Steve Ward's Universal DRSSTC Driver is the board most amateur coils are built on, directly or through a clone. Knowing which revision you are holding matters, because the part designators moved and the schematics did not always follow.
What it is and why
One board doing the four driver jobs: feedback from a current transformer through a comparator, a phase lead made of a series L + R, an enable from the interrupter, current protection from a second transformer, and an output stage into a gate drive transformer.
The reason it is worth knowing as a line rather than as a product is that the logic core did not change from 2.1 through 2.5 to 2.7. Modifications published against one revision transfer legitimately to another. What does not transfer is the part numbering.
The revisions
- UD2.x, the classic. Comparator,
L+Rlead, GDT. - UD2.7, from loneoceans. The visible change from Ward's bill of materials is the lead inductor family: Ward specified Coilcraft Slot Ten parts, 10 mm, and UD2.7 moved to the 7 mm Slot Seven 7M3. Everything downstream, UD2.9, WaskaLabs and the Chinese clones, inherited the 7M3 rather than the Slot Ten.
- UD2.8A, from Eastern Voltage Research, documented in a March 2014 datasheet. Unique in the line for supplying the lead as swappable inductor modules rather than a core-tuned shielded can.
- UD2.9 and UD29X. Pulse skip added, UVLO by resistor, one optical receiver. The WaskaLabs UD2.9-X is the open branch the clones derive from.
- UD3, from Netzpfuscher. Programmable: interrupter on board, a CLI, USB-MIDI, autotuning, telemetry, master and slave. This is where the line gives up on the inductor and sets the lead as a time instead of an angle, in nanoseconds.
- UD+. Freewheeling: over current it lets the current circulate rather than cutting the bang.
What will get you
Removing the L+R section is not a simplification. A builder who rebuilt a UD2.7 without it reported hard switching with substantial overlap of current and voltage in the devices, which is the whole failure the network exists to prevent.
The numbers
- Feedback burden on UD2.7: 51 Ω. The companion resistor in that divider is 1 k.
- Lead inductor footprint from UD2.7 onward: Coilcraft Slot Seven 7M3.
- UD3 lead setting: nanoseconds, entered directly, typically 1000 to 2000 on large slow bricks.
- The ceiling on any
L+Rlead: a quarter of a period, becausearctancannot exceed a right angle. At 490 kHz that is 510 ns; at 72.5 kHz it is 3.45 µs. Derived here, not stated by any source, and it is why a digital driver can offer a big brick a lead an inductor cannot.
What is not established
- Who designed UD2.5. Ward's published design files end at UD2.1 revB, March 2011, and a later folder on his own site holds a single 1.1 kB text file. The absence of UD2.5 files there is not evidence that he did not make it: the two forum threads that would settle it return server errors and remain unread. The honest form is that the question cannot be answered from what is online now.
- How much processing the job actually needs. One builder abandoned an FPGA driver because digital phase lead wanted something like 450 MHz; UD3 does the same work at 64. Nobody has reconciled the factor of seven.
What goes wrong
- A clone will not start on settings copied from a UD2.7 thread. The feedback diodes, above.
- A modification does not fit the board. The designators moved; the logic did not. Trace the function.
- Hard switching after a "cleanup" rebuild. The
L+Rsection was left out. - The lead is right at one frequency and wrong at another. An
L+Rnetwork holds an angle, and the hardware needs a time. On a wide sweep that is the technique running out, not a component fault.
Where next
- Inside the driver, what the board is doing and why.
- What a phase lead actually buys.
- Choosing a phase lead inductor, which is the part these revisions keep changing.
Attribution: the revision facts here are from published design files, vendor datasheets and forum threads by the people named. Where a claim is ours rather than theirs, it says so.