02-08-2012, 07:20 PM
Hello John, the bit about the diode dropper I wrote in haste, I was trying to say that if using a diode dropper and if for some reason it goes short circuit there will be consequences, hence the crowbar diode, for anyone not familiar the crowbar diode takes the full AC negative output from a short circuit diode dropper and blows a suitably rated fuse, the "cathode" of the crowbar diode is connected to the rectified side of the diode dropper, normally the "cathode" The anode of the crowbar diode is connected to chassis/mains return, a suitably rated fuse is connected in line with the "anode" of the dropper diode.
If the diode dropper is conducting on positive peaks as designed then it's business as normal but if the dropper diode goes short circuit then the negative part of the AC waveform will be shorted to chassis via the crowbar diode and thus will blow the protective fuse in the dropper diodes "anode" circuit. From this it can be seen that any additional dropper resistor reqiured is normally connected in circuit after the diode dropper.
Hope this helps.
Lawrence.
If the diode dropper is conducting on positive peaks as designed then it's business as normal but if the dropper diode goes short circuit then the negative part of the AC waveform will be shorted to chassis via the crowbar diode and thus will blow the protective fuse in the dropper diodes "anode" circuit. From this it can be seen that any additional dropper resistor reqiured is normally connected in circuit after the diode dropper.
Hope this helps.
Lawrence.
