Two 120V transformers in series for 240V ?

[Bernhard]

Is this a risk ?

My thought was that if one transformer sees much more current on the secondary ( in case of circuit failure), primary impedance will fall and the other transformer will see more than 120V and the circuit on the secondary will get overvoltage.

[pinkmouse]

More importantly, the insulation of the 120V traffos is probably not rated for 240V.

[Tony]

otherwise, if they are loaded indepentdently, you can have problems.

you can connect a fuse say 2amps, between the primaries of the two transformers, if the fuse blows you got the phasing wrong, reverse one and try again, this time the fuse should not blow.

if you can megger the primary coils with say 1kv test voltage, then you should be able to use them.

[peranders]

Originally posted by Tony
if you can megger the primary coils with say 1kv test voltage, then you should be able to use them.

.... mm, but in Europe they must cope with 4 kV....

[clem_o]

Originally posted by Bernhard
Is this a risk ?

Depends how the outputs of the two transformers are loaded, as you noted. In an Onkyo Power amplifier with dual independent transformers (designed for 110V), we wanted to get the amplifier operational on 220V. The primaries were put in series whilst the secondaries (outputs) were put in parallel, producing a single transformer equivalent rated at 220V and secondary with 2x the current rating. The now 'single' output is used to drive the bridge rectifiers and filters of the two inpdendent amplifier channels.

This probably dropped the channel separation of the Onkyo a tad, but what the heck...

Oh, BTW isn't an autotransformer used in the same way, i.e. in the case of a step-down, the load is on 1/2 the single winding. So putting two transformers in series should be safe as long as the current loads do not go above (near?!) the specs of each...


Cheers!

[Tony]

Oh, BTW isn't an autotransformer used in the same way, i.e. in the case of a step-down, the load is on 1/2 the single winding.

hi,
just to remind you, autotransformers has both induction and conduction of current owing to that single winding from which primary and secondary taps are taken from.

so some current is conducted and some induced.

besides what the thread starter wanted to do was connect two 120 volt primiries as a single 240 volt winding, which imho can be done so long as the secondary windings are either in series or in parallel and not independent of one another.

@peranders,
i said 1kv becuase i thought 240volts ac summed up to 1kv in absolute dc terms. i was unaware of the 4kv requirement.

[Bernhard]

The amp is dual mono, got independent rectifiers and caps.

The question is if it may happen that one secondary gets overvoltage while the other has overcurrent.

There are no rail fuses for the output stage.
Only for the input stage and protection.
So the primary fuse in front of the transformers has to blow.

What could happen ?
If one output device blows, the protection will switch off speakers, no problem.
If both N & P output devices blow and make a short across the rail, it is the question if that primary fuse will blow fast enough, and also if it will blow at all because of the second series transformer.
Same thing if a rectifier diode blows...

I use a 320W stepdown transformer right now.

Other question: The stepdown got two 120V primaries and two 120V secondaries. Can I use a single secondary 120V winding with 320W load or do I have to connect two in parallel ?

[AndrewT]

Hi Bernhard,
you need to use all the windings to get the full VA rating of the step down transformer.
Wire the two primaries in series and the two secondaries in parallel.

My opinion on the series transformers; don't, for the reasons you and others have stated.
The load on the secondaries MUST be equal for the series primaries to work.
Your dual mono does NOT guarantee equal loading. Music guarantees that the loading will not be equal 99% of the time.

[clem_o]

Originally posted by clem_o


The primaries were put in series whilst the secondaries (outputs) were put in parallel, producing a single transformer equivalent rated at 220V and secondary with 2x the current rating. The now 'single' output is used to drive the bridge rectifiers and filters of the two inpdendent amplifier channels.

Originally posted by AndrewT
Hi Bernhard,
you need to use all the windings to get the full VA rating of the step down transformer.
Wire the two primaries in series and the two secondaries in parallel.

There you go - same advice. I think it should be pretty safe if you do it this way.

Cheers!