John et al - Consulted the reference you gave (page 14-4 in the 6th edition) and can see ```where the figure comes from. The value of 4.2% quoted is, as you said, the RMS value of the ripple / mean output voltage. The RMS value is not really appropriate for this purpose and in fact is rather meaningless; ie. providing a DC supply to a PA. The circuits in the book assumes a series DC choke - something the heavy electrical people would use routinely but unnecessary for our purposes. The actual instantaneous value of the full wave rectified waveform using the 6 diode equivalent of a bridge rectifier, (no connection to the star point) is as follows : Minimum value is 1.5 Vpk at the crest of one phase, current returning to ```the other two phases at their 30 degree points, at amplitude -0.5. Maximum voltage is at the 60 degree point of two phases, where the ```respective amplitudes are SQRT(3) / 2 Vpk and -SQRT(3) / 2 giving 1.732 Vpk. The third phase has a value of 0 here. Hence max possible ripple is 1.732 - 1.5 = 0.232 Vpk, pk-pk having a fullwave rectified shape at 300Hz. Mean output voltage is obtained by integrating over this 1.5 to 1.732 waveform, giving 1.65 Vpk, and a Peak / Mean ratio of the 1.05 quoted in the Data Book. Can't be bothered calculating the RMS ripple value as it is of little value, (and is complicated to do) but it looks as if a figure of ```4.2% would be in the right sort of region. ```If using a capacitive PSU input, the unloaded voltage would just be Vpk, and ripple would be determined by capacitance and load current, but now at 300Hz prf meaning lower values caps. Using the standard mean UK supply of 234V rms nominal (405V phase voltage, peak = 573V) the rectified supply would be 573 volts max, so my earlier statement that 600V FETs could be used was of course rubbish, as the margin is much too low. (Unless using a half wave rectified supply giving the same 340V as available from a single phase + neutral supply, with 150Hz ripple ```frequency - not a good idea) 800 - 1000v FETs are only just beginning to appear and are very expensive ```compared with 500V devices, so the idea of using full wave rectified 3 phase ```supply for a mosfet PA is definitely not on. ```However, US and Canadian stations with their 110V mains (supplied as effectively 6 phases - 3 phase with plus / minus each plus centre tap / ground, seven wires in total, if I understand it correctly based on looking at pictures of power cables in US towns) would result in a 270 V rail so, much more realistic and safe using low cost 500V devices. Andy G4JNT ``````-----Original Message----- From: John Rabson [mailto:[email protected]] Sent: 2001-06-25 19:05 To: [email protected] Subject: LF: 3 phase ripple (was Re: Re: RE: LX1PD) Sorry, I meant rms. For a 3 phase full wave bridge ```RMS ripple/mean OP = 4.2% ```(Ref Data for Radio Engs 5th ed p 13-5) 73 de John Rabson G3PAI `````` -- The Information contained in this E-Mail and any subsequent correspondence is private and is intended solely for the intended recipient(s). ```For those other than the recipient any disclosure, copying, distribution, or any action taken or omitted to be taken in reliance on such information is ```prohibited and may be unlawful. ```