Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

For the purpose of learning, I have tear down a small mosquito zapper racket.

enter image description here

I am now trying to understand how the circuit is working (which goal is to produce high voltage).

enter image description here

(yellow = resistor, black = transistor, red = led, gray = diode, blue = capacitor, brown = switch or wire) Battery is on the left, high voltage output on the right.

How I think it works : the transistor generate pulse (square wave?) which is fed into the transformer. The transformer multiply voltage, then voltage is rectified trough the diodes, and finally stored in the big capacitor. Can anybody confirm this ?

Also some other things are still unclear to me :

  • How does the 6 pins transformer works ? I already saw 5 pin transformers (http://en.wikipedia.org/wiki/Center_tap) but never that one.
  • How does the transistor generate the pulses. I have seen some led blinking circuits but they always involve a transistor and a capacitor.
  • What is the purpose of the two resistors in parallel with big capacitor? (to limit voltage, for safety ?) and also why do we need the small blue one ?

EDIT1 : I tried to reverse-engineer the circuit. Here is the result. I'm no sure about transistor configuration (eg : if base, collector and emitter are correctly set) and same about transformer.

EDIT2 : I open racket again. I found transformer pin-out by measuring it with multimeter (as jippie suggested). I also found transistor configuration by reading number information on it and checking datasheet. It seems what Spehro Pefhany suggested was right.

schematic

simulate this circuit – Schematic created using CircuitLab

share|improve this question
    
Can you reverse-engineer the board and post a schematic, including the power supply and the connections to the HV screen and the yellow thing (inductor?) on the right? –  EM Fields Jul 13 at 19:20
    
OK, as suggested, I will try to upload a schematic. –  tigrou Jul 13 at 19:50
    
What's the part number on the transistor? It looks as I expected in my answer except for the transistor leads mix- I think your emitter should be base, your base should be collector, your collector should be emitter. –  Spehro Pefhany Jul 13 at 23:14
    
Did you manage to measure the transformer's pinout or did you guess? It took me a second to realize D4 is an LED. –  jippie Jul 14 at 5:32
    
For the transformer pinout : I guessed it, maybe it is totally wrong. I did not think about finding pinout by measuring, that's a great idea, I will try to do that. Things that are still not cleared : brown capacitor capacitance and max voltage (nothing was written on it) and final output voltage of the circuit. I wanted to measure it but my voltmeter is rated 600V max. I do not want to fry it (output is probably several kV). –  tigrou Jul 14 at 7:57

2 Answers 2

up vote 5 down vote accepted

Looks like a self-oscillating converter, probably a primary winding, a feedback winding and a secondary winding (6 pins). It would be similar to this, but with many more turns on the secondary:

http://www.next.gr/uploads/135-9320.png

I think this is a blocking oscillator with the transformer primary in the emitter of the transistor and the feedback winding blocking the base voltage.

The transistor is probably a cheap high-current BJT such as an 8550 with a base resistor and nothing else. The blue capacitor and the brown film capacitor form a voltage doubler (there is room for parts for more multiplication, but they're not populated). The two resistors are to discharge the capacitor- one is a 22M and the other is a 20K. When they use the tripler configuration they probably use two 22M or a 22M and a 10M resistor. Th

The LED is just across the input power (after the tact switch) with a resistor in series.

These things have to make with BOM adding up to not many pennies, so everything is minimized. The output capacitor is probably being run at way over rated voltage, the transistor will probably burn out if you hold the switch down, the transformer insulation is unsuitable for anything but brief momentary operation. Minimalist, fully Muntzed, design philosophy.

share|improve this answer
    
Thanks for your help. I still have one question. In zapper circuit I shown, the voltage double is clearly something similar to this : coolcircuit.com/circuit/voltage/vd2.gif. I try to reverse the other alternative circuit (with tripler). What I got is this : s3.postimg.org/fc41qnfr7/tripler.jpg. I'm not sure if this is correct. It does not look like voltage tripler you usually see (eg : coolcircuit.com/circuit/voltage/vt.gif). What kind of voltage tripler did it use ? Does my schematic looks correct ? –  tigrou Jul 18 at 21:49
    
There are different ways of making a tripler: Here is the same one as yours: sub.allaboutcircuits.com/images/03283.png Your schematic looks okay. What marking is on the transistor?? –  Spehro Pefhany Jul 18 at 23:15
    
The exact information written on it is H SH 965 7L R. I posted link to datasheet in answer. –  tigrou Jul 19 at 8:16
    
Thanks, interesting. That part originated for photoflash applications, so that's quite possibly where the zap circuit HV generation oscillator evolved from. –  Spehro Pefhany Jul 19 at 11:51

It's most likely using the transformer in flyback mode to make high voltage pulses. With 6 leads, it is probably a true transformer. This sort of thing can be done with a auto-transformer too. It's probably as simple as driving the primary with pre-determined fixed-length pulses.

To get more detailed answers about this particular circuit, trace out the connections and show us the schematic. The first schematic you create will be a mess as you draw parts someplace on the paper then draw connections as you discover them on the board. We don't want to see that one. Redraw it with logical layout. Try to put high voltages at top, low votlages as bottom, and logical flow left to right as much as this is reasonable to do.

Since the transformer has 6 pins, it probably has a center-tapped primary and secondary. That may be a generic transformer used by that manufacturer in other products. It may not be optimal for any one product, but useable in all and cheaper at the higher volume.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.