Dual Bridge Rectifier - Why? - Page 1
Forum section: electronics / projects

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: September 21, 2020, 01:48:30 am
content: I've been looking at simple bridge rectifier PCBs on AliExpress and have found some that are dual rectifier circuits providing +V, 0V and -V outputs.I am wondering the point, particularly how they are better than a single bridge on a center tapped transformer (or dual windings in series.)Also some advise applying each of the dual windings to each bridge without onnecting in series. This seems wrong because returning current from +V to -V needs go hrough 4 diodes rather than 2. Windings in series avoids that.BTW I don't have an application - just some nice transformers I want to find a use for.

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2020, 02:01:34 am
content: Asymmetric positive and negative supplies require separate secondaries and bridge rectifiers. The special case of taking 0V from the center tap of a bridge rectified secondary feeding positive and negative rails through a single bridge rectifier only works when it is the truecentertap.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: September 21, 2020, 02:19:27 am
content: Yes, if you mean that trying to use Ground as 0V doesn't work like a "true" center tap.But I'm thinking of transformers with center taps or dual windings in series. Any advantage there?

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2020, 02:23:15 am
content: Nope.

author: BravoV
author_rank: Super Contributor
timestamp: September 21, 2020, 03:06:20 am
content: That situation is better, if the current draw between the (+) and (-) to ground (0v) are different, say V+ rail will be supplying much higher current like power application, compared to V- rail, say just tiny current for supplying control circuitry like op-amps.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: September 21, 2020, 04:15:32 am
content: QuoteIn the asymmetric load case a single bridge alternates the loads between the windings each half cycle spreading the power between them, but the dual bridge puts all of the greater load's current to a single winding (and a single bridge.) I don't see an advantage there.

author: BrianHG
author_rank: Super Contributor
timestamp: September 21, 2020, 04:20:06 am
content: You can use a different winding thickness, or smaller transformer for the negative supply. Or even different voltage if HV but low current is required.

author: greenpossum
author_rank: Frequent Contributor
timestamp: September 21, 2020, 04:37:46 am
content: Just think of it as two bridge rectifier circuits of opposite polarity with a common lead. You are free to use them in a symmetrical configuration or have separate windings/transformers/voltages/current capacities.

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2020, 08:32:21 am
content: However it would be far more versatile if they kept the two rectifier circuits separate right up to the output terminals as then you could strap them 'as-is' or as independent floating supplies which would be more useful if you were feeding a pair of positive voltage regulators and wanted to be able to stack their outputs, or if you needed a floating supply for metering or you regulator's control circuit.Anyway lets see links to the actual PCBs so we can see if they good enough to be worth bothering with or whether you'd do better to wire up the configuration you actually want point-to-point on tagstrip.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2020, 09:25:28 am
content: When the windings are equal, there is no advantage, indeed it is disadvantageous as the voltage drop is doubled.Nonetheless, these circuits show up often in audiophile circuits. Which, well... needless to say, it's a community not well known for rigorous proof.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2020, 09:20:06 pm
content: In the version with 2 rectifiers the voltage rating of the rectifiers can be lower (e.g. half) - however this is usually not a problem today. There may be a slight difference in the layout and it may be a little easier to avoid ripple from a poor placed ground point. The added resistance of the extra diodes can improve the power factor a little - there are however more effective methods.It does make a difference when the AC windings are also used for a charge pump.

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: September 21, 2020, 10:37:18 pm
content: QuoteI'm glad it wasyouwho said that. I posted this because when I searched I found a lot of explanations that were vague or didn't really make much sense and they were mostly audiophile sites. Idohave a lot of respect for audiophiles, especially the elites who only use gold plated USB cables.

author: Ian.M
author_rank: Super Contributor
timestamp: September 21, 2020, 10:47:17 pm
content: Quote from: scuzzyTerminator on September 21, 2020, 10:37:18 pmWhat, respect for the depth of their wallets, the ease with which they open them and the vigor with which they defend their purchases in the absence of any objective evidence?

author: scuzzyTerminator
author_rank: Regular Contributor
timestamp: September 21, 2020, 11:52:33 pm
content: QuoteI went ahead and bought two of these without having any reason. I'm like that. A budding audiophile?Dual Bridge Dual Polarity Unregulated Power Supply PCB.https://www.aliexpress.com/i/32545683258.htmlThey arrived on a fast boat from China and they do appear excellent quality. I would by from that store again.There are several examples around, many using 25XB80 type bridges for higher current.Rectifier Filter Power Supply Board Dual Power Parallel Output PCB Bare Board Dropshiphttps://www.aliexpress.com/item/33013351811.htmlOops. That's a single bridge.2pcs/lot 6*10000UF power level power supply board PCB empty plate sound big pond bridge rectifierhttps://www.aliexpress.com/item/32487129857.htmlI think I might like this for a higher power supply.GHXAMP Single Power Rectifier Filter Board DIY Kits Fever Large Current For 1969 Amplifier 50Ahttps://www.aliexpress.com/item/32823022629.html

author: Ian.M
author_rank: Super Contributor
timestamp: September 22, 2020, 10:09:11 am
content: That looks reasonably nice. Its also using discrete diodes rather than bridge rectifiers so you've got a lot of flexibility in how you configure it. e.g:Omit diodes D2, D4, D5 and D7, cross jumper AC1 to AC3 and AC4 to AC2, and fit a terminal for the center tap to the ground end of the D4, D5 footprints, and you've reconfigured it as a single bridge fed by a center tapped secondary, as per your original post.