Ametek Sorensen DCS M51 Series, Programming Manual, Page: 13 / 14

SECTION 3
THEORY OF OPERATION
3.1 General
The M51 interface board consists of two functional blocks, the isolation amplifier with input
buffer amplifier and associated switching circuitry, and the isolated power supply, which
provides power to the input side of the isolation amplifier.
3.2 Isolation Amplifier
The isolation amplifier U1, has a capacitively isolated input stage, which prevents any D.C.
currents from flowing between the programming source, and the power supply. It has a fixed
overall unity gain, which allows for simple interfacing of the external program signal and the
internal control points. U3 amplifies and buffers the input signal, with R8 and R11 providing
range and offset adjustment points, while R7, R6 and R5 in conjunction with SW1-7 and 8 set
the appropriate program scale.
Quad analog switch, U2, connects the output of the amplifier to either the voltage program line
(J2-9) or the current program line (J2-8), depending on the setting of SW1-2 and SW1-3 (see
Table 2-1 for settings). The control line that is not being programmed is connected to the R4,
R3 voltage divider, which provides a fixed full scale (5V) program signal. SW1-1 sets the power
supply in the remote control mode by pulling pin J2-4 low. In this way the single isolation
amplifier can be used to program either the output voltage or current, while the other non-
programmed line is held at the full scale level, independent of where the front panel control
knobs are set.
3.3 Isolated Power Supply
The isolated power supply is required to p12V and -5V to the input side of the isolation
amplifier, while maintaining electrical isolation between this section of the M51 board and the
DCS unit output.
The power supply is a low power flyback type circuit, controlled by U5, a 100kHz current mode
pulse width modulator (PWM). This PWM controls the on time of the switching transistor Q1, a
N-channel MOSFET, which controls the power delivered to the secondary T1, the isolation
transformer. The transformer winding voltage is sensed at the control circuit by rectifying and
filtering the output of the control winding at T1 pin 9 and 2. This voltage is divided by R13 and
M360002 Rev D 3-1