Using an Audio Transformer for Power

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bugmenot

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I connected my 15VAC power supply to a small audio output transformer (part of a kit), and whoops! blew a fuse. The secondary of the transformer was disconnected (ie, an open circuit).

I'm very confused as to why the fuse blew. My understanding is that very little current can flow in a primary if the secondary isn't connected to a load. If that's true, why did my fuse blow? If it's not true, how does any power transformer work? Does it make a difference that the transformer was intended for audio, not power, use? Why?

(For completeness: I also had a volt meter hooked up to the primary. And I may have thought the primary leads were secondary and vice versa.)
 
Are you sure it was a 15VAC power supply and not a 15VDC power supply? A DC supply will damage the transformer if enough current flows.
Even if it was an AC supply, all transformers have a maximum voltage rating that can be applied to the primary winding. They also have a minimum and maximum frequency that they can operate at. Exceed these ratings and excessive current will flow and damage the transformer and/or blow the fuse.
 
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By operating at a lower frequency than the transformer is designed for, this will decrease the Z of the primary substantially, thereby increasing the current. To go with Kchristes comment, audio transformers operate at much lower voltages than power transformers. However, audio transformers will have a higher DC resistance than power transformers, so you probably won't damage anything by hooking it up to DC. But it also depends on the resistance of the coil. Simply ohm it out to verify. I use transformers for phone line communications, so yours may be different. So you probably used AC, and with the higher load the audio transformer provides, blew the fuse. That would be my guess.
 
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The input current into a transformer with no load is only small if the back-voltage generated by the changing magnetic field is nearly the same as the input voltage.

The magnetic field is proportional to the input current.

The back voltage is proportional to the rate of change of magnetic field.

The magnetic field cannot go on increasing for ever. It stops increasing at a point called saturation, where the core cannot take any more magnetic field. When this happens, there is much less rate of change of magnetic field, so much less back voltage. If the input voltage is fixed, the current increases dramatically.

Saturation will happen if the input voltage is too high or the frequency is too low. It sounds like that was the case with your audio transformer on 15 V at 50 Hz
 
All of the above info is correct.

to add a few numbers, most output transformers have 0-4-8-16R outputs.
depending on the power, that corresponds to the following voltages
5W 0-4.4-6.3-8.9v
10W 0-6.3-8.9-12.6v
25W 0-10-14.1-20v
50W 0-14.1-20-28.3v
100W 0-20-28.3-40v

so it depends how big the transformer was and which taps you used. as you can see, a lot of the above options won't accept 15V

as for frequency, then half the frequency means you have to run at half the voltage. depending on the amp, the output tranny would 'probably' go down to 50Hz - certainly if it was hifi, and most guitar amps do too.

depending on the transformer the secondary resistance may only be 1R or so, so if it was a DC supply, 15A would be drawn. fortunately you blew a fuse, so the tranny is probably ok.
 
The primary side is normally rated to 100W.

The question is why would you want to do this?

I take it that this was just an experiment.

If the secondary has gone open circuit then the chances are it's knackered.
 

What size fuse?

I asked this same question back in college, because theoretically you could have a transformer with one turn in the primary and two in the secondary, but you know this one would also blow fuses.

The cranky old teacher told me that you first have to decide what core flux density you want and the rest of the design falls out from that.

You could measure the AC impedance of your winding by hooking it to a doorbell xformer and measuring voltage across and current through. Your arrangement may work on some lower voltage.

I used an audio xformer to deliver a small voltage from the secondary with 120vac on the primary, when I was a kid.
It got rather warm over time so I know it wasn't happy, but it did work. I probably lucked out.
 
A power transformer has enough inductance so that it draws a very low current when it is not loaded.
An "audio" transformer might haver a response that begins at 300Hz and at 60Hz it is a dead short.
 
I used this transformer because I had none other, and I didn't know it would break things. The fuse was 1.25A. The supply was clearly AC (labeled as such, confirmed with DMM).

Can someone explain a bit more why at lower frequencies, the transformer can't handle a voltage? How can current flow in the primary, if it's not flowing in the secondary?

How can I test the transformer to see if it's fried?
 

Even though there is nothing connected to the secondary, you still have a primary coil that is connected to your source and drawing current. That's all it really is, is a coil with an open secondary. Think of an inductor connected to an AC source. It's a closed circuit, therefore current flows. It's not the voltage that's the issue directly but the Z of the audio primary windings that is drawing excessive current. You could have an audio transformer that can have a higher Z at 50/60 Hz 15Vac and not blow anything (Although I would never use an audio transformer for power).
Something you haven't addressed yet, is why are you using an audio transformer? I know you said you don't have anything else, but what are you trying to use it for? What's the application?

To address your question in detail, the formula for inductive reactance is:
Xl = 2πfL

2π (2pi) = 6.28 radians per cycle
f = frequecy
L = inductance
You can see that the Xl is directly proportional to the frequency. So as f decreases, Xl decreases thereby lowering the Z. That is Z = sq(R² +Xl²). Sorry, not sure where the square root symbol is. I hope this clears things up.
 
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The core of the audio transformer might saturate when it is fed a signal frequency that is at a lower frequency that it can handle. When the core saturates then the primary winding is not an inductor, it is a low value resistor that burns.
 
The rule is main transformers are rated in VA and output transformers in Watts

(well it's my rule anyway)
 
It was the 100W transformer that was mentioned and I figured it to be a power transformer.
 
rezer said:
You don't really see a wattage rating for transformers. They are rated in VA. "Apparently" close enough (pun intended).
Click to expand...

Sorry I meant 100W.

rezer,
The square root sign is on the left hand cornder of the symbols panel.
 
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rezer said:
Something you haven't addressed yet, is why are you using an audio transformer? I know you said you don't have anything else, but what are you trying to use it for? What's the application?
Click to expand...

The application is to build a simple RC filter to remove noise from a signal. To create a noisy signal, the project manual (Art of Electronics Student Manual) says to hook up a transformer's secondary (powered by an AC power supply) through a function generator's high freq. sine wave output. The RC filter is supposed to remove the 60Hz noise from the transformer and give you a clean sine wave signal.

Does anyone have a simple, alternate way of creating a noisy signal? I have a simple function generator, and a bunch of standard simple components.

Thanks for all the great help here. I have one remaining unanswered question: how can I test the transformer to see if it's been destroyed?
 


If you know the winding resistance of the pri/sec, simply meas it with an ohmmeter. Otherwise, you can still meas it, but you won't really know if a few windings have become shorted or not.
So what you are working with is a series RC high pass filter, correct?
If all you need is a 60Hz sine wave, why not use the 15Vac transformer. If you have two of them, you can put them in series to drop the voltage to about 1.8V if the power level is a concern. If your function generator has dual outputs or if you have access to two, use the other for your 60hz signal (You may want to isolate them with a resistor.).
Can you post a block diagram of your project?
 
rezer said:
If you know the winding resistance of the pri/sec, simply meas it with an ohmmeter.
Click to expand...
The kit shows it as 500 Ohm/8 Ohm. My measurements are a fraction of that.


Otherwise, you can still meas it, but you won't really know if a few windings have become shorted or not.
Click to expand...
If some windings have been shorted, what will be the properties of the transformer? Can I still use it in audio circuits, or will it act as a short circuit?

If all you need is a 60Hz sine wave, why not use the 15Vac transformer.
Click to expand...
I need a 60Hz sine wave with a faster sign wave riding on top of it. My function generator and AC power supply are part of the same kit and share the same ground, so I know of no way to mix the two without using a transformer. (I'm sure it could be one via some analog mixing circuit, but I'm not up to that yet).

Can you post a block diagram of your project?
Click to expand...
How?
 
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