Philips PM2519 - Page 26

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2.3.2.6.
RX
2.3.2.7,
Diode measurements
Diode measurements and measurements of semiconductor junctions are performed in the same way as for
resistance measurements in the 1000 Q range, except for the input of the ADC. The unknown voltage across
the diode is routed via R1110 to the ADC. This is done to get a quick response for the bleeper measurements.
The value displayed is the voltage in forward or reverse direction across the diode in the highest range of the
ADC. In the diode measuring range, the constant current derived from the OO 0063 is 1 mA (see previous
section).
CURRENT SOURCE
TO ADC
v1553 vi554
R1501 750+1k5
82V46
€2v0
I
vist _|v1552 ‘ -——_
750+ 1k5
RISO2z
BAXIZA JBAXIZA R151
TEKS —+
TP1003 R510
TP 1004
jo
crp04 [1505
Fh ties Ton
ry Vv
-9v
ST4403
840713
Fig. 2.6. Ohm measurements
Temperature measurements (°C)
When the °C is selected, the constant current Irx (1 mA) is routed from pin 2 of the probe connector X1004,
which is connected via the probe to one end of the Pt-100 resistance thermometer. The other lead is
connected to earth. This current gives a voltage drop which depends on the resistance value. across the
Pt-100 probe. The voltage drop is measured via two other points of X1004 (4-wire measurement), 0 °C will
give a voltage of 100 mV. The 100 mV offset is subtracted in the microcomputer so that 0 °C will be
displayed.
X1004
RI513
{300k
he
ST4404
840713
Fig. 2.7. Temperature measurements