DC on AC supply

The saving grace for the RCD and ADS generally  is that during any fault of significant current fault the diode or triac, switch more power supply chip  or whatever usually stops diodeing and becomes a near short, as the semiconductor gets hot and all the carefully placed doping chemicals diffuse and mix together so 'P' and 'N' regions are no longer discernible.

So, rather like all small children's paint mixing experiments result in brown, and all Scout's cooking looks burnt, all overheated semiconductors, even expensive computing ones, tend towards a rather mediocre low value  resistor. Of course in a high current fault the metallisation gets blasted off the chip surface or eventually  the bonding leads just evaporate, so above a certain I2t it does its own ADS.


Also unless there is a smoothing capacitor, any fault  waveform will trip  the RCD type A kind, and many type ACs not anything more expensive.

(and if the smoothing cap is electrolytic, and it nearly always is, then once the diode has gone and it is being subjected to the full AC, you only have a short time to duck before the content of the capacitor follows it. Teenage years of TV repairs and similar leave memories that are too deeply etched to forget.)


As an aside

Given that a smoothing cap will hold the voltage up you can make a 3 level heater with a diode and a C, at least for low wattages like soldering irons,

1) pre--heat,single  diode in series, half power, soldering iron on stand, I have gone for a break

2)run full waveform (either mains direct or  bridge rectified, heater takes either) indoor normal soldering of small parts

3)boost full wave rectifier plus smoothing, DC of ~ 330V - ideal for soldering large parts or outdoors...

The saving grace for the RCD and ADS generally  is that during any fault of significant current fault the diode or triac, switch more power supply chip  or whatever usually stops diodeing and becomes a near short, as the semiconductor gets hot and all the carefully placed doping chemicals diffuse and mix together so 'P' and 'N' regions are no longer discernible.

So, rather like all small children's paint mixing experiments result in brown, and all Scout's cooking looks burnt, all overheated semiconductors, even expensive computing ones, tend towards a rather mediocre low value  resistor. Of course in a high current fault the metallisation gets blasted off the chip surface or eventually  the bonding leads just evaporate, so above a certain I2t it does its own ADS.


Also unless there is a smoothing capacitor, any fault  waveform will trip  the RCD type A kind, and many type ACs not anything more expensive.

(and if the smoothing cap is electrolytic, and it nearly always is, then once the diode has gone and it is being subjected to the full AC, you only have a short time to duck before the content of the capacitor follows it. Teenage years of TV repairs and similar leave memories that are too deeply etched to forget.)


As an aside

Given that a smoothing cap will hold the voltage up you can make a 3 level heater with a diode and a C, at least for low wattages like soldering irons,

1) pre--heat,single  diode in series, half power, soldering iron on stand, I have gone for a break

2)run full waveform (either mains direct or  bridge rectified, heater takes either) indoor normal soldering of small parts

3)boost full wave rectifier plus smoothing, DC of ~ 330V - ideal for soldering large parts or outdoors...