26-04-2012, 01:40 PM
Hmm: that I do not like the look of. 
The HT bridge rectifier will produce at least 300v. across its reservoir capacitor. Between that cap. and the Zener diodes we have R5: 470 ohms. Calculation gives the power dissipated in R5 in the region of 100 watts! 
And a current through it of about 450 mA. On no-load (or a low load current) that 450 mA will flow through the Zener diodes. So each one will dissipate about 13 ~ 15 watts.
Unless my arithmetic is wrong, alarm bells are now starting to ring! I can just about accept that it will be necessary to mount the Zeners on a very substantial heat-sink, but what about R5? The problem is clearly the 'secondary' of the mains transformer: it needs to be a 12v - 120v item.
Could there have been a mistake in the drawing?
Al.
The HT bridge rectifier will produce at least 300v. across its reservoir capacitor. Between that cap. and the Zener diodes we have R5: 470 ohms. Calculation gives the power dissipated in R5 in the region of 100 watts! And a current through it of about 450 mA. On no-load (or a low load current) that 450 mA will flow through the Zener diodes. So each one will dissipate about 13 ~ 15 watts.Unless my arithmetic is wrong, alarm bells are now starting to ring! I can just about accept that it will be necessary to mount the Zeners on a very substantial heat-sink, but what about R5? The problem is clearly the 'secondary' of the mains transformer: it needs to be a 12v - 120v item.
Could there have been a mistake in the drawing?
Al.
