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Old 8th May 2012, 09:54 PM   #1
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Default Reversing Interstage Transformer

I have an interstage transformer with ratio 1+1 : 1.5+1.5k

Current handling capability is 15mA series and 30mA parallel

Could I reverse this transformer to 1.5+1.5 primary and 1+1 secondary ?
In this reversed configuration, is the current handling still the same ?
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Old 8th May 2012, 10:07 PM   #2
DF96 is offline DF96  England
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If the transformer is a step up of 1.5 in impedance then this means that turns ratio is sqrt(1.5) - about 1.225. Assuming the current limit is caused by core saturation rather than copper heating, you can put 12.25mA or 24.5mA in reverse mode.
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Old 8th May 2012, 10:17 PM   #3
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The turn ratio is 1+1 :1.5+1.5
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Old 8th May 2012, 10:25 PM   #4
DF96 is offline DF96  England
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OK, you didn't say which ratio. A turns ratio of 1:1.5 would mean current is limited to 10mA or 20mA.
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Old 8th May 2012, 10:28 PM   #5
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Should not be too much of a problem.
When secondary DCR is about 1.5 higher than primary DCR the same wire gauge has been used; when secondary DCR is much higher than 1.5 x prim, take care because secondary wire will be thinner and have less current capability.
These specs are normally within not too tight margins when you have a quality unit as far as core saturation is concerned; I'd say give it a try.

Last edited by pieter t; 8th May 2012 at 10:32 PM.
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Old 8th May 2012, 10:35 PM   #6
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Quote:
OK, you didn't say which ratio. A turns ratio of 1:1.5 would mean current is limited to 10mA or 20mA.
Could you tell me how did you come up with these current figures ?

I wouldn't think the current capability correlates with turn ratio.

Last edited by HP8903B; 8th May 2012 at 10:43 PM.
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Old 9th May 2012, 06:36 AM   #7
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Quote:
Originally Posted by DF96 View Post
If the transformer is a step up of 1.5 in impedance then this means that turns ratio is sqrt(1.5) - about 1.225. Assuming the current limit is caused by core saturation rather than copper heating, you can put 12.25mA or 24.5mA in reverse mode.
Are you sure this makes sense?

The current through the windings is not the same thing as the magnetising current. For any given frequency and voltage, the higher the current due to the load, the less likely the transformer is to saturate.

The magnetising current depends on the frequency, the voltage, and the primary inductance.

Your calculations seem OK. Basically, the power handling will be the same, and you are changing the reference from one winding to the other.

The more crucial question here is, will it have the same bandwidth? This is where that initial confusion between winding ratio and impedance ratio makes me wonder.

Essentially, the bandwidth will shift if the impedance seen by the windings is changed. Consequently, reversing the transformer and keeping everything else the same will change the frequency response.
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Old 9th May 2012, 09:08 AM   #8
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But, when I woke up next day, I thought interstage, valve, DC current.

That's another thing that might have been more clear. The current limit is for DC bias current. To assume this relates to core saturation rather than power dissipation is unnecessary: it's a certainty. No secondary current arises from DC primary current, and so it is all magnetising. Flux density is proportional to turns*current, so the DC limit for the reversed transformer will be 1/1.5 times what it was.

Points made about bandwidth remain.
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Old 9th May 2012, 10:56 AM   #9
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Quote:
Originally Posted by PlasticIsGood View Post
But, when I woke up next day, I thought interstage, valve, DC current.

That's another thing that might have been more clear. The current limit is for DC bias current. To assume this relates to core saturation rather than power dissipation is unnecessary: it's a certainty. No secondary current arises from DC primary current, and so it is all magnetising. Flux density is proportional to turns*current, so the DC limit for the reversed transformer will be 1/1.5 times what it was.

Points made about bandwidth remain.
I'm still having trouble with this:

Wire is rated strictly on current carrying capacity.
If only the primary is meant to carry the extra DC current (on top of the AC),
then one of the transformer maker's concerns will be current carrying capacity of the wire,
and he will use heavier wire in the primary, possibly even if the ratio were 1:1.

When you reverse an interstage xformer,
you are transferring the DC standing current to the other winding,
the one which could easily have smaller wire, especially if its a step-up.

Although the transformer doesn't care which side is given the source
of the AC signal, and which side passes it on, (reverse the general signal flow
and core saturation stays the same: think time reversal).

The transformer wire may care which side is carrying
the additional D.C. current, which is the whole reason
the transformer is needed (isolation).

It seems to me this must be checked on a transformer by transformer basis.
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Old 9th May 2012, 11:48 AM   #10
DF96 is offline DF96  England
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Core saturation will be shown by a reduction in inductance. Copper heating will be shown by a rise in DC resistance. It is therefore possible to do some measurements to see where the limits are.
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