µTracer (uTracer), part 8: Testing the DIY heater supply

As when testing the µTracer’s internal PWM heater supply, a dummy load with 4, 8 and 16Ω resistance is used. Voltage and current measurements are performed by a multimeter.

Output voltage accuracy:

Heater PSU set to 12.6V, and loaded with 16Ω. Output is very close to the nominal value.

12.6V, 8Ω. Pretty much spot on. This looks promising so far. What I do notice, however, is a decrease in output voltage as the XL4005 heats up.

12.6V, 8Ω, after leaving the power on for a while, to let the XL4005 get warmer. The voltage decrease is noticeable, but after all, this is just 0.3% below the nominal value.

6.3V, 4Ω. Voltage loss in the wires become more prominent, but 0.6% below nominal value is fully acceptable.

Conclusion: This heater supply provides significant accuracy improvement over the PWM solution, especially at low voltages. There is some voltage decrease as the XL4005 heats up, probably due to temperature coefficient the internal 0.8V voltage reference. This is after all a cheap buck converter, and it would probably possible to improve the temperature stability by using a more advanced buck converter chip.

Wire loss can be reduced by using thicker wire, and keeping the wires as short as possible. To fully compensate for the wire loss, I would have to extend the buck converter’s voltage feedback loop all the way to the tube sockets. This, however, would mean adding a voltage feedback line from each tube pin number, and the patch panel (described in next post) would need 2-pole connectors instead of 1-pole.

Current limiter

When designing current limiter for the heater PSU, I calculated resistor values to give a max selectable current limit of 2.5A. One concern was whether the current limiter would be stable, i.e. not start to oscillate. It’s time to find out whether it works as planned.

With 12.6V nominal voltage, 8Ω load, and current limit set to max (2.5A). It may be hard to see on the picture, but measured ripple is about 17mV RMS.

With 12.6V nominal voltage, 8Ω load, and current limit set to 1.3A. The current limiter is now active, and limits the output voltage to 10.6V. The ripple stays at the same low level as before, so there is no sign of oscillation.

With 12.6V nominal voltage, 4Ω load, and the current limit set to max. The current limit kicks in and limits the output to 10V/2.5A, as planned.

From the pictures above, we can see that green LEDs around the voltage potentiometer are lit when set voltage is the limiting factor (current limiter is inactive). When the current limiter is active, yellow LEDs around the current potentiometer are lit. Orange LEDs around the selector switch are lit when the heater output is turned on.

When comparing current values in the display with multimeter values, I noticed a small discrepancy. This was calibrated by changing shunt resistor value in the code.

Next post is about putting the pieces together in an enclosure.

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