Using Two Identical transformer to form Centre Tap Configuration

[Gyro]

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).

[eneuro]

No simulator needed for that. 

Why not let computer double check and test other things.
I coudn't sleep if don't checked it in software 

Yep, when we dissconnect transformer tap we have classic transformer secondary rectified and no supprise, there are different +/-V, so you were right 
Simply, 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 

[paulie]

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?

[eneuro]

Now, put 10R on +V and only 1k on bottom -V and... difference between two bridges and one bridge cleary visible 

Both transformers saturated-there is very small load on -V and this will never happen in the case when two bridges were 

[paulie]

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....".

[basit701]

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.

[bktemp]

Now, put 10R on +V and only 1k on bottom -V and... difference between two bridges and one bridge cleary visible 

Both transformers saturated-there is very small load on -V and this will never happen in the case when two bridges were 

I 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.

[Gyro]

@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.

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?

paulie, 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!

[Gyro]

@bktemp

No, 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.

[paulie]

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.

Considering 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.

[Gyro]

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. 

[basit701]

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?

[IanB]

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?

I 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.

[basit701]

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?

[Gyro]

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).

[basit701]

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).

Yeah my load can be unbalanced and the safest solution is to use two bridge rectifiers.

[eneuro]

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.

Where do you see half wave rectifier in my simulation? It is full-bridge-watch +V current scope on 10R load 

If you use two separate bridge rectifiers you will have the disadvantage of doubling the diode drop.

Synchronous rectifier driver can be used (like on PC motherboards close to 12V CPU power supply ) and forget abput diodes drops 

Anyway, 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 amplifier


More interesting magnetic amplifier stuff and cores saturation with small DC curent at this nice page 
http://sparkbangbuzz.com/mag-amp/mag-amp.htm

BTW: Probably will use such magamp to drive my 1kW flat-iron in DIY reflow oven 

[oldway]

If you use two separate bridge rectifiers you will have the disadvantage of doubling the diode drop.

It 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).

[Kalvin]

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.