Sencore FE149 - Page 13

sistor used to calibrate the low ohms scale will, of course, affect the tolerance or
accuracy of the low ohms readings.
NOTE: If a high resistance fuse is used in place of the low resistance type rec-
ommended, the low ohms scale will be effected, and the low ohms cal would have to
be readjusted.
CIRCUIT DESCRIPTION FOR THE ЕЕ149
The Sencore FE149 isthe ultimate in field effect meters anduses the FET to achieve
the high input impedance with a differential amplifier system to provide the utmost
in stability and accuracy. Тһе push button switches select the mode or function of
the amplifier system and the range ofresistance for the proper measurement. The
FE149 employs two printed boards to make servicing easier. The large board on
the top near the meter contains the FET amplifiers and function switch. Тһе bottom
board contains the range switch and all of the range resistors.
When the plus or minus DC VOLTS button is depressed, FET's ТК1 апа TR2 (See
schematic Figure 5 ) form a differential amplifier with the meter in the source.
ёре DC BAL control and ZERO ADJ set the amplifier up so that with no input, the
rent through the FET's are equaland no current flows through the meter. When
a DC voltage is applied tothe input, it is applied across the voltage divider consis-
ting of R43 to R50. Тһе range switch is depressed and selects one of the points so
that the voltage applied to the gate of ТКІ is . 4 volts or less to cause the meter to
read full scale. Шап excessive voltage is applied to the input, say the 5 volt range
is depressedand 1500 volts is applied, then diodes CR12 and CR14 will conduct and
keep the voltage at the gate and source of TR1 from reaching breakdown. ‘The voltage
that does appear at the gate will be aboutonly one-fifth of the breakdown voltage and
the meter will only be overloaded about 2 to 3 times so that no damage will result.
When the OHMS function isdepressed, ТКІ and TR2 are again usedas a differential
amplifier. Тһе unknown resistor is placedin series with the standard resistor R52
through R58 by the range switch. R51 is used to calibrate R52 for an exact 6 ohms
center scale on the low range. БІ is in series with the unknown and standard re-
sistor so thatin case excessive voltage is applied to the ohms ranges, the fuse will
blow to protect the resistors. Тһе voltage developed across the standard resistor
is proportional to the unknown resistor. ‘This is applied to the gate of TR1 through
R32, R33 and R34. Thisdivider divides the voltage tothe gate sothat full scale de-
flection will occur at .4 volts. Also, as the ranges are changed from КХ10 meg
and down, the zero ohms adjustchanges very little, eliminating time consuming re-
zeroing of the meter.
In DC volts and OHMS, TR3 has no function and is inoperative at this time.
When the AC VOLTS button isdepressed, TR2 and TR3 become the differential am-
plifier and TR1 becomes a source follower. Тһе AC voltage is applied through the
voltage divider as before to the gate of TRI.
The AC voltage from ТЕ] is applied through СЗ to the peak to peak detector СКІ and
CR2. The detected output is applied to the gates and is the result of the actual peak
to peak voltage of the input AC signal. Тһе resultant unbalanced DC voltage causes
the meter inthe source circuit of TR2 and TR3to read. If excessive voltage is ар-
plied tothe inputas іп DC, diodes CR12 and СК14 willagain conduct and protect the
the input, the gates of the transistors are balanced with equal currents through
e FET's sothat the meter readszero. This compensates for any unequal leakages
and components in the circuit.
9. transistor ТЕЛ. Тһе control R22 is set ир so that with no AC voltage applied
The АС currentranges use те same amplifier system as used for AC voltage. Тһе
FE149 is connected in series with the circuit where the AC currentis to be measured.
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