Bug Zapper [repaired] - measuring high voltages and reverse engineered

[ornea]

Cheap Fluro based bug zapper has only ever mildly sizzled bugs often non-fatally since new. 

I reverse engineered the circuit and thought I would share the schematics and related images for the curious or someone wanting to do a similar repair. 

This turned out to be a fault lug and screw termination which was found after 2 repair attempts.  Its now zapping bugs with a satisfying zap-crack as I write this.


Reverse engineered schematic which looks like a typical Cockcroft–Walton multiplier.  Output approx 1432VDC.


schematic with annotated nodes to help reverse engineer


pcb annotated


high voltage - voltage divider


Zapper Picture

My second repair attempt revealed that the lug - screw connection had gone high resistance with burn marks on the lug.  They use a screw and the lug and it also  bites into the grid.  However, the screw would need to be the perfect length to simultaneously tighten down on the lug and bite into the grid.  Obviously, this was not the perfect size, so did not mate properly.  I almost didn't catch this fault.  Just about to re-assemble and thought I would just do a quick ohms test from output of multiplier to the grid.  One path was good, the other was open circuit even thou it passed the visual inspection. 

I measured approx 84VDC using the voltage divider circuit above which equates to approx 1432VDC.  I didnt analyse the voltage divider circuit, I simply fed in 30VDC and measured 1.76VDC and used this as the multiplier ratio i.e. 30/1.76 x 84 = 1432VDC.  This seemed low compared with other figures quoted on the web of 2kvdc-5kvdc so I began to assume it was just a poorly designed circuit until I found the fault.

[golden_labels]

Are you sharing your work or is the question missing?

⁂

The 2 kV figure is not a lie. It seems to “only” be missing an important detail!

It feels like the designer didn’t fully understand the operation of the circuit. It seems as if somebody thought that in a Cockcroft-Walton multiplier a single stage is a single capacitor and a single diode, and what they make is a 5-stage multiplier giving 5× higher voltage.

But a single stage is an entire assembly of two diodes and two capacitors. So first we have a proper voltage quadrupler: C1…C4, D1…D4 in your schematic. Depending on conditions it produces solid 1.2–1.4 kV. After that there is a “funny half-stage” (C5, D5). This adds a sine wave of 600–650 Vp-p.

I guess your measurement is 1.2-1.3 kV plus 300–320 V from that sine wave, slightly sagging under the load. Measured as DC, so the average value. Marked as “M” on the chart below:

⁂

As for the divider, mind accuracy. Circuits that have high output impedance are a bit tricky.

For this setup it didn’t matter that much, because it is already all over the place. But let’s take it as an example. Ignoring a minor influence from the DMM, this divider presents about 9 MΩ. Since the voltage multiplier has high output impedance (in the order of 100 kΩ), the measured voltage will sag by a good percent or two just from that measurement.

You also calibrated the divider with a known voltage source. That is a good choice! But if you didn’t and just went with 8:1 ratio, there is a trap. The DMM itself is about 10 MΩ, which would be in parallel with 1 MΩ. It is a small influence, but not something that can be completely ignored.