UTRACER AND HEATER REGULATOR BOARD
A while back I built a vacuum tube curve tracer designed by Ronald Dekker which measures tubes and produces curves to compare to the spec sheets to assess tube health and performance. One weakness of this design was in its ability to drive the filaments, which is overcome by a heater regulator board design from Stephan Lafferty.
REGULATOR BOARD BUILT
The regulator board made use of a very accurate 2.5V reference source to precisely control the voltage of the filament supply through an opamp feedback loop, with extreme precision resistors used to set the voltage points to generate 5V, 6.3V and 12.6V output levels at up to 5A.
It also is a four wire delivery system, meaning that there is a separate sense wire for both ground and + voltage levels. Thus any voltage drop on the supply wires to the tube socket is detected because the sense voltage is lower than the target. The heater regulator board design is controlled to drive whatever voltage is necessary to produce the exact filament voltage as seen by the sense wire.
The buck converter used to drop the 19V incoming supply voltage to the desired filament voltage was a board sourced from eBay that was built around the XL4005 IC. This board was manufactured in China and provided 5A buck voltage regulation with either constant voltage or constant current control, but at an extremely low price, thus it was not worthwhile for Stephan to design his own converter.
The part specified in the design of the heater regulator board was no longer available on eBay or Amazon. I found some similar boards, based on the XL4015 chip, whose specs and layout appeared to match. I chose to buy those but when I tested the overall heater regulator board, it didn't work. I lazily assumed that XL4015 was a more modern and potentially slightly cheaper chip that the makers had switched to.
I finally switched on my brain recently and downloaded the spec sheets of the two chips, XL4005 and XL4015. Two differences were apparent - switching frequency and the set point for the regulation.
The original chip operated at 300KHz while the newer regulator chip operated at a lower 180KHz. This might introduce some remnants into the uTracer which weren't filtered by the component values on the heater regulator board.
The fatal difference, however, was in the set point of the chip. The XL4005 used 0.8V as the feedback level, while the newer XL4015 dropped that to 0.2V. The entire design of the heater regulator board was oriented around monitoring the filament output voltage and selecting a divider network to produce 0.8V exactly when it was on target, that being driven through the opamp to control the XL4005 board.
I dug around once I realized this and found some different buck regulator boards that did used the XL4005. The first order I placed on Amazon disappointed as the boards came with XL4015 and the vendor had sneakily updated the item description on Amazon so that it now claimed to use XL4015. Luckily it wasn't a very high cost item so I simply dumped them in the trash.
The next try was successful, the boards were indeed built on XL4005. The modification for the heater regulator board opens the onboard feedback from a small potentiometer and instead connects the feedback pin to the opamp output. The board works perfectly.
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