AboutDave Nyce Expertise I have been an electronics engineer for 25 years. I can answer questions on analog and digital circuits and my specialty is sensors.
Experience I am the inventor on 23 US patents, and also some foreign ones. Developed sensors for over 25 years. Licensed private pilot (airplane and rotorcraft), have HAM radio license. I'm not an expert in computer networking.
Expert: Dave Nyce Date: 6/13/2008 Subject: Half Wave Voltage Multilier Problem
Question Hi Dave, I hope you can help me with this problem. I have one of those Tennis Racket electronic bug zappers that runs off 2 C Batteries. The circuit is fairly straight forward. There is a 2 transistor oscillator into a small transformer that produces approx 350 V on the output which then goes through 3 stages of conventional half wave rectification - two capacitors and 2 rectifiers per stage. The chain of 3 stages ends up producing 1500VDC across which is placed a 0.1 Uf cap and a 22M resistor as a load. It works fine for mosquitoes and samll flies but I needed a little more zapping power. So I wired a 0.5Uf (at 2KV) across the 0.1 Uf cap. I get one or two nice zaps and then one or more of the rectifiers goes short. (All of the caps are rated at 1KV except the output which are 2KV; all of the rectifiers are 1A 1000v 1N4007).
I replaced the rectifiers 3 times and each time one or more of them shorts out after between 1 and 3 discharges. The caps are all fine.
I am assuming that when the cap discharges, the current is very high (>1A for short time). But since the rectifiers are reverse biased I did not think they would be affected. If anything, I would think that the caps might over heat.
Finally I put a 1500 ohm wirewound 5 w resistor in series with the output to limit the current. This works and the diodes are fine but the current limiting produces a discharge which is way less than the unmodified unit with the 0.1Uf output.
Any ideas or suggestions?
Thanks and regards, Murray
Answer If you have 1500V, it looks like the diode rated voltage is too low, at 1000V. You need to change to diodes to ones rated at 1800V or higher. The 1A rating should be OK.
The manufacturers of consumer goods are very frugal with parts cost. It seems they must have found that the 1000V rated diodes could survive if the capacitance was only 0.1 uF. When you increased the capacitance, then the stored energy became sufficient to destroy the diodes when exceeding the reverse voltage rating.
