Calibrating a GW Instek GPD-3303S lab power supply

Stock photo of a blower control unit, from

I recently purchased a second-hand GW Instek GPD-3303S three-channel power supply. It’s a great unit and comes at a considerably lower weight and size than my previous bench supply while supplying the same amount of power with a much more fine-grained voltage and current control. Before putting it to use, I wanted to see how well it was calibrated (the voltages looked good, but the currents were slightly off). Locating the service manual on the web was not a big problem, but the instructions did not really line up with my unit.

Entering the calibration menu

First of all, I recommend not to have anything connected to the outputs at this stage. The configuration stored in memory location 1 will be available at the output for the brief moment between steps 6 and 7; in the worst case, that’s 32V@3A. Enter the following sequence of keys:

  1. Press (MEMORY) 1 key
  2. Press PARA/INDEP key
  3. Press CH2 key
  4. Push the CURRENT knob
  5. Press (MEMORY) 4 key
  6. Press OUTPUT key
  7. Push the VOLTAGE knob

If your GPD-3303S is on the same firmware as mine, the following characters are now shown on the display:

    32.000  32.000
     C.A05   C.A05

Voltage calibration

Initially, the unit is in voltage calibration mode, as indicated by the illuminated FINE signal next to the voltage knob. The active channel is marked by means of the correspondingly illuminated button LED, and can be changed by pushing the buttons CH1 and CH2.

Connect a (calibrated) meter to the CH1 output pins, set to a DC voltage mode that can tolerate 32V. Turn the CURRENT knob to toggle between the calibration points (shown on the current displays). The setpoint voltages are shown on the voltage display of the channel that’s being calibrated. They are:

Now adjust the voltages one by one: If there’s a mismatch between the voltage shown on your meter and the voltage display, turn the VOLTAGE knob to adjust the output voltage as close to the setpoint as possible. Then hit the SER/INDEP key, which turns on the fine-tuning mode. Continue setting the output using the VOLTAGE knob. Keep in mind to give the value some time to settle (there’s a capacitor in the output). Once you’re happy with the setting, store it by pushing the OUTPUT button. I recommend to unset the fine-tuning mode by hitting SER/INDEP key again before proceeding to the next calibration level.

Repeat this step for all voltage calibration setpoints on channel 1. Then connect the meter to the channel 2 output, push the CH2 button, and repeat the same procedure for the second channel.

Current calibration

To move on to the current calibration mode, unplug the meter and push the CURRENT knob on the lab power supply. The FINE LED next to the current knob will come on, indicating that you’ve entered current calibration mode. It’s important to note that a current will already be present on the output, but the display won’t change. Connect your meter in DC current measurement mode to the channel to calibrate, using an input that can deal with 3 amps.

By turning the VOLTAGE knob, the different calibration modes can be accessed (the display will show them when turning the knob). Similar to the voltage calibration levels, they are:

This time, the CURRENT knob is used to modify the output current, and again fine-tuning is possible by pushing the SER/INDEP key; the step size then reduces from about 15mA to approximately 0.5mA. Once the output current equals what’s shown on the current display, hit the OUTPUT button to store the setting.

Again, repeat the sequence for all current calibration levels and both channels (push the CH2 button and connect the meter to the second pair of output terminals to repeat the same procedure for the second channel).

Parallel mode

Hit the PARA/INDEP button to calibrate the current when operating the power supply in parallel mode. The display will show the following characters:

    CA.001  PA.r
     1.500   1.500

Turn the VOLTAGE knob left to switch to CA.000 mode (no output current). Click the CURRENT knob to move to the current adjustment (there’s no need to tune voltages here); the FINE LED next to the current knob must activate. Now connect the meter to the channel 1 output terminals, and set it up to measure DC currents up to 3 amps. Rotate the voltage knob to select CA.001 and adjust the output current using the CURRENT knob (possibly fine-tuning with SER/INDEP again) to 3 amps. Once this value is reached, click the OUTPUT key to store the calibration point and switch off the device.

Encountered issues

Sometimes the power supply gets stuck outputting a fixed voltage of 1.5V or no voltage at all, with no output current. In such cases, pushing the PARA/INDEP button twice in quick succession reverts to the voltage calibration mode C.A05. Simply re-try accessing whatever other function you wanted to calibrate from there.

Also note that these are most certainly not the manufacturer’s recommendations to calibrate this device, but a documentation of my reverse-engineered approach to the calibration process because the instructions in the service manual did not work for me.