Fluke 8508A, Service Manual, Page: 38 / 120

8508A
Service Manual
2-6
measurement across the unknown resistor. Configuration relays on the A1 DC PCA
perform the change in terminal connections between two- and four-wire measurements.
The A3 Ohms PCA includes an alternative high-resistance measurement feature for
resistance measurements above 20 M
Ω
. These measurements use 10 times the standard
stimulus current and result in a 10 times increase in the voltage across the resistor. This
high-voltage resistance feature enables the Multimeter to make quiter and faster
measurements. It also enables the inclusion of a 20 G
Ω
resistance range.
Another feature included on the A3 Ohms PCA is true-ohms. This feature eliminates the
input offset-voltage from the measurement circuit by continuously reversing the direction
of current flow through the unknown resistence. The stimulus current reversal is timed so
that the current flows continuously thus maintaining identical and constant self-heating of
the unknown in both forward and reverse measurements. To ensure same-polarity
measurements, voltage sensing of the unknown resistor synchronously reverses with the
current.
The true-ohms ratio function uses a similar current reversal process. However, the current
is applied to both unknown resistors simultaneously.
Power Supplies
The Multimeter has two types of power supplies, out-guard supplies and in-guard
supplies. The out-guard supplies provide power for the digital (earth ground) side of the
Multimeter. This includes the A4 Digital PCA, the A5A1 Display (and keyboard), and
the opto couplers on the A1 DC PCA. The in-guard supplies provide power for the analog
(measurement) side of the Multimeter. This includes the A1 DC PCA, the A2 AC PCA,
and the A3 Ohms PCA.
Two chassis-mounted transformers (T1 and T2) are the source of all the various voltages
required for operation of the Multimeter. See Figure 2-2. Transformer T2 is the fused
line-powered (mains) transformer. It has a single 24 V center-tapped secondary which
provides ac power for both the out-guard and in-guard power supplies. On the A4 Digital
PCA, the 24 V ac drives three conventional rectifiers and regulators to p5 V,
+8 V, and +17 V sources for the out-guard circuits. The +5 and +8 volts provide power
for out-guard digital logic circuits; the +17 volts, along with a driver on the A4 Digital
PCA, supplies the power (17 V ac) to drive the primary of the in-guard transformer T1.
Transformer T1 has 3 secondaries which drive a series of conventional rectifiers and
regulators located on the A1 DC PCA. These T1 supplies provide the following in-guard
operating voltages:
•
+5V dc regulated
•
+8 V dc un-regulated
•
±15 V dc regulated
•
±36 V dc regulated
•
250 Vac
All of the in-guard PCAs use these in-guard voltages, with the exception of the 250 V ac
which goes directly to the A3 Ohms PCA where it is rectified and filtered for use in
making the high-voltage ohms measurements. The 250 V dc supply is active only when
the High Voltage Ohms function is selected.
A few other in-guard voltages, which are uniquely developed and used on the A3 Ohms
PCA, i5 V, -15 V, ±6 V, and +5.5 V. The in-guard voltages on the A1 DC PCA
are the original source driving all of these uniquely developed voltages on the A3 Ohms
PCA.