Using one transformer to step down another.

[Shoog]

What do people think about using one single supply line transformer to step down a bigger transformer. I have this setup on a power supply for a GC. It converts a +/-35V AC transformer into a +/-13.7 V AC transformer. Seems to work well. The first step down transformer can be relatively small as it doesn't supply much current.
I have a massive tranformer with +/- 60V AC which at the moment is without a use. I was think of using it (stepped down to say +/-30V AC) to driver a higher power gainclone or subwoofer.
Can anyone sujest a good reason not to try this ???
Am I correct in thinking that the potential current that the second transformer can supply is increased by the ratio of the step down, ie a 60V 10A transformer could supply 20A if supplying 30V.
Am I missing something.

Shoog

[peranders]

Are you clear about the VA ratings? Your smallest transformer must be big enough for your needs.

The disadvantage of this is three things:
1 Weight
2 Physical size
3 Efficiency (more losses)

You have 230 V primary voltage?

If you want to step down, the secondary voltage of the first transformer must be 100-150 V. I suspect that you have mixed up something here.

Can you give us some sketch over your thoughts?

[Shoog]

The one I have set up at the moment works, I will take a look at it tomorrow to remind myself exactly how I wired the two up. The first transformer is a toroidal and the second is a big EI, they must be similar power ratings.

The weight and size problems are not huge issues when compared to the saving in been able to use otherwise redundant transformers.

My understanding on the VA rating of the first transformer is that if the second transformer is expected to supply 4A and there is a 4 times step down from first to second, then the first one only needs to supply 1A, meaning the first one can be rated 60VA (step down from 240V to 60V). If the second transformer is supply 15V (again 1:4) then its 4A input could supply potentially 16A at 15V (240VA).
This logic doesn't seem quite right to me- would the first and second transformer have to have the same VA rating ???

I will get a full plan of my current setup and post it tomorrow.

Shoog

[pinkmouse]

Originally posted by Shoog

This logic doesn't seem quite right to me- would the first and second transformer have to have the same VA rating ???

I am quite possibly wrong, but I suspect the first traffo would need to have a larger VA rating, both to account for the losses in the second, and because it is driving a purely inductive load, so there may well be back EMF issues.

[Steve Eddy]

Originally posted by pinkmouse
I am quite possibly wrong, but I suspect the first traffo would need to have a larger VA rating, both to account for the losses in the second, and because it is driving a purely inductive load, so there may well be back EMF issues.

Driving a purely inductive load? How so? The load it will be driving will be the load the second transformer is driving, modified by the square of second transformer's turns ratio, no?

se

[pinkmouse]

As I said, I'm not sure, but that is what I recall being told once when specc'ing some custom lighting control kit. And this was an engineer from Avolites, one of the best companies in the business, so I have no reason to doubt him. But then again, we were driving these traffos from heavily chopped dimmed 240V power, so that may have had something to do with it.

[haldor]

Originally posted by Shoog
What do people think about using one single supply line transformer to step down a bigger transformer. I have this setup on a power supply for a GC. It converts a +/-35V AC transformer into a +/-13.7 V AC transformer. Seems to work well. The first step down transformer can be relatively small as it doesn't supply much current.
I have a massive tranformer with +/- 60V AC which at the moment is without a use. I was think of using it (stepped down to say +/-30V AC) to driver a higher power gainclone or subwoofer.
Can anyone sujest a good reason not to try this ???
Am I correct in thinking that the potential current that the second transformer can supply is increased by the ratio of the step down, ie a 60V 10A transformer could supply 20A if supplying 30V.
Am I missing something.

Shoog

Hi Shoog,

A 60 V, 10A transformer has secondary windings sized to supply 10 A of current. Dropping the voltage to 30 doesn't make the secondary wires bigger in diameter which is what would be required to handle 20 A.

Transformers are rated using VA instead of Watts to account for reactive loads. That 600 VA transformer will supply 600W into a purely resistive load. Since it has a 60V secondary rating then the max current it can ever safely supply is 10 A. With a reactive load the peak current will not coincide with the peak voltage which means that a Watt meter will lie (since it is only measuring the resistive component of the power disappated in the load.

Sorry to burst your bubble.

Phil

[Shoog]

"Dropping the voltage to 30 doesn't make the secondary wires bigger in diameter which is what would be required to handle 20 A."


This is informative, but in itself it doesn't make the idea any less useful than before. Having a 10A 30V transformer is a hell of a lot more usefull in most situation than havving a 60V 10A one.

The point about matching VA ratings is much more critical - anyone with a definative answer on this one.

Shoog

[peranders]

Originally posted by Shoog
The first step down transformer can be relatively small as it doesn't supply much current.
I have a massive tranformer with +/- 60V AC which at the moment is without a use.

I think you have missed what VA rating is. A transformer with 1 A and 400000 V huge and 100A, 0.1 V is small.

Your 60 V transformer can only be used if:

1 remove some of the winding, which decreases the VA rating to 50-60% of the original value.

2 Connect a 230 to 115 transformer in front of the 230/60. With this you can take out less than 100% of the VA. This is a heavy non-optimation.

I think the whole idea is not very good if not have the two transformers and/or space and weight is of no importance. Sorry....

[fdegrove]

Hi,

The point about matching VA ratings is much more critical - anyone with a definative answer on this one.

Haven't followed this from start to finish but the VA ratings are meaningless in this case unless you also specify the voltages you're using.

As a basic rule of thumb the first xformer is going to determine what the next one can ultimately deliver but there's no point in feeding xformer two with a 1KVA xformer if xformer 2 is only designed for 100VA and the circuit is drawing more than that.

If, conversely, the circuit draws less than 100VA (W) and xformer one can deliver more than, there's no harm done, at best you'll have better regulation.

Hope this clarifies it a little or I have misread the issue altogether,