Tapered transformer and two rectifiers! Need help! - Page 1
Forum section: electronics / beginners

author: 001
author_rank: Super Contributor
timestamp: December 16, 2019, 02:23:28 pm
content: Hi!I have tapered transformer. Can I use two bridge rectifiers this way to get two different DC voltage +A and +B?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2019, 02:43:03 pm
content: Certainly. But will they be the voltages you were expecting?Tim

author: 001
author_rank: Super Contributor
timestamp: December 16, 2019, 02:49:53 pm
content: I`m looking for about +50V and +35V

author: 001
author_rank: Super Contributor
timestamp: December 16, 2019, 02:51:54 pm
content: Quote from: T3sl4co1l on December 16, 2019, 02:43:03 pmIs it mean "ok"? I`m worry about accidentaly paralleled "LOW" diodes of rectifiers

author: Andy Watson
author_rank: Super Contributor
timestamp: December 16, 2019, 03:56:53 pm
content: Quote from: 001 on December 16, 2019, 02:51:54 pmIt is ok to parallel those diodes, however, by connecting the grounds together you will have two supplies of 50V. The 30V tap becomes redundant in this situation.

author: 001
author_rank: Super Contributor
timestamp: December 16, 2019, 04:40:52 pm
content: Quote from: Andy Watson on December 16, 2019, 03:56:53 pmWHY?

author: Andy Watson
author_rank: Super Contributor
timestamp: December 16, 2019, 05:25:27 pm
content: Ignore the B+ diode bridge. You have a 50V supply for A+. There is also a path via one of the diodes in the B+ bridge that provides half-wave rectification, at 50V, using a diode in the A+ bridge as the return path.

author: 001
author_rank: Super Contributor
timestamp: December 16, 2019, 06:29:42 pm
content: Quote from: Andy Watson on December 16, 2019, 05:25:27 pmIs it joke?

author: MarkL
author_rank: Unknown
timestamp: December 16, 2019, 06:56:38 pm
content: This bridge and tap arrangement probably doesn't have the output waveforms that you want. It's not symmetric on the half-cycle on the B+ (assuming the two windings are in phase (end-to-end output of 38VAC)).Below is a quick simulation.

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 01:17:51 am
content: Quote from: 001 on December 16, 2019, 06:29:42 pmNo, Andy was not joking, but be sure to consider what MarkL said about the waveforms.

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 01:20:37 am
content: Quote from: MarkL on December 16, 2019, 06:56:38 pm"It's not symmetric on the half-cycle..." Mark, please explain more about this. I would like to learn about it. Thank you.

author: MarkL
author_rank: Unknown
timestamp: December 17, 2019, 02:29:30 am
content: Quote from: t1d on December 17, 2019, 01:20:37 amTake a look at the simulation plot. What I mean is that the +B output (labeled Bout) does not have its peaks at the same level through a full cycle of the AC input, like you would expect of a single full wave bridge rectifier being fed by a single or isolated transformer winding.Perhaps it's ok for the rest of 001's circuit, which is not drawn, but we don't know. Presumably, the next stage is some large filter capacitors. Because the peaks are unequal for the whole cycle, you would only be charging up a filter capacitor once every cycle at the highest peak (instead of twice per cycle on two peaks). This could lead to unacceptable ripple under load conditions.In contrast, the +A output (labeled Aout) *does* have the typical bridge rectifier output waveform. Note that it is overlaid by the green Bout plot where they are equal.001 was also trying to derive two different DC voltages at the +A and +B outputs, but the plot shows that it doesn't do that as intended either. Because the peaks are equal, the outputs after the unmentioned filtering capacitors would be equal too.Hope this makes it a little clearer. I'm assuming the simplest intentions in 001's circuit.

author: t1d
author_rank: Super Contributor
timestamp: December 17, 2019, 04:48:21 am
content: Great tutorial, Mark. Now, I get it. Those are problems I would not have thought of, being a bit noob-ish. Thanks for the effort to explain it.

author: gbaddeley
author_rank: Regular Contributor
timestamp: December 17, 2019, 07:17:34 am
content: A full wave bridge rectifier block assumes the AC input is ‘floating’ with respected to the rectified +/- outputs, so that the diodes can effectively ‘swap’ the AC inputs every half cycle. The way you have it, the AC inputs are not isolated for both rectifiers, if you have both - outputs commoned to earth. A solution would be to split the tap between the 8 and 30 volt windings. Does the transformer allow you to physically do that?

author: 001
author_rank: Super Contributor
timestamp: December 17, 2019, 08:18:25 pm
content: Quote from: gbaddeley on December 17, 2019, 07:17:34 amno

author: 001
author_rank: Super Contributor
timestamp: December 18, 2019, 12:38:33 pm
content: Can I get two DC voltages from this transformer? What circuit I need?

author: Gyro
author_rank: Super Contributor
timestamp: December 18, 2019, 12:47:59 pm
content: Only half wave rectified (if you want them both referenced to a common ground).IFyou can separate the connection between the 8V and 30V windings, then you can simply use two bridge rectifiers one on each winding.

author: Rigolon
author_rank: Regular Contributor
timestamp: December 18, 2019, 04:16:45 pm
content: I'm not really into the power aspects of electronics.But if I remember correctly there is a better way to do what you want. You should use your tap wire as neutral (gnd).Sorry for my drawing, hope you guys can understand. And as I said, not really my thing, I might be dead wrong with this.EDIT:Another idea, that I'm sure I've seen somewhere. In OP's drawing you could connect B+ to GND of bridge A. And GND of bridge B becomes B-.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2019, 06:44:19 pm
content: You could half-wave double the extra end. That keeps power factor okay, but the output still has 50/60Hz ripple, as the name suggests.Otherwise you can FWB the full winding and use buck converters to derive other rails.Tim

author: gbaddeley
author_rank: Regular Contributor
timestamp: December 19, 2019, 12:51:48 pm
content: The drawing will produce about 84v peak across the output resistor, as it is a half wave voltage doubler on the 30vac winding. The 8v winding doesn’t contribute anything, as it’s diodes will alway be reverse biased, against what is produced by the 30v winding.

author: 001
author_rank: Super Contributor
timestamp: December 21, 2019, 03:57:32 pm
content: Thanx a lotmy transformer suck