Using Two Identical transformer to form Centre Tap Configuration - Page 1
Forum section: electronics / projects

author: basit701
author_rank: Contributor
timestamp: May 22, 2015, 03:34:23 pm
content: Hello Guys!My requirement for project is 24_0_24 volt and 2amp each tap. Unfortunately 24_0_24 volt center tap in my area is only available with current rating of 800mA. To meet the 2amp requirement I bought TWO 24v 2 amp transformers and connected their outputs in series to form 24_0_24. The two transformers are identical i.e. same manufacturer, same rating etc.The two transformer configuration is working perfectly, but when I discussed about this method with my friends. Their reply was not in favour of this, most of them said that if one of transformer gets short circuited it will short the other transformer and both will not work. A couple of them said that it will damage the components like ICs etc.What are they saying is correct or not?

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2015, 04:12:59 pm
content: Your solution is perfectly fine. Your friends are misinformed.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2015, 04:32:30 pm
content: The solution is fine, even if the transformers are not identical. The downside is, that 2 smaller transformers usually are larger, more expensive and have slightly lower efficiency.

author: basit701
author_rank: Contributor
timestamp: May 22, 2015, 05:03:29 pm
content: So this configuration will work fine as centre tap will?

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2015, 05:12:02 pm
content: Yes, it will work. I have taken apart some OKI printers, and there they used 2 separate 120VAC transformers, with the mains voltage switching being to connect the primaries in series for 230VAC and in parallel for 120VAC use. Secondaries were used like you did to make a centre tapped winding giving 24VDC for the hammer drives, and another 2 secondaries to give 8VDC for the 5v rail and 12V rails for the RS232 interface.

author: retrolefty
author_rank: Super Contributor
timestamp: May 22, 2015, 05:12:32 pm
content: Quote from: basit701 on May 22, 2015, 05:03:29 pmYes it will. However be aware you have to make sure you wire the two secondary windings so they 'add' and not 'subtract' causing zero voltage output.

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 06:04:13 pm
content: Small point... It does depend a little on what you are doing with the outputs. If you are planning to put a bridge rectifier across the two 24V outputs to create a center tapped DC output (as I suspect you are) then you'll be fine as long as the load on both DC rails is equal.If you were planning to use the transformers in a traditional 'full-wave' configuration - 2 diodes, to generate a single rail then you would get DC saturation of the transformers.As I say, probably not the case here, but something to be aware of if blanket replacing a single transformer with two.

author: IanB
author_rank: Super Contributor
timestamp: May 22, 2015, 06:07:31 pm
content: Quote from: Gyro on May 22, 2015, 06:04:13 pmWhy would two transformers in series behave differently than a single centre tapped transformer in this configuration? Is it because of unbalanced current in the primaries?

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 22, 2015, 06:09:09 pm
content: For what it is worth, your friends are at least partly right. If the primary of one transformer shorts it will kill both outputs. So what. If you had a single center tapped transformer and shorted the primary it wouldn't work either.

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 06:11:31 pm
content: Yes, in a single transformer (ie. single magnetic core) then imbalances between the secondary windings cancels. Not necessarily the case if you have 2 separate cores.Somebody please tell me I'm talking rubbish here

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 22, 2015, 06:15:31 pm
content: Quote from: Gyro on May 22, 2015, 06:11:31 pmHow is that any different than using a single transformer with a single half-wave rectifier? Its been done a million times with no apparent ill effects.And using dual transformers like @basit701 proposes is done all the time as well. I don't see what all the fuss is about?

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 06:21:34 pm
content: I am probably wrong- I just can't think whether an unbalanced current draw after the (presumed) bridge rectifier relative to the center tap would cause any DC saturation in the transformers.Using a separate bridge on each transformer and combining the center tap at the DC side avoid any potential issue anyway.

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 06:26:33 pm
content: QuoteYes it has been done a million times, but half wave rectifying the output of a transformer seriously de-rates its output current vs full wave rectification.

author: basit701
author_rank: Contributor
timestamp: May 22, 2015, 06:49:56 pm
content: I am using this rectifier circuit.

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 07:15:40 pm
content: This is the approach I was suggesting (ignore extra components, I just pulled it off Googe images).It may be overkill, but It addresses the possible issue that I raised.

author: basit701
author_rank: Contributor
timestamp: May 22, 2015, 07:24:36 pm
content: Quote from: Gyro on May 22, 2015, 07:15:40 pmThe circuit you posted contains independent bridge rectifiers for each tap. What the significance of using this over a single bridge rectifier for both taps as in the circuit which i have posted.

author: eneuro
author_rank: Super Contributor
timestamp: May 22, 2015, 07:46:41 pm
content: Quote from: basit701 on May 22, 2015, 07:24:36 pmFor example in your single bridge design if one of the capacitors will change its capacity due to many factors than you can end up with different +V and -V since they are charged in series-additionaly one of them can be overcharged to higher voltage than nominal if this capacity change will be higherSo, in simple mosfet driver design like this where I have two caps in series and ~24V accross them, I have additional resistors to balance capacitors voltage and keep its voltage at the same +/-VBut you have a few amps outputs there, so such simply balancing is not an option, so you'll probably have to pray and keep those caps at good condition, so that is one of the reasons I better like solution with two bridges and separate floating PSUs to form +/-V

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 08:05:03 pm
content: Sometimes, especially in a one-off application it is simpler to do something that you know will absolutely work than spend too much time wondering about a possible issue (like the possible DC imbalance with unequal load that I was talking about). I may well be completely wrong but I would need to analyse it in detail to be completely comfortable. As I say, I know for sure the dual bridge circuit will work as expected.That said, feel free to make me slightly uncomfortable.

author: dom0
author_rank: Super Contributor
timestamp: May 22, 2015, 08:10:37 pm
content: You are aware that the dual-bridge-rectifier configuration is identical to the standard center tap configuration, except you need double the diodes and have doubled the rectifier losses?@eneuro: this isfalse. The maximum potential imbalance across the filter caps is determined by the transformer, not their capacitance. What you say would be the case if you did not connect the center tap.

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2015, 08:12:42 pm
content: The dual bridge rectifier circuit offers no benefits for a single, centre tapped transformer but has the benefit of preventing core saturation if two transformers are used and the DC currents are significantly unbalanced.

author: eneuro
author_rank: Super Contributor
timestamp: May 22, 2015, 08:14:33 pm
content: Quote from: Gyro on May 22, 2015, 08:05:03 pmYep, you have two floating PSUs.Voltage imbalance I was talking above-probably this is not what we want-made quite bad scenario where one cap capacity changes +20% while other goes down -20% and bingo +20V vs -30VFundamentals friday

author: eneuro
author_rank: Super Contributor
timestamp: May 22, 2015, 08:19:24 pm
content: Quote from: dom0 on May 22, 2015, 08:10:37 pmHaven't got more time today to put into simulator whole circuit, so you can be the first to proove you are right

author: dom0
author_rank: Super Contributor
timestamp: May 22, 2015, 08:21:59 pm
content: No simulator needed for that.

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 08:25:59 pm
content: @dom0I've gone from a nagging doubt to sketching this out on the back of an envelope....For the single bridge rectifier case, take an extreme case of full load on the +ve rail and zero load on the -ve rail. Now look at the current waveforms (and therefore the core magnetisations) on each of the transformers. Now compare with the secondaries (overall magnetization) on a single core (ie single transformer). I think they are different.What do you think?

author: dom0
author_rank: Super Contributor
timestamp: May 22, 2015, 08:41:31 pm
content: I see your point there, for large imbalances that becomes an issue (or a really large, fuse-blowing one with toroidal transformers).

author: Gyro
author_rank: Super Contributor
timestamp: May 22, 2015, 09:00:40 pm
content: Interesting one isn't it We end up with 2 half wave rectified transformers driving alternate half cycles. As I say, it started out as a nagging doubt until you made me work it out (thanks, I learned something today!).As indicated I took an extreme case, but at some level of imbalance they are going to start buzzing and warming up - as you say, with toroidals that could be quite dramatic (I'd lay odds that it might happen sooner than expected given their very low headroom and effectively gapless core).

author: eneuro
author_rank: Super Contributor
timestamp: May 22, 2015, 09:09:39 pm
content: Quote from: dom0 on May 22, 2015, 08:21:59 pmWhy not let computer double check and test other things.I coudn't sleep if don't checked it in softwareYep, when we dissconnect transformer tap we have classic transformer secondary rectified and no supprise, there are different +/-V, so you were rightSimply, when saw those two caps in series I was back with memory to this circuit and forgot we have taped transformer, while I had only 24VAC secondary, so I had to try balance those caps to ensure TLP351F will provide gate voltage +/-12V

author: paulie
author_rank: Frequent Contributor
timestamp: May 22, 2015, 09:37:33 pm
content: So the conclusion is unless conductors open AND drastic mismatch in components the 4 diode circuit works fine. Silly to consider the 8 diode version unless you suffer from OCD?

author: eneuro
author_rank: Super Contributor
timestamp: May 22, 2015, 09:39:10 pm
content: Now, put 10R on +V and only 1k on bottom -V and... difference between two bridges and one bridge cleary visibleBoth transformers saturated-there is very small load on -V and this will never happen in the case when two bridges were

author: paulie
author_rank: Frequent Contributor
timestamp: May 22, 2015, 09:55:13 pm
content: It would be very nice if you attached pictures to the forum like a normal human being instead of linking those gay 2nd party sites. I really don't want to know if "Miley Cyrus bikini pics make her look like a....".

author: basit701
author_rank: Contributor
timestamp: May 23, 2015, 06:33:05 am
content: What is meant by Core saturation and due to current imbalance.I am novice towards transformers and inductors (i know simple things like mutual inductance and turns ratio etc) and will study these things in 4th semester.

author: bktemp
author_rank: Super Contributor
timestamp: May 23, 2015, 06:54:20 am
content: Quote from: eneuro on May 22, 2015, 09:39:10 pmI do not see anything in the simulation results that proves both transformers are satured.The magnetic field in the core leading to saturation is mainly determined by the primary voltage and therefore the magnetisation current. I think you are mixing output current and magnetisation current.If you use only a half wave rectifier (as in your example), this DC load current leads to a DC shift in de primary voltage because of the uneven voltage drop in the primary winding. Only this small DC offset is visible to the transformer. If the core starts to saturate because of this offset voltage, it starts to draw more current and the voltage drop increases. This compensates the DC offset voltage. Therefore a transformer can tolerate quite a high DC output current without beeing fully saturated.In theorey it is possible to saturate a transformer using a half wave rectifier, but this is unlikely unless you are drawing a high current. It is more likely to burn the transformer by simply overloading it due to the righ peak current and the resulting higher power dissipation compared to a full wave rectifier.

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 09:35:02 am
content: @basit701...Core saturation is when you get un-equal current on the positive and negative cycles of the AC waveform (like if you half wave rectify with one diode). This causes rhe core to magnetize by different amounts in each direction, effectively you're developing a DC bias on the core material. Now a choke can handle this condition because it has an air gap in the core to stop it saturating, a transformer has no gap so the offset will start to saturate it, dropping the inductance and increasing the primary current significantly - it will get hot and buzz. I hope this provides a clearer explanation.Quotepaulie, That's certainly not my conclusion! I won't speak for dom0 now he sees what I'm getting at. The OP is asking this question because he wasn't sure whether the 2 transformers will act like one. What we have established it that they will only do so if there is equal load on the 2 rails. Maybe he intends to fuse his 24V rails? maybe his load won't be balanced? WE don't know. If he loses connection on one of his rails (fuse, loose wire, whatever) or his load gets too unbalanced then his transformers WILL saturate. I didn't say anything about "AND drastic mismatch in components" that makes no difference.If his transformers are toroidal then it's probably the best case because this will instantly blow the mains fuse. If they are not, and the primary fuse matching isn't too good then he will get heat, if unattended then maybe smoke or fire before it blows. He will probably get the opportunity to go and purchase new transformers. Remember we're not talking about an output overload here, just an open circuit or imbalance in load.I am talking about SAFE design here, we don't know enough about the OP's load to know. The cost and extra voltage loss (remember we're talking 24-0-24 here, not 5) of an extra bridge is worth it for the additional safety that it brings. That is my 'conclusion' anyway. I believe we should be advising the OP on the safest option if he doesn't understand the hidden 'gotcha' in using 2 transformers in place a single one.Maybe you consider that silly and OCD, maybe I've spent too long designing products for customer safety but I'd prefer not to be the one who causes him to burn his house down (extreme but possible) because I didn't disclose some fact that I personally considered something that I wasn't worried about. Let's let the OP decide his preference!

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 10:05:35 am
content: @bktempNo, I don't see anything in the simulation either, but that seems to be focused on different capacitor values rather than load current. (actually eneuro's second simulation does show it!)I think we basically agree that DC offset causes increased primary magnetizing current and increased heat. The level of saturation will depend on the transformer construction - with a toroidal it will be close to brick-wall. With an EI then it will be slower - and I think that's a bad thing in this case because it will let it get much hotter without blowing the mains fuse.The general rule of thumb (remembered from an old NS book and just found again on a Hammond transformer pdf on the web is Idc = 0.28 x Iac for a capacitor filtered half-wave rectified. In reality no one is going to design in a transformer that's nearly 4 times the rating just to avoid using a bridge rather than a single diode. I think we have to assume that the OP is planning to use the majority of the transformers' output rating so unintentional half-wave rectification would take him pretty deep into saturation.

author: paulie
author_rank: Frequent Contributor
timestamp: May 23, 2015, 10:42:01 am
content: Quote from: Gyro on May 23, 2015, 09:35:02 amConsidering how cheap those rectifiers are I would probably agree. Maybe better if I had added the phrase "and running near the limit". In my experience vast majority of applications do utilize less than 1/4 the current capability. But like I said your points are well taken and appreciated and for the extra pennies good advice.However "burning down the house" maybe a good Talking Heads title but IMO a little overboard here. That is unlikely with a properly fused and otherwise protected design.

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 11:01:54 am
content: Agreed maybe a little overboard (I did say extreme) but still remotely possible and something that I woudn't risk in a volume product (or my own come to that). I'd still consider it 'unsafe under reasonable (single failure) fault conditions'. Glad you see the point anyway.We still don't (at least I don't think we do) know the OP's intended use - I suspect an amplifier, in which case they it would be pretty much coasting most of the time (unless class A of course), but not knowing, I think we have to assume the worst.Anyway I think we're agreed on the (at least our) conclusion now.

author: basit701
author_rank: Contributor
timestamp: May 23, 2015, 01:57:28 pm
content: After checking all the posts, i am thinking to break centre tap connection and connect a rectifier to each of the transformer output and then connect rectifier output in series to get 24+24. What you people say? Is this more better than my previous configuration or just same?

author: IanB
author_rank: Super Contributor
timestamp: May 23, 2015, 03:11:40 pm
content: Quote from: basit701 on May 23, 2015, 01:57:28 pmI think what you showed in post #13 would be perfectly fine as long as the loads are reasonably balanced between each side. If you use two separate bridge rectifiers you will have the disadvantage of doubling the diode drop.

author: basit701
author_rank: Contributor
timestamp: May 23, 2015, 03:18:33 pm
content: Current Imbalance will occur because when postive part will be connected with load and negative part without load current will not be balance & regarding diode voltage drop i need 20 volt max from IC so 4 volts in in excess. So is it fine na?

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2015, 04:40:40 pm
content: If you know that your load is going to be unbalanced (I think that's what you just indicated?) then you certainly need to two rectifiers, connected as you say, no alternative in that situation (and safest in all situations).

author: basit701
author_rank: Contributor
timestamp: May 23, 2015, 04:44:08 pm
content: Quote from: Gyro on May 23, 2015, 04:40:40 pmYeah my load can be unbalanced and the safest solution is to use two bridge rectifiers.

author: eneuro
author_rank: Super Contributor
timestamp: May 23, 2015, 05:25:52 pm
content: Quote from: bktemp on May 23, 2015, 06:54:20 amWhere do you see half wave rectifier in my simulation? It is full-bridge-watch +V current scope on 10R loadQuote from: IanB on May 23, 2015, 03:11:40 pmSynchronous rectifier driver can be used (like on PC motherboards close to 12V CPU power supply ) and forget abput diodes dropsAnyway, apropos saturation.This simply simulator I've used showed really bad looking unbalanced load, so it is very close with such transformer sconfiguration to... magnetic amplifierMore interesting magnetic amplifier stuff and cores saturation with small DC curent at this nice pagehttp://sparkbangbuzz.com/mag-amp/mag-amp.htmBTW: Probably will use such magamp to drive my 1kW flat-iron in DIY reflow oven

author: oldway
author_rank: Super Contributor
timestamp: May 23, 2015, 05:31:56 pm
content: QuoteIt is the price to pay for not using a center tap transformer with unbalanced loads.Transformer may saturate with high half wave rectified secundary current.For example, this happens when you use a welding transformer to weld aluminium.Secondary current is asymetrical because aluminium oxyde acts as a rectifier.If not correctly protected, you will burn the primary of your transformer by overcurrent. (saturation of the core).

author: Kalvin
author_rank: Super Contributor
timestamp: May 23, 2015, 05:37:14 pm
content: I would go for two separate full-bridge rectifiers with associated filtering capacitors, as the transformers can be then considered as two separate power supplies connected in series.