Two transformers

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rayma

rayma

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Joined 2011
gyro said:
if you value your really nice quality transformers, then a second bridge rectifier, as in payloadde's drawing is a small price to pay to protect them from unbalanced (or one rail open) load conditions.
Click to expand...

Yes, an extra bridge is cheap insurance, and fewer grounding problems as well.
The rectifier current pulses are confined to each winding, and are not in a
common center tap to ground.
 
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andyr

Member
Joined 2003
Mark Whitney said:
Measure the resistance to confirm A and B, if you get it wrong you may damage the transformer.
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Thanks, Mark - let's assume I have measured the secondaries and have been able to determine which 2 wires belong to one and which 2 wires belong to the other.

So let me re-state my question; assume the 25v secondaries have ends marked:
* A1 and A2
* B1 and B2.

Am I correct in thinking that, if I connect 'A1' with 'B1' and read around 50v, that shows the 2 windings are in the same polarity.

Whereas if I measure around 0v then that shows I have joined the secondaries in opposite polarity and if I connect 'A1' with 'B2', the windings will be in the same polarity?


Thanks,

Andy
 
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gyro

Member
Joined 2004
DF96 said:
Yes. I realise that undermines my argument!
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Haha, sorry about that.

Adding load to a transformer does not push it into saturation. On the contrary, most power transformers are near the limit of saturation when unloaded and loading moves them away from saturation. The current limit is usually set by heating (core and copper). Very cheap transformers will sometimes saturate unloaded, but work fine with a load. I agree that accidentally going into half-wave mode will heat up a transformer, but this is not saturation.
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Yes, I can understand that increased secondary loading will 'eat' the flux generated by the primary and of course the core and copper losses for a symmetrical load The only bit I'm not convinced on is why a half-wave load won't shift the overall core magnetization in one direction and move it into saturation on one half of the cycle... and the concept of the primary being capable of pulling more current on one half of the cycle (being an isolated AC device).

I guess that one is going to come down to more reading and some experimentation (easy to do) but at least the consensus seems to be moving towards 'play it safe' as far as the number of bridges goes.
 
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andyr

Member
Joined 2003
rayma said:
Right. What you don't want to do is connect the windings in parallel, but with opposite polarities.
That would do damage.
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Thanks, Ray. All I want to do is be able to work out which are the 'starting' ends of the 2 secondaries - so I can use each one on its own diode bridge, wired in the same polarity. "In real life" I won't actually be connecting the 2 secondaries in series.

Someone (Mark?) made an earlier comment about a simple test (which gives twice the voltage or ~0v) to make sure that the single secondaries belonging to 2 different transformers could be wired in the same polarity ... I was merely trying to extrapolate this idea, to see whether the same concept could be used to identify when 2 secondary windings belonging to the 1 transformer were in the same polarity.


Regards,

Andy
 
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AndrewGM

Member
Joined 2008
OK... after some speed reading I realise you are all in another world to me !:eek:

However, I really like the two full wave bridge route - seems risk free, gives me plenty of what I really want plus I can't wreck anything by having a wiring mishap.

I can also insert plenty of smoothing caps, bypass caps and a choke or two also - yes I called them inductors.
I was thinking CLC en route to nice clean DC for the output stages

Thank you again for this comprehensive and educational set of ideas

Andrew
 
rayma

rayma

Moderator
Joined 2011
andyr said:
I was merely trying to extrapolate this idea, to see whether the same concept could be used
to identify when 2 secondary windings belonging to the 1 transformer were in the same polarity.
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Certainly, that's right. The two secondaries are floating, and it doesn't matter whether
they're on the same core or not, for this purpose.
 
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DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
gyro said:
and the concept of the primary being capable of pulling more current on one half of the cycle (being an isolated AC device).
Click to expand...
Think of the transformer as being simply an impedance transformation device. For simplicity assume 1:1. Whatever impedance you put on the secondary will be seen reflected in the primary; whatever current is drawn from the secondary will be present in the primary. If that impedance is asymmetric then so will the current be. As the same current is present in the primary and the secondary, any DC component will cancel just as any AC component will cancel.
 
Mark Whitney

Mark Whitney

Member
Joined 2013
andyr said:
Thanks, Mark - let's assume I have measured the secondaries and have been able to determine which 2 wires belong to one and which 2 wires belong to the other.

So let me re-state my question; assume the 25v secondaries have ends marked:
* A1 and A2
* B1 and B2.

Am I correct in thinking that, if I connect 'A1' with 'B1' and read around 50v, that shows the 2 windings are in the same polarity.

Whereas if I measure around 0v then that shows I have joined the secondaries in opposite polarity and if I connect 'A1' with 'B2', the windings will be in the same polarity?


Thanks,

Andy
Click to expand...

Correct, if in doubt try this: Powering Up Your Radio Safely With a Dim-bulb Tester.
 
Mark Whitney

Mark Whitney

Member
Joined 2013
If you do make a mistake when testing and short the secondaries the light bulb will protect the transformer. The bulb is in series with the primary and as the current increases the voltage over the transformer decreases.

But if you enjoy the anticipation of sparks, smoke and bangs then don't let me stop you.
 
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gyro

Member
Joined 2004
DF96 said:
Think of the transformer as being simply an impedance transformation device. For simplicity assume 1:1. Whatever impedance you put on the secondary will be seen reflected in the primary; whatever current is drawn from the secondary will be present in the primary. If that impedance is asymmetric then so will the current be. As the same current is present in the primary and the secondary, any DC component will cancel just as any AC component will cancel.
Click to expand...

Yes, I follow and agree with all of that, thanks. Still very curious about where the core magnetization is sitting in absolute terms though - I'm off to do some experimenting on a torroidal and see how asymetrical I can push it in practice. :)
 
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andyr

Member
Joined 2003
Mark Whitney said:
Correct, if in doubt try this: Powering Up Your Radio Safely With a Dim-bulb Tester.
Click to expand...

Just did the experiment, Mark, with a 160VA toroid having 2x 26v secondaries. (No lightbulb needed. :D )

Secondaries are marked on the tranny as:
* Red – yellow
* Blue – grey.

From the above, one would assume that Red & Blue are the same points – say, the start – of the secondaries.

So when I connect them in series (there are 2 possible orientations) – I get 2 different voltage readings:

* Connect yellow to blue and measure between red & grey 55.8v (not 52v, as the trannie is unloaded)

* Connect yellow to grey and measure between red & blue 0v!!

So it’s very easy to make sure the secondaries are in phase! :) Simples! :D


Andy
 
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