Huge cap inside battery charger

Just for the fun of it I opened the battery charger that came with my Atlas Copco (AEG/Milwaukee) cordless power drill. I was expecting to find some sort of SMPS but found a SCR instead (a TC820). But what astonished me most is the huge 25 uF 420/470 Vac cap.

What I wonder about most is why this cap? Surely there are cheaper alternatives...


  • Charger pcb top.jpg
    Charger pcb top.jpg
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  • Charger pcb bot.jpg
    Charger pcb bot.jpg
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I think that the SCR is used in this circuit to keep the voltage to the desired level as it buzzes loudly while charging (being switched on and off constantly). When no battery-pack is connected, there seems to be no power drain from the mains (or is below the detection threshold of a Voltcraft energy monitor). Without the pack, there's only 1.4 V across the terminals suggesting the battery-pack itself becomes part of the circuit and is not just connected to it.

I measured the power consumption during and after finishing the charge cycle and you can clearly see that it goes into trickle charge mode. Without having to set anything on the charger, it takes battery packs ranging from 7.2 to 18 V. All this with such few components!...

The IC in the background is just a quad comparator, BTW.
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