Back to Back Transformers

[eccerr0r]

I guess I have to clarify that the two windings on the "primary" of the "secondary" transformers need to be carefully connected if you're using both of them and yes phase matters since you're connecting them together on the same core to the same source. Though I do find this weird, usually double primary winding transformers were meant to be 120/240 and you should be using both windings in series for 240V... weird.

[petehall347]

just though of a simple test .. strobe disc on a record player and a bulb into the transformer output . that will say for sure if frequency has doubled . if it has do more load tests ..you may have come across something here ..

[eccerr0r]

Yes have to agree that the extra 50Hz is noise your meter is somehow picking up, and will go away when loaded down. That being said you should be able to see the noise in a scope...

[pintglass]

Originally Posted by redwire

This schematic diagram might help. If OP can verify.

Phasing is important for the dual-primary tranny but more so if they were autotransformer connected which is what sam is talking about.
Adding a small load will get rid of the frequency doubling OP is seeing.

I want an earth ground connection and don't see it.

Yes the schematic is how I have it wired up.

I don't have an earth connection.

I will test later today with a load.

I have an image somewhere of how, what is now my output transformer was wired up orignally in the charger, if i remember correctly it could be switched from slow to fast charging, which I assume would mean the transformer is wound in parallel for higher current when fast charging.

[eccerr0r]

do investigate, usually for 'more' current, it increases voltage. How? Reduce number of windings on primary... kind of strange, but not like it has more secondary windings to increase...

[pintglass]

I've just tested and indeed when connected to a load the frequency does drop to 50Hz

Just for testing purposes am I ok to connect my oscilloscope on the output transformer, do I connect the ground clip or leave of?

[eccerr0r]

If the isolation transformer is really working as an isolation transformer, the whole point is that you can connect your scope straight to both the isolation transformer output pins. The point of an isolation transformer is that you can now probe any points after the isolation without worrying about shorting your scope to ground.

But do measure to make sure your isolation transformer is indeed isolating.

[pintglass]

I've attached an image from my scope.

This was captured with no load.

Yellow trace is connected to the output transformer.

Blue trace is connected to the 12V side.

I don't seem to be getting a great waveform is this to be expected using transformer's back to back.

[eccerr0r]

yeah that would do it, those little blips would cause you to get 100Hz.

[redwire]

The squiggle at zero-cross which is making your multimeter read double-frequency, I think is from the power meter's internal power supply (capacitive-dropper). If you disconnect the meter's power it would prove it. Having a small AC load also gets rid of it.

You might not have the transformers quite right.
If you overdrive the last transformer, let's say it is a 6VAC winding being fed 12VAC, its core will be saturating on the peaks. If everything runs hot with no load then this is likely the case.
You could connect the isolation transformer unit backwards (feed mains into the output, have a fuse) and see what symmetry you get on the waveforms.

Connecting transformers back-back usually gives meh results because transformers are overwound to make up for their losses. They are actually made as a 10-20% higher output voltage part which then sags under full load.

[petehall347]

it might have worked better with matched transformers

[pintglass]

Originally Posted by redwire

The squiggle at zero-cross which is making your multimeter read double-frequency, I think is from the power meter's internal power supply (capacitive-dropper). If you disconnect the meter's power it would prove it. Having a small AC load also gets rid of it.

You might not have the transformers quite right.
If you overdrive the last transformer, let's say it is a 6VAC winding being fed 12VAC, its core will be saturating on the peaks. If everything runs hot with no load then this is likely the case.
You could connect the isolation transformer unit backwards (feed mains into the output, have a fuse) and see what symmetry you get on the waveforms.

Connecting transformers back-back usually gives meh results because transformers are overwound to make up for their losses. They are actually made as a 10-20% higher output voltage part which then sags under full load.

The transformers do get hot after a while with no load especially the input transformer. I think when I was testing before I got more of a voltage drop with them the other way around.
Disconnecting the meter does improve the waveform.

Originally Posted by petehall347

it might have worked better with matched transformers

I agree but finding two the same is not easy when trying to do it on the cheap, as this is only a hobby for myself.

[eccerr0r]

That's not a good sign that they're getting hot with no load... oh well, depends on how long you keep it connected and idling, I guess?

I have a bona fide 300W iso transformer, it gets a little warm, but not hot.