Sorry Robert, Both Wojciech and I are wrong. His suggestion about adding a choke is basically the same as mine of using a transformer. The idea of both is to add a dc path to ground at the rectifier inputs. The difference is that the transformer adds DC isolation - which if you include your bandpass filter - you do not need. Sounds like the thing to do but sadly, the simulations show the reality! A choke does not do what you want and neither does a simple 1:1 transformer. However, if you use a 1:1:1 transformer then it all comes together. You can use a transformer with a 1:2 turns ratio, centre tapped and keep to the original half wave rectifier scheme. If you use a three winding transformer of 1:1:1 then you can use two bridge rectifiers. Using bridge rectifiers doubles the ripple frequency so allows lower smoothing C for the same ripple voltage. The attached (not very good quality) pdf shows the non-working choke and 1:1 transformer ideas and the working 1:1:1 transformer versions. Note the 1u smoothing capacitor values. These were reduced to make the simulation reach a steady state sooner than with the original 100uF values. Andy. signality.co.uk On 22 June 2011 01:12, Wojciech Kazubski <wk0@xxxxx> wrote: >> Hello all, >> >> I would appreciate some expert advice. >> >> I have a system which rectifies a sine wave input signal of 20Khz after >> a LC filter (see Rectifier_sim.jpeg) >> Everything works fine if LOAD_1 and LOAD_2 are equal. Vx is then >> (almost) the same as Vin. And Vcc and Vss are equal to the positive or >> negative part of the sine wave (less the DC losses) (Vss = -Vin_top and >> Vcc = Vin_top). >> BUT if LOAD_1 and LOAD_2 are not equal (like in Rectifier_sim.jpeg) it >> seems that Vx is lifted (DC component added) and Vss moves to the 0V and >> Vcc is lifted to twice the value I would expect (Vss = 0 and Vcc = >> Vin_toptop) (see rectifiersmp.eps). >> Our real life prototype shows the same behaviour as the simulation. >> >> I need this set-up for my system to work and I can not guarantee that >> the two loads always will be equal. >> Vin can be anything between 10Vtt and 90Vtt. >> >> I have tried adding a resistor from Vx to ground and that seems to help >> but increases the current drawn from the source (V1) to a unacceptable >> level. It should be a low power solution. >> If I short-circuit C1 everything works fine again (V1 has a low >> resistance output) but of course will disable the filter, which we don't >> what. >> >> Is there anyone here who can explain to me how and why this is happening >> and if available can anyone suggest a solution to me. >> >> I have been wrestling with this problem for a couple of days now, so any >> help will be very much appreciated. >> >> Many thanks, >> Robert > There is no DC patch from Vx to ground. If both loads are equal, both > rectified curreant are equal and cancel one another. If the loads are > different, the imbalance current charges Vx node until both output currents > become equal again. To avoid this place a choke between Vx and ground. > > Wojciech Kazubski > > > _______________________________________________ > geda-user mailing list > geda-user@xxxxxxxxxxxxxx > http://www.seul.org/cgi-bin/mailman/listinfo/geda-user >
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