There are currently eight trimming potentiometers (trimpots) on the current supply circuit, labeled K1, K2, K3, K4, K5, K6, R26, Load_Set. This is a short explanation of the function of each and how they should be adjusted, ordered appropriately. Also included are some suggested values for the components that do not have listed values in the schematic.
In this document, enabling the current output refers to SW1, and the manual short refers to SW2 (diode is shorted when switch is in down position).
Description: The circuit is meant to protect the laser diode by shorting the output whenever the circuit is not in use. In particular, it shorts the output whenever the current is set to “almost zero”, the meaning of which is set by K2. The output will remain shorted unless the current set point is higher than the value set by K2.
Setting: Disable SW1 and enable SW2. Turn K2 all the way down. The output enabled indicator (LED3) may turn on. If it does, turn K2 up just enough that LED3 turns off again. Enable the current, and set the coarse and fine current potentiometers to zero. Turn K2 up just enough that LED3 turns off, and then turn a little more. Increasing the current setpoint slightly should cause it to turn on.
Description: The current supply circuit senses its current output by measuring the voltage across a 0.1Ω resistor (the SR20). This is buffered by a INA128 (whose gain is set by K4) and sent to the PID servo and current monitor. For the TA controller, we want 1V=1A on this monitor.
Setting: To set K4, connect a voltmeter up to the current monitor BNC and an ammeter up to the current output. The feedback loop ensures that the voltage monitored at BNC2 is the same as the setpoint. Use the pots to set the monitor to a relatively large voltage (like 2.000V—you should be able to set it to at least three, maybe four significant figures). Next, adjust K4 until the 1V=1A condition is met. The current monitor should not register any change in voltage if the feedback is working corretly. If you can't reach this condition, you might need more gain on the OPA549, set by R26. (If you only have one multimeter, short the current output and set the current monitor voltage, then use the ammeter to measure current as K4 is changed.)
Description: This trimpot adjusts the voltage to the LCD readout.
Setting: Enable the current (output shorted or unshorted, but if unshorted, there needs to be something for the current to flow through, such as an ammeter). Set the current to some nice value, using either the ammeter or a voltmeter on the current monitor output, and then adjust K3 until the reading on the display matches the current. At this point, it's nice to double check that the various outputs scale together. Hook an ammeter up to the output, a voltmeter to the current monitor output, and set various currents. Hopefully, the current monitor output, ammeter, and LCD should all read the same values for pretty much any current setting.
Description: The current modulation allows you to adjust the current setting about the set value, and K1 adjusts its scaling. This modulation feature is not necessary in typical operation of a TA, but is a legacy part of this controller. It should be noted that the modulation does not pass through the same protective ramp as the rest of the current supply, so large modulation amplitudes may be harmful to the diode.
Setting: Set the current with nothing connected to the modulation input (or with the input shorted). Apply a voltage across the modulation input, and use K1 to set the scale. The scale is nominally 50mA per volt, so try something like setting the current to 1A, putting 1V across the modulation input, and setting K1 so that the current was 1.05A. Again, check to make sure it works at other scales.
Description: There is a current limiting feature on the board, with an indicator light (LED1) to notify when you attempt to exceed that limit. The circuit that works this is a simple active rectifier (using IC4) with a “zero” determined by Load_Set (buffered by IC3). A comparison between the input and set point (done with a second op-amp on IC3) is used to trigger the indicator (LED1).
Setting: Short the output manually, enable the current, and turn Load_Set up all the way. Set the current to a little over the desired maximum value. Turn Load_Set down until the indicator turns on, and the current decreases. Use that to set the current to the desired maximum value. Test by turning the current down below the set value (make sure LED1 turns off), and then turning up above that value—LED1 should turn on, and the current should not exceed the set value.
Description: The OPA549 also has a current limiting feature, which is controlled by the resistance between pins 6 and 8. This is used as a secondary current ceiling to protect the diode.
Setting: Short the output manually, enable current, and adjust the set point to a little above the point where LED1 turns on. Adjust K6 until the output current just starts to decrease.
Description: Since it is detrimental to operate the TA while there is no seed light, the circuit includes a safety to automatically disable current output if the signal from a photodiode which monitors the seed is too low. This monitor is compared to an adjustable voltage set by K5. Since this is an adjustment that will be made after powering up the TA and determining what power to use in the seed, the trimpot is front-adjustable through the rackmount face.
Setting: Send a buffered photodiode signal (positive voltage) to BNC3 and adjust K5 so that the voltage on pin 3 of IC10 is just below that on pin 2. To operate the circuit while
Description: This trimpot controls the gain of the OPA549, and is really in place in case more oomph is needed to output the amount of curent desired.
Setting: In some sense it amounts to doing the same thing as K4, and really only needs to be adjusted if it's set so low that the 1V=1A condition of the current monitor cannot be met (or whatever other equivalence you want to use). Adjust in a similar manner to K4. It is possible that the gain of the OPA549 could have some bearing on the noise of the output (we didn't observe this, but now might be a good time to check it out if you'd like).
In addition to the main trimpots, there are a few other adjustable components.
Rlimit sets a maximum current set point—the maximum current you can get by turning all the knobs up full (assuming the overload protection does not kick in). The table on the schematic outlines some resistance values and their associated current maximuma, and also shows how to compute additional values. The full formula for computing these Rlimit values is: Imax = (6.95V)/(G*Rsense)*Rcoarse/(Rcoarse+Rlimit), where 6.95V is the nominal voltage from the LM399, G is the gain of IC9, and in the current design G*Rsense~1.