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102808_CAPAC_MANUAL_MODEL_90W12D_SYSTEM.doc
Controller
The 90W 12 D controller functions to provide the correct amount of output current to the
anode so that the desired cathodic polarization potential is maintained on the metallic
surfaces exposed under water. The controller operates on an input voltage of 12V DC, (i.e.
batteries ore rectified or rectified and filtered direct current source) and consists of one
integrated circuit, seven transistors and associated circuitry configured to provide the
following functions:
1) Polarization level control.
2) Current limiting.
3) Automatic shutdown for open or short-circuited reference electrode.
Polarization level control is provided by integrated circuit I1, transistors Q3, Q4, Q5 and Q6.
these devices are connected as a closed loop, gain-controlled amplifier/comparator whose
output is connected to the anode. An adjustable reference set point voltage is connected to
one input of the integrated circuit. This represents the desired polarization potential for the
metal to be protected (i.e. 650 millivolts for bronze, 850 millivolts for steel and 950 millivolts
for aluminum). The other input of the integrated circuit is connected to the reference
electrode.
If the sensed reference electrode potential is below the set point potential, the amplifier
output is increased thereby supplying current to the anode and increasing the polarization
on the immersed metal. If the sensed reference electrode potential is above the set point
potential, the amplifier output is decreased, reducing the anode current and the polarization
on the immersed metal. In this manner, the correct polarization potential on the cathodic
surface is automatically maintained.
Current limiting is achieved by means of transistor Q7 whose input is connected across
current sensing shunt resistor R19. As the anode current through R19 increases and the
voltage drop across it increases thereby turning on transistor Q7 which shunts the drive
current to the darlington connected output stage comprised of Q5 and Q67 this reduces the
output current from this accordingly so as to restrain it to a maximum of approximately 7.5
Amperes.
Automatic shutdown is achieved for open or shorted reference electrodes by two different
means. If the reference electrode should become open-circuited, resistor, R2 causes the
reference electrode terminal of the controller to increase in potential thus simulating an over-
protection condition which causes the control amplifier to reduce the output current to the
anode. If the reference electrode should become short circuited, transistors Q1 and Q2 are
turned on, thereby shunting the drive signal at the input of transistor Q3. This action also
turns off the controller output current fed to the anode.
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