Chinese desoldering gun S-993, S-995, S-998

[Vytautas]

Hello, guys,
recently I was using my S-998 desoldering gun and suddenly, BUM, electronics exploded inside the handle. Happily, no injuries.
I opened it up and found four components dead: two resistors (47k and 51k), a 22V zener, and a FR107 diode (all indicated in the schematic and the picture attached).
Some tracks have blown up also. I suspect, one of the devices failed and shorted. Replaced the affected components, used two jumpers instead of blown up tracks and voila, its working again.
Just for the sake of interest, I took my Cat S60 phone and looked at how it works through the thermal camera. Noticed that the resistor R5 is heating up like crazy, to almost 110 degrees Celsius.
For the sake of more reliability installed a 2W resistor instead of the original 1/4W. But I wonder, why is such resistance for current limiting is chosen? What bad could happen if I increase the value to, say, 100K? It would dissipate less heat, but I wonder would it create any problems?

[theHWcave]

Hmm,  according to the datasheet for the BZX79C-22 it needs a zener current of 5mA. At 240V AC it means that resistor is dissipating the remaining voltage of 240V-22V -1.2V  = 217V and at 5mA that amounts to 1.09W.  The 1.2V btw is the drop over D4.  No wonder that the original 0.25W resistor failed.  The value of the resistor is 217V / 5mA = 44K or so. To answer your question, no you should not increase the resistor, because the Z-diode needs that current to properly work, but as you already done, put a 2W type or more in, and make sure it is mounted in a way that it can get rid of the 1W-worth of heat it produces.

[Vytautas]

Thank you very much! Now everything is clear.

[Ian.M]

Its certainly *NOT* running at 5mA zener current.  Assuming 230V 50Hz mains supply,  the maximum average current through a 47K resistor in series with a diode is 2.2mA.  As the output voltage is in the 20V-22V range, the current through the resistor will be about 10% lower as it doesn't 'see' the lowest 20V or so of each positive half-cycle.  After you subtract however much current the OPAMP uses, the Zener current is probably under 1mA.

If you've got enough space and can rearrange the circuit a bit, you can reduce the dropper resistor dissipation to 0.1W.  To do so, you need to convert to a capacitive dropper, which only has a resistor to handle surges and fast spikes.  The voltage drop across the resistor should be approx 10% of the supply voltage and, in normal operation, the capacitor handles the current limiting.

To convert it, you'd need to replace the 47K resistor with a 4K7 resistor in series with a 68nF class X2 capacitor, and move the cathode connection of the Zener D3 to the anode of D2.  I'd also recommend increasing C1 to 22uF.  A 68nF class X2 capacitor should be approx 18x5x11mm, and there should be enough space to mount it track-side.  To rewire D3, its probably best to also move it to the track side, near D4.

See attached LTspice sim.