Parallelling power transformers

[Nihilist]

I have Qty 4 of the same transformer.

I plan on building a pair of mono amplifiers that will require more current than a single transformer.

If I were to parallel the secondaries of the transformers , would I be better off to :

A) Directly connect the transformers secondaries together ( + to + , -- to -- ), and then run the parallelled secondaries to a single rectifier ?

OR . . .

B) Keep the secondaries seperate from each other, connect them each to their own rectifier, then connect the load side of the rectifiers ?


Thanks.................................Blake

 

[star882]

Using separate rectifiers helps even sharing of current. However, it might cost a little more.

 

[Nihilist]

Thanks for the reply.

I'm not worried about the extra cost of the additional rectifiers , but could you elaborate on "Using separate rectifiers helps even sharing of current." ?

My thoughts were that if I ran the transformers coupled after rectification that it would offer some isolation between the transformers, being that they weren't directly coupled.

However, my thoughts also lead me to wonder if perhaps the coupled transformers might be more forgiving of voltage/current variances than the parallelled rectifiers .


Thanks............................Blake

 

[wg_ski]

I'm not worried about the extra cost of the additional rectifiers , but could you elaborate on "Using separate rectifiers helps even sharing of current." ?

Thanks............................Blake

If there is a difference in reactance there will be a circulating current between the trafos. Combining post-rectifier, there can't be any circulating current - just perhaps a slight difference in harmonic currents (and conduction angle). Two rectifiers are better than one for absorbing surge current at turn on. Because rectifiers can have different Vf, the currents can be unequal if directly paralleled. The trafo reactance effectively "ballasts" the rectifiers just like separate emitter resistors on parallel output transistors. So both the trafos and rectifiers benefit if combined post-rectifier.

 

[Nihilist]

Excellent info ! That is what I needed to hear . Thank you both for your help.



.....................Blake

 

[Elvee]

If there is a difference in reactance there will be a circulating current between the trafos. Combining post-rectifier, there can't be any circulating current - just perhaps a slight difference in harmonic currents (and conduction angle). Two rectifiers are better than one for absorbing surge current at turn on. Because rectifiers can have different Vf, the currents can be unequal if directly paralleled. The trafo reactance effectively "ballasts" the rectifiers just like separate emitter resistors on parallel output transistors. So both the trafos and rectifiers benefit if combined post-rectifier.

If there is a reactance difference, there will be an uneven sharing of the load, but no circulating currents.
A voltage difference is required for that.
If the transformers are industrially made, exact to the turn, then there is no probleme paralleling them.
If there is a suspicion of a difference, the dual rectifier option is safer. It is easy to check for unequal turn ratio: feed them in parallel, connect together one side of the secondaries and put a voltmeter between the two remaining wires. If you read a voltage < 100mV, it is OK.
Note that if you use the dual rectifier option, you should bolt them on the same heathsink, because diodes have a negative tempco, and require equalizing resistors and a good thermal contact to properly share the currents. The copper of the transformers will act as resistors, you just have to ensure a good thermal contact.

 

[AndrewT]

I don't know how much current you are expecting.

Using a single rectifier on a ClassA amplifier ran the rectifier very hot.
Using twin rectifiers with half the current going through each run them cooler than warm.

I noticed no difference in the transformers whether paralleled before the rectifier or paralleled after the rectifier.

Do not parallel separate transformers, unless you know they are identical. That does not equal reading the label.

 

[Nihilist]

Here are the transformers I am talking about :

http://img33.imageshack.us/img33/4083/magnavox169aapowertrans.png

This spec comes from a Sams Photofacts , FYI.

They are from (and currently in) some Magnavox mono amplifiers that were from a console stereo. I have Qty 4 , thinking of parallelling them so I can run more or different output tubes.


Another question I have is about the current rating of the "Sec. 1" , which is rated at 612V CT @ .082A. Would this mean that after rectification I would have approximately 82ma , or would it be 164ma ?


Thanks again................................Blake

 

[star882]

If you bridge rectify it, it would be 82mA. If you center tap rectify it, it would be 164mA. (Beware that you'll need to use HV rectifiers with a rating of at least 1.6kV.)

 

[Eva]

The rms AC current rating of a transformer is not the same as the amount of DC rectified and smoothed current that you can get without overheating it.

Rectification draws 2 to 3 times the DC load current from the transformer during 1/2th to 1/3th of the time, and nothing the rest of the time. This results in higher than expected rms current (2 to 3 times) and heating in the windings.

A transformer rated for 80mA rms may only be able provide 40mA DC after rectification and smoothing, unless the 80mA figure is actually the DC rating taking all into account (probably from a smart manufacturer).

BTW: If the transformers happen to have exactly the same turns ratios (manufacturing tolerances may result in +/-1 turn error or more) and same winding polarities are used, they may be freely paralleled. Otherwise use separate rectifiers.