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Omnichrome 150 Laser Power Supply

ALPS 14B revB PCB

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Updated circuit, 2 errors in drawing corrected; value of R10 changed to 91k, current control input changed to pin 6.

The accompanying circuit was drawn after reverse engineering the power supply as no information for this particular PSU seems to be available; the later Omnichrome 150R is well documented in Sam Goldwasser's Laser FAQ site see the section Omnichrome 150R power supply and 532 head for these circuits.

The circuit does not show the low voltage power supplies (+/- 12V for the isolated circuits and +/- 15V for the non-isolated circuits) as these are simple linear regulator based supplies fed from 2 independent windings on the power transformer. Also not shown are the HT rectifier and associated 3 x 2200uf filter capacitors and inductor, the boost supplies, the filament (cathode) transformer or the pre-heat delay circuit as these are all near identical to those in the 150R power supply (and I couldn't fit them onto 1 A3 drawing sheet!). All component references are my own as the PCB has no component references anywhere; note also that the function references next to the 5 preset pots have been deduced from the circuit and some may be incorrect.

Description of operation

This description is based on my own understanding of this PSU and may be partially or completely wrong! Let me know if you think I've got it wrong.

The circuit is split into 2 sections, the isolated external loop which is connected to the remote interface connector on the front of the PSU and the non-isolated inner loop which is connected to the HT supply; the two loops are linked by optocouplers OK1 and OK2.

Inner loop operation

Initially the outer loop is inactive and all regulation takes place within the inner loop, this is also the case when the power supply is set to standby mode via the remote connector. R47 is in series with the anode of the laser tube, when current flows, the current mirror consisting of Q1 -Q5 produces a voltage across 100R resistor R51, this is approximately 130mV /A of tube current. This monitor signal is fed into IC3d which then drives optocoupler OK2 which feeds the current signal into the isolated outer loop; the current signal is also fed into the feedback amplifier consisting of IC3c, IC3a and associated components; this amplifier has a flat frequency response between 0 and 1Hz then rolls off by 3dB to 10Hz and is flat again to 10KHz where it then rolls off at 3dB /octave. Diode D8 appears to be present to clip the signal should fast transients appear in the current signal, without the diode fitted, the tube and power supply become unstable above 8A (more on this later). IC3a is AC coupled and feeds back the noise component of the tube current back to the 1st amplifying stage via R28, the noise control adjustment. R27 sets the standby current to, nominally, 4A; R26 controls the gain of the circuit when regulating on the outer loop. OK1 is initially switched off so the top of R26 is nominally 0V therefore the feedback amplifier output depends only on the setting of R27 and the tube current signal. The feedback signal output from IC3c feeds into one of the error amps in IC4, a TL494 SMPS controller; IC4's outputs are in parallel and drive IC5, a DS0026 MOS inverter /driver which, in turn, drives the MOSFETs Q6 - Q8; as a result, IC4 is operated in an unusual inverted mode whereby the switching FETs are conducting when the output of IC4 is off, so the feedback signal goes into the non-inverting input of the error amp. The internal oscillator of IC4 runs at 100KHz, determined by R54 and C18 and because the output control pin is grounded, the PWM output from the device is also at 100KHz. Note that the ground of IC4 is connected to the -15V power rail not 0V, this means that the operating region for regulation is when pin 16 is between -11.5V and -14.5V; when the voltage here is greater than -11.5V, the outputs of IC4 switch off causing the FETs to switch on and thus send maximum current through the tube; this current is limited only by the internal resistance of the HT supply circuit, the Rds of the FETs, the 2 current sense resistors (1 in the laser head) and the cabling resistance.

Prior to tube ignition, IC3c's inverting input is negative w.r.t. the non-inverting input, the actual voltage depending on the setting of R27. IC3c runs open loop at DC so the output swings to the +15V rail therefore causing the switching FETs to be turned on; this protects the FETs from any stray transients from the igniter circuit and the high di/dt waveform through the inductors. When the tube strikes, the current very rapidly rises to around 25A then falls slightly to about 18A; as a result the current monitor signal is at around +2.3V and IC3c's inverting input now becomes + w.r.t. the non-inverting input, therefore the output of IC3c starts to fall to the -15V rail at a rate largely determined by C8 as D8 is now conducting and damping the response of IC3c (see starting problem below); when the voltage reaches -11.5V the SMPS starts to regulate and the tube current reduces until the inverting input of IC3c reaches 0V, the PSU is now in closed loop control at the standby current.

Outer loop operation

The current monitor signal, isolated by OK2, feeds into the current signal amplifier IC2d whose gain is set to produce 100mv /A of tube current; this signal feeds out to the remote current monitor, pin 26, of the remote connector and also into IC2b. IC2b is a threshold comparator with its inverting input held at +180mV derived from 9.1V zener diode D4 with R23 and R21 acting as a potential divider; when the inverting input becomes higher than +180mv (1.8A of tube current), the output of IC2b goes high causing the current detect signal, pin 12, on the laser head connector to go to approximately +14V via diode D3; this signal is not used in the ALC60X or Omnichrome 532 head, but is used in larger models, particularly Krypton filled types. IC2a acts as an integrator with a time constant determined by R19 and C5; when IC2b's output goes high, the output of IC2a will go low approximately 100ms later, this pulls the cathode of OK1's LED low and enables OK1 thus putting the PSU into outer loop regulation. OK1 can be disabled by putting the remote standby input, pin 34, on the remote connector to a voltage of less than 0V (e.g.-12V on pin 24), thereby putting the PSU into standby operation at approximately 4A of tube current.

IC2c forms a reference voltage source that supplies the front panel power control pot and the power control pot in the laser head with a +5.1V reference; the reference voltage is adjusted with R23.

The current signal from IC2d is also fed into comparator IC1b where it is compared to the voltage across R11; R10 and R11 form a potential divider reducing the 1A /V current control signal on the remote current control, pin 6, of the remote connector to 100mV /A, therefore when the current in the tube exceeds the current set at the remote current control input, IC1b's output goes low. R9 and C2 reduce the HF gain of this stage to improve stability.

IC1c amplifies the light output signal from the laser head's optical pick-up and this signal is then fed out on the remote light output, pin 29, on the remote connector. The light output signal from the laser head also feeds into IC1a where it is summed with the inverted remote light control signal from IC1d, thus when the light output of the laser exceeds the level set at the remote light control input, pin 3 (or pin 5), on the remote connector, the output of IC1a goes low.

IC1a and IC1b are connected, via diodes D1 and D2 respectively, to the anode of the LED in OK1. When either (or both) IC1a or IC1b's outputs are high, current flows in OK1, causing the voltage across R25 in the inner loop to become negative; this voltage feeds into the feedback amplifier IC3c where it increases the current demand thus increasing the duty cycle of the switching FETs. As the duty cycle of the FETs increase, the light output and current in the tube rise until both IC1a and IC1b's outputs go high again, thus regulation is achieved via the outer loop. Current or light control modes can be used by leaving the unused input open or the two modes may be combined by supplying drive signals to both inputs, whichever is set highest will take control.

Starting problem

When the tube first ignites, the current very rapidly rises to a high value for approximately 1ms; if the current remains at this high level, around 18A, the plasma is highly unstable and the discharge is blown out, Chart 1 shows a typical ignition current waveform. The problem is that the current demand signal at IC4 pin 1 falls very slowly as a result of D8 in the feedback loop of IC3c, taking 35ms to reach the regulation region of -11.5V to -14.5V (Chart 1 shows the control signal with the control loop modelled in Spice), therefore the arc becomes extinguished before the PSU goes into regulation. Removing D8 causes pin 16 to reach this region in less than 200us as measured with my oscilloscope (the Spice model shows closer to 650us, see Chart 2) and the tube ignites first time every time, but if the current is increased above 8A, the plasma becomes very unstable and the power supply breaks into oscillation resulting in rapid current rise and, usually, the plasma extinguishing. If any EE has a suggestion as how to improve the stability of the loop without compromising the time it takes to start regulating, please contact me, thanks.

Last updated 28 March, 2004