multiple transformers: where to put diode bridge? - Page 1
Forum section: electronics / projects

author: ondreji
author_rank: Contributor
timestamp: July 19, 2014, 11:36:24 am
content: Hi,I had a trouble to find multi-tap 40V/1A (or 2A; at least 5 taps) transformer to have >15V bipolar output. Hence I ended up with two transformers (see schematics).Relay switches coils between parallel and series operation (both coils must be used simultaneously) which helps with power dissipation in the linear regs and pass thru transistors down the track.Finally, the question is where to put the diode bridge? I see two options:a) as I have it nowb) or rectify transformers output after relays independently and then create create common ground, plus, minus terminals(or is there an another better option?)Thanks!

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: July 19, 2014, 07:48:18 pm
content: some use 2 bridges with isolated matched secondaries when a dual supply is wantedcost - extra diode drop, extra diodesbenefit - can use same polarity Vreg circuit for both sides, save costs on volume, stocking, and can choose the better performing polarity regulatoranother sometimes claimed benefit is that the CT would couple line noise through the xfmr pri-sec parasitic C to the output gnd all the time, but dual bridges, 2 secondaries are "disconnected" except when the diodes conduct - although there is still some parasitic diode C even in reverse bias

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 19, 2014, 07:53:08 pm
content: You have to use a 4PDT relay (or a pair of DPDT as shown) to do it in AC, at least, to get true series/parallel wiring.If you don't require parallel operation, you can connect all windings in series (to get your 5 taps), and just live with not having double current capacity on the low range. And the voltage can be switched with a single DPDT, in AC or DC. You can also use TRIACs. (TRIACs are probably not as good with the series-parallel case because, with so many, it's going to be more expensive and cost more voltage drop.)Tim

author: ondreji
author_rank: Contributor
timestamp: July 20, 2014, 12:12:19 am
content: TIm,Quote from: T3sl4co1l on July 19, 2014, 07:53:08 pmApparently, that's not possible. Manufacturer claims that windings must be use together in series or parallel. Transformer windings are not designed to be used independently.O.

author: ondreji
author_rank: Contributor
timestamp: July 20, 2014, 12:22:23 am
content: Quote from: f5r5e5d on July 19, 2014, 07:48:18 pmCost shouldn't be an issue for this one-off project. I'm working on my first 4 quadrant PSU / SMU -- LT1970A based. Here is the original schematics:http://m.eet.com/media/1135516/10804di.pdf

author: johansen
author_rank: Super Contributor
timestamp: July 20, 2014, 12:45:38 am
content: Quote from: ondreji on July 20, 2014, 12:12:19 amthey said that because not everyone is the brightest bulb in the box, and they will try to get the full rated wattage out of only half the secondary.

author: tautech
author_rank: Super Contributor
timestamp: July 20, 2014, 12:46:33 am
content: QuoteI did this successfully using 2 12V (non CT)transformers for a series/parallel output in a single output PSU by DPDT switching on the LV AC.One has to get the phasing correct though. Can be easily checked with a DMM.

author: ondreji
author_rank: Contributor
timestamp: July 20, 2014, 02:40:45 am
content: Quote from: johansen on July 20, 2014, 12:45:38 amLOL, I was thinking that there is going to be some scientific explanation... Something along the lines outputs can be unbalanced due to unsymmetrical currents currents flowing in the coils because of unbalanced loads. My 4 quadrant PSU (under construction) has this characteristics (unbalanced load)Basically, this is the reason why I thinking to use two transformers but I'm not sure where to put diode bridge...

author: WarSim
author_rank: Frequent Contributor
timestamp: July 20, 2014, 03:05:30 am
content: Yes there is an issue with unbalancing when coil load is non symmetrical. Basically you will not achieve maximum load if you do not share the load. Also depending on the transformer design other factors maybe present but I see no reason to explain them all here.Unless you are using an E C or I core transformer with notation that the secondaries or primaries are not distributed in some way the losses will likely not effect your use if you stay under maximum rate load. If any of the coils are denoted as non distributed then pay head to it or rush frying the transformer.Besides if you are designing to use the maximum ratings you should get a transformer with built in fused links and thermocouples.Sent from my iPad using Tapatalk

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 20, 2014, 03:27:08 am
content: Connect the transformer to be center tapped, then put a bridge rectifier on the output. Use a relay to select between using it as a center tapped full wave or a bridge. Repeat for the other rail.

author: ondreji
author_rank: Contributor
timestamp: July 20, 2014, 04:56:32 am
content: Hi,Quote from: NiHaoMike on July 20, 2014, 03:27:08 amSomething like this, right?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2014, 02:41:39 pm
content: That'll work, though you use double the voltage drop of doing the switching on AC, plus the added size and expense of an additional FWB.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 20, 2014, 03:53:40 pm
content: Rectifiers are cheap compared to an extra relay, at least at those power levels. Or just use some buck converters.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 20, 2014, 04:15:55 pm
content: Doesn't take an extra relay...Well, for the "5 taps" case. Unless there's something fancier.But note that FWCT operation gets you sqrt(2) times the current of a single winding, not the full double you get from two in parallel. So it's better than all series, but not by all that much. Probably still worthwhile.Tim