values measured at PJ4 are:
pin 1: +5V ± 0·2V
pin 2: +5V ± 0·2V
pin 3: 0V
pin 4: -5V ± 0·2V
Measurement Control Signals
The control signals required external to the DMM IC, IC2, are latched in IC7. The relay is a single
coil latching type.
Input Signal Conditioning Guard Tracks
All input sense paths which would be susceptible to board leakage are protected by a guard track
driven by IC5B.
Input Signal Conditioning and Protection
IC2 is a DMM IC and contains the A to D converter and analogue gates to allow different ranges
and functions to be selected. The 10MΩ input resistor is part of the input attenuator in voltage
measurement mode and also provides protection. On mV ranges the signal passes via PTC2
which together with Q1 and Q2 provide protection.
On AC voltage ranges IC2 routes the signal via the Trms AC/DC converter IC6. IC6 is driven
differentially; the other input is connected via IC5A and is to compensate for the on-resistance of
IC1B.
VDRs V1, V2, V3 provide overvoltage transient suppression to further protect the A to D.
The A/D Converter
IC2 contains the A/D converter. For voltage measurements the reference is 655mV.
R43,R44,C15,C16 and an internal op-amp form a 2-pole filter which can be switched in and out.
R41 is the integrator resistor and C14 the integrator capacitor which must be polypropylene.
Current Ranges
Q7 is off for 4mA and on for 400mA ranges. The 10A range is via a separate socket to R24.
IC3A selects the range. IC3B selects the normal reference on VR9 for the 4mA range and a
slightly lower reference at R47 for the 400mA and 10A ranges to allow independent adjustment.
Resistance Measurement and Diode Test
Resistance is measured ratiometrically using one of the input attenuator resistors as R-known.
R-known is routed via PTC1 to the input terminal and gives protection. The voltage on the input
terminal is measured via PTC2 and gives input protection.
For diode test Q6 is turned on and current flows through R6 and PTC1 and the diode under test.
The resulting voltage is measured via the 10M input resistor.
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