Full-wave bridge

[abj1]

Quick dumb question: Rectifier spec sheets talk about max RMS voltage per plate for standard full wave setups which is fine but what is the situation with using them with a hybrid (with SS - Graetz bridge?) bridge from say a 300-0-300 traffo - if for argument sake I was to use a 6X5 (with a 350V max spec) as the hollow-state component? Is the max RMS from the centretap to each end (happy to create a bipolar supply if needs be) or from end to end (ie would the RMS seen by the rectifier be 300 or 600V)?
TIA,
Andrew.

[Dave Cigna]

In a bridge rectifier (hybrid or not) the 'top' and 'bottom' pairs of diodes work independently in the same way as a FW CT rectifier. If you use a CT in the transformer you'll get a bipolar supply. If you don't use the CT (or there isn't one) then you can think of the two supplies as being stacked on top of each other.

Hope that made sense. The short answer: the 6X5 would see 300 Vrms.

[av-trouvaille]

Hello,

The 480 – 0 – 480 V connection on my Bartolucci mains transformer W-272 can be used for a hybrid full wave bridge. I have two questions about this as.
a. Can the two 480 V windings simply be wired in parallel so more current can be drawn? As there is only one 0 connection for the two windings (for a traditional CT connection I think) the phase cannot be changed.
b. Can the GZ34 be used as a rectifier? This full wave rectifier accepts 450 – 0 – 450 V as a maximum on the plates. I my understanding of the previous posts in this thread the solid state diodes would take half of the 480V. Which would mean that the GZ34 only sees 240 -240 V on the plates. Is this correct?

In the attached picture I have drawn the schematics. I intend to use a choke input filter for optimal current draw (well, with a small input C may be to dampen the voltage peaks a bit).

Can you comment please?

Best, Arjen.

[ray_moth]

No, Arjen, I'm afraid you can't do that. It puts a dead short across the transformer secondary that will destroy it. Even if you could disconnect the center tap and use the two halves in parallel, it wouldn't give you any more current with a bridge rectifier than you could get with a conventional 'full-wave' rectifier using the center tap in the usual way. Don't forget that a bridge rectifier places a load on the winding for 100% of the time, while a 'full wave' rectifier only loads each half of the winding 50% of the time.

The 'full wave' center tap approach is your your best option, IMHO. You can overcome the voltage restriction by using a pair of TV damper diodes instead of the GZ34 and the result could even be less 'noisy'.

[av-trouvaille]

Hello Ray_moth, others,

Thanks for your clear reply.
I already was afraid the existing wiring would not allow this because of the phase connection.
The transformer is all-epoxy potted, so no rewiring option.

I will use some 12D4 damper diodes with controlled heater warm-up instead with the 'full wave' center tap approach. It will cost another socket and indeed there is the reward of obtaining a less 'noisy' result. They can handle some 155 mA and that will be enough for the application.

For the sake of learning the second question remains relevant: Suppose the transformer would have allowed to rewire the secundairies into the correct phase to use them in parallel in a hybrid bridge rectifier, would the GZ34 see only 240 -240 V on the plates? If so would there be any other risk involved that prevents the use of such a configuration?

Best, Arjen.

[ray_moth]

Hi Arjen,

See Dave Cigna's response earlier in the thread. If you have a 480v rms winding then each diode will see 480v rms. A bridge arrangement does not 'share' the voltage between the diodes.

[av-trouvaille]

Hello Ray-moth,

Thank you for the confirmation.

Best, Arjen.

[Dave Cigna]

Actually, I was wrong in my old post. With both FWCT and bridge rectifiers each diode sees the voltage of the entire winding, from end to end.

But, Ray got it right despite my original mistake; with a 240-0-240 transformer each diode sees 480Vrms.

-- Dave