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3H Spectrometer Gradient Coil Programmable Power Supply Unit (PPSU) General overview The programmable power supply is intended for use with the gradient coils used on the 3H spectrometer electro magnet. There are three sets of coils on each pancake, X,Y & Z the resistance of these coils are as list below Z large coil Z medium coil Z small coil X pair of coils Y pair of coils 17 5 3 8 8 ohms ohms ohms ohms ohms It is important to realise that the PPSU is designed as a constant current (up to 2amps) power supply with a compliance voltage of 20V thus without a load (a gradient coil) the output will be at approx 20V and adjusting the SET AMPS control will not vary this open circuit voltage. Since the compliance voltage is 20V the maximum current into the 17 ohm large Z coil is typically 1.0amps. The PPSU is capable of supplying the maximum current into all the other gradient coils (see IMPORTANT conditions that follow). IMPORTANT :- The power op amp used in the design incorporates a thermal protection circuit that shuts down the op amp when the internal heat dissipation of the device is exceeded and this condition is indicated by a lit red LED on the front panel. The heat dissipation of the device will be exceeded if a current, typically in excess of one amp, is output into a low resistance load i.e. the 3 ohms of the small Z coil, for a period longer than a few seconds. Once the heat dissipation has reduced as a result of device shutdown the op amp will be automatically re-enabled, this cycle will be repeated resulting in a flashing red LED until the current is reduced to a lower level by the user. This automatic protection results in a distorted output voltage/current and could result in damage to the op amp therefore a flashing red LED must be taken as a requirement for user intervention to reduce the current to a safe level. NB if an oscilloscope is connected to the output ONLY one black terminal can be referenced to the scope earth at a time. Do not connect the scope earth to both black terminal at the same time. Do not connect black terminals together. Do not connect units in parallel Circuit description The OPA549S acts as a constant current source. The output current passing through the load develops a voltage across the 0.1 ohm sense resistor R3 and when this voltage equals the program voltage on pin 4 of the OPA459S the constant current is established. The maximum current for the unit is 2 amps and since the value of R3 is 0.1 ohms the programming voltage is a maximum of 200mV. This program voltage is derived from REF1, R8, R9 and R4 when the INT/EXT switch is in the INT position. When in the EXT position the programming voltage is derived from a buffered external source via U2 and associated components. Potentiometer R10 is used to set the voltage at the junction of R11 & R12 to 200mV for the maximum current. R10 is by default set for a 10V external voltage to give maximum current. U3 and associated components buffers and amplifies the sense resistor voltage in order to give a current monitor output at the 4mm sockets on the back panel such that 1V equates to 1A. Manual and remote enable/disable of the output current is achieved by Q1, R5, R2, D2, R16 & C7. The OPA549S output is disabled when pin9 is held lower than 2.4V with respect to pin6. Pin9 is thus pulled low and OPA549S disabled when Q1 is turned on. This is achieved when the ENABLE/DISABLE switch on the front panel is in the DISABLE position due to the 5V6 applied to Q1 gate via D2, R16 and the +15v supply. When in the ENABLE position the gate of Q1 is held off via R5 & R2 and therefore the output current is available unless a REMOTE CONTROL voltage of approx 5V disables the output via the rear panel DIN 6 pin EXT PROG/ENABLE socket. The OPA549S is protected internally from excessive power dissipation such that if the junction temperature exceeds 160 degrees centigrade the device is turned off and the voltage on pin9 drops to less than 200mV thus indicating that the OPA549S has shutdown. The voltage on pin 9 as well as being controlled by Q1, as discussed earlier, is also monitored by Q101, Q102 & associated components. Since the OPA549S may only be turned off for a few milliseconds at a time by its shutdown circuitry, Q101 etc stretch the pulse using the stored charge on C100 to light the LED Q100 via Q102 in order to provide a visual indication that the OPA549S is in shutdown. The LED is also lit as a result of the ENABLE/DISABLE front panel switch being placed in the DISABLED position. The power supply consist of a simple un regulated high current 22V supply composed of T1, Q200, & C200 which provide the power for the constant current source. A low current regulated 15V supply for the reference circuit and low power op amp circuits is provided by T2,D200,C201,U200. Operating Instructions Front Panel Controls INT / EXT In the INT position the output current is set using the appropriate multiturn SET AMPS control. In the EXT position the output is set using an external programming voltage applied via pin2 (OUTPUTB) / pin4 (OUTPUTA) of the EXT PROG/ENABLE socket on the rear panel. 1V programming voltage = 0.2A output current. ENABLE / DISABLE In the ENABLE position the output current is enabled and the red LED is off. The ENABLE position also allows control of the output current via the external remote control such that a 5V high level on pin1 (OUTPUTB)/pin5(OUTPUTA) of the EXT PROG/ENABLE will disable the corresponding output and a 0V level will enable the corresponding output In the DISABLE position the output is disabled and the red LED will be lit. The DISABLE position will also override an externally enabled output. RED LED On when output disable via front panel switch or remote input. Off when out output enable via front panel switch or remote input. A flashing LED in the ENABLE state indicates that thermal shutdown of the unit is occurring. See IMPORTANT under General Overview. SET AMPS Multiturn dial control to set corresponding output current OUTPUTA Screw/plug output terminals for connection to gradient coils. MAINS ON Mains power switch. Mains is applied and unit is active when in down position and switch is illuminated. Rear Panel AMPS MON A 4mm white socket outlet for monitoring a voltage proportional to output current on OUTPUTA 1V = 1A AMPS MON B 4mm red socket outlet for monitoring a voltage proportional to output current on OUTPUTB 1V = 1A COMMON 4mm yellow socket outlet providing reference common for AMPS MON A & AMPS MON B EXT PROG/ENABLE 6 pin DIN socket input for external input of programming voltage and remote enable/disable of OUTPUT A & OUTPUT B. All inputs have 100K input resistance. 1 2 3 4 5 6 Remote enable/disable B Programming voltage B Common Programming voltage A Remote enable/disable A Common Programming voltage 1V = 0.2A Remote enable = 0V Remote disable = 5V