Download 3H Spectrometer Gradient Coil Programmable

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