What happens when Connecting step-up and step-down secondary Transformer coils?

[toneboy1]

This website is a great idea. (I'm a thread vergin)

Not sure if its standard but where I live 'step up' is secondary = higher voltage
Opposite for 'step down'.

What would the result be if you connected one secondary with a high voltage and low current in series with a secondary that had a low voltage and high current respectively, so that the positive terminal of the step-down secondary was connected to the negative terminal of the step-up terminal?

(Note: Both transformers may or may not be the same ratio (but inversed) and the primaries are connected in parallel and/or the magnetic metal core's of the transformers are the same. P.S. the order of secondary connection can change if this makes a difference)

I'll be honest, my hypothesis is high voltage and high current would result, but intuitively that seems too good to be true?

Alternatively I am also curious as to what would happen if they were in parallel and rectified so the voltage didnt flow from step up to down with the consequence of making it half wave DC. For this I have attatched a drawing to help picture what I mean.


Thanks heaps and kind regards,

Rob

 

[Sciurus]

Hi, toneboy1.

What would the result be if you connected one secondary with a high voltage and low current in series with a secondary that had a low voltage and high current respectively, so that the positive terminal of the step-down secondary was connected to the negative terminal of the step-up terminal?

What determines the current through a secondary coil?

 

[toneboy1]

Hi, toneboy1.


What determines the current through a secondary coil?

The fact that it has less turns around the alternating magnetic core of the transformer than the primary.


Regards,

Rob

 

[Sciurus]

The fact that it has less turns around the alternating magnetic core of the transformer than the primary.

I always looked at it as the load determined the current, but I guess in an ideal sense, there is also a relationship between turns and current.

What would the result be if you connected one secondary with a high voltage and low current in series with a secondary that had a low voltage and high current respectively, so that the positive terminal of the step-down secondary was connected to the negative terminal of the step-up terminal?

Is this your circuit?

 

[toneboy1]

I always looked at it as the load determined the current, but I guess in an ideal sense, there is also a relationship between turns and current.

Is this your circuit?

Oh, yes, just in a simple ideal sense of the set-up. No wonder I didn't really understand your question.

Yes, T2 being the step-down transformer. That is a perfect drawing, maybe I should have drawn it, sorry, I'm going to get a scanner soon.

Regards,

Rob

 

[Sciurus]

OK, cool. Since T1-secondary, T2-secondary, and R_L form a closed loop with no branches, the current through those components is the same. The voltage on R_L is the sum of the voltages from the T1-secondary and T2-secondary (the AC has the same phase), and by ohm's law, you can work out the current through the resistor. The current through the primaries then relates to turn ratios of the transformers, as in the other diagram.

 

[toneboy1]

Ah, I see, so with this set-up you could potentially get very large voltages and currents through the resistor?

What did you think of the drawing I attatched to the original question, the one where they are in parallel?


Thanks

 

[Sciurus]

Ah, I see, so with this set-up you could potentially get very large voltages and currents through the resistor?

What did you think of the drawing I attatched to the original question, the one where they are in parallel?

Well, you can get high current through any transformer by decreasing the resistance of the load. (Sometimes it's called increasing the load since the energy the transformer supplies increases). Of course, at some point, either a fuse blows (if you're lucky), or something starts on fire. The ability of a transformer to handle high current depends on the design -- mostly its ability to dissipate heat.

The effect of your two-transformer circuit could be achieved with just one -- the second transformer just adds a little more voltage to the resistor, you could do that with more winding on the first.

I've got to go, I'll take a look at the other diagram in a bit.

 

[toneboy1]

Unfortunately I don't think I can agree there. Even if you short circuited a step-up transformer the current still wouldn't be as high as a step-down transformer. Hence your explanation of just adding more voltage misses the point, the point of a step down transformer is more current.

I heard it be explained like if you have a marbel and a bowling ball suspended, the bowling ball a foot high and the marbel over the top of a building. They both the marbel has more 'voltage' but the bowling ball has more current, like the bowling ball is a step down transformer.

My question (to me) seems to be able to make high voltage and high current but I'm not so sure in my gut, seems like it might put out more power than it takes in (but I haven't done the maths).

I think I still need more 'input' :P

thanks though

 

[Sciurus]

Unfortunately I don't think I can agree there. Even if you short circuited a step-up transformer the current still wouldn't be as high as a step-down transformer. Hence your explanation of just adding more voltage misses the point, the point of a step down transformer is more current.

Hmm...I must misunderstand something. To be honest, my experience with this stuff is from a technician point of view, not from an engineer's, so I often think in terms of mental short-cuts so I can troubleshoot quickly. In this case they may have gotten me into trouble somewhere.

 

[Don Kelly]

Sciurus is right. All you get is a small increase in open circuit voltage as well as in the internal impedance so that you may get a bit more or less current in a load than with the step up transformer alone. What you won't get is the higher voltage of the step up transformer along with the higher current capacity of the step down transformer. Note that the current capacity and the impedance of the step up transformer are limiting factors. These are both lower than those of the step down transformer
You would be better off simply adding some turns to the step up transformer as that is effectively what you are doing, or possibly by using an autotransformer.
any EE energy conversion text will deal with this

 

[toneboy1]

H'MM, so what your saying is that the step-down transformer couldn't fulfil its function because it would be limited due to that it would have to flow through the internal impedance of the step-up transformer, (which is high) ? Makes sense.

Also what about if there were diodes there like in the attatched drawing?


Thanks heaps guys,

Rob

 

[Don Kelly]

The load current is determined by the voltage and the circuit impedance- NOT by the current rating of the transformer. Suppose the output voltages were 100 and 10V and the load impedance was 100 ohms. Ignoring the transformer impedance, you would get a half cycle current of 1.414 Apeak (141.4Vpeak) followed by a half cycle peak of 0.1414A (14.14V peak). Not much better than a half wave rectifier using the HV transformer only. I don't think that this is what you want.