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PostPosted: 24 Nov 2016, 15:58 
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Joined: 23 Oct 2015, 12:35
Posts: 10
Location: Czech Republic
Hi!

I found a small toroidal transformer in scrap bin at work. It's rated at 110V/11.2V. Since I live in europe I'd be feeding it 230V and should in theory get about 23.4V on the secondary, which is convenient for my aplication.
What I'm concerned about is the higher voltage damaging it. Should I be worried? Is the frequency difference a problem too?

Thanks for any reply and be good :))

Potucek


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PostPosted: 24 Nov 2016, 16:10 
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Joined: 25 Jan 2015, 11:51
Posts: 54
Location: Netherlands
Good evening!

For that Voltage the count of windings simply will be not enough Potucek.
Even without any load it will get hot very fast and then will burn and be not usable anymore, I am afraid :(
I tried the same with a 110Volt transformer and it even tripped my (old style) wire-fuse...

Mathew


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PostPosted: 25 Nov 2016, 02:34 
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Joined: 23 Oct 2015, 12:35
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Location: Czech Republic
What a shame... Thanks for your reply. Well, there is two of them, so i guess that i'll use them both and just connect them in series.

One more question: could the transformers be used as output transformers for a tube headphone amp? Everything I've ever built with tubes was hybrid and I'd love to try something tube onlyish. I dont know the number of windings, but the secondary is rated at 0.75A.

Potucek


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PostPosted: 25 Nov 2016, 03:46 
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Location: Netherlands
Hm

Putting both in series (both primaries and secondaries) will give you 112 / 1100 * 240 = 24.5Volt by 0.75A
Using them as output transformers most probably will give an interesting result.
I expect it will give you the ultimate BASS experience :)
Only the frequencies round 60Hz will come trough; Your sound-spectrum will be "flat" between 30Hz and 200Hz or so.


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PostPosted: 01 Mar 2017, 01:23 
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Joined: 28 Dec 2010, 22:07
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I have used transformers designed for power transmission and they are decent for communications-quality audio. Whether or not they are flat 20 Hz to 20 kHz is doubtful, but I haven't made any measurements.

If your transformer is 110v - 11.2v, the turns ratio is 9.82, which means the impedance ratio is 96.5-ish. If you have headphones with 32-ohm impedance, the primary impedance will be 3087 ohms. Most tubes that can conduct 10-15 mA can put a few milliwatts into 3087 ohms.

The way to design an amplifier is to start at the load. If you want a headphone amplifier, you start with 32 ohms or whatever your headphone load is.

EXCEPT

The transformer changes your load. If you're using the transformer mentioned above, you're not looking at a 32 ohm load but a 3087 ohm load. Now, let's say you want an amplifier with a max power of 1 watt into 3087 ohms (although, with headphones, it would be much less).

1 watt into 3087 ohms is 18 mA RMS and 56v RMS. This could be done by most receiving tubes. A 6AQ5, EL84, or 6V6GT biased at 28 mA would be more than capable of providing headphone power. If you have sensitive headphones, you could run a 12AT7 or 6DJ8 with 10 mA or so into the primary of that transformer. Some headphones are power hogs and need about a watt of power. For that, I'd recommend 200v of B+ and 30 mA of standing current. That means 6 watts of plate dissipation, which is in the realm of most receiving tubes.

You'll get better bandwidth with a transformer designed for audio, but the low power requirements could work in your favor.

Ed

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PostPosted: 01 Mar 2017, 08:37 
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A few things:

a) Many transformers do not care about the input voltage as much as the current on them.
b) And some few transformers are bi-directional as well.
c) What is far more important is the current the transformer is carrying.
d) Most individuals stop when they get to the correct voltage, and do not consider the current required for the device under consideration.
e) Watts is watts.
f) Then, there is Power Factor - the losses in converting from one voltage to another. "Unity" would be a power-factor of 1, meaning no losses to heat, conversion or other issues.

With all that in mind:

1. What is the demand for the device attached in watts, continuous?
2. What is the rated power of the secondary of the transformer you have, continuous? This will be in VA (volt-amps) which is Volts x Amps x Power Factor. For the purposes of your use. I suggest a power factor of no more than 0.85.
3. What quality (best guess) is the transformer.
4. Safe output watts for that transformer at 240V input will be 1/2 the rating for 120V.

So, after determining all of the above, you will know if the transformer *might* be suitable for your intended use.

If it falls within the realm of possibility, do the following:

Using about a 1A fuse, connect the transformer primaryto 240V, no load. Run it, while supervising, for at least four (4) full hours. The transformer should NOT get hot. It should barely get warmer than room temperature. If it does get hot, reject it immediately. If it does not, you should be home-free. BUT, make sure it is properly fused.

On the 60 Hz to 50 Hz, if you are well within the operational parameters otherwise, the losses will be negligible. If you are on the margins, recalculate at a power factor of 0.80. NOTE: A 50 Hz transformer will run cooler and more efficiently at 60 Hz. And why I suggest the 4-hour test before passing on calculations alone.


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PostPosted: 01 Mar 2017, 12:35 
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Joined: 28 Dec 2010, 22:07
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If you are wanting to use this power transformer hooked up to a 240v line, I'd connect a light bulb in series with the primary. The lamp should be rated for 240v at some higher wattage than the transformer rating. When you turn on the power, one of two things will happen. The transformer primary will either draw excessive current, causing the lamp to glow brightly, or the primary will successfully impede the 50/60 Hz current and the lamp will glow dimly or not at all.

The preferred outcome is the second one. If your secondary is not connected to anything, the primary should show a high impedance to AC and little current will flow. If you were to short the secondary leads or connect them to a heavy load, it will draw power from the primary, allowing current to flow. You will notice the primary lamp lighting up as you short the secondary. The lamp will protect your transformer from excessive power from the mains.

The winding of a transformer is designed with a certain voltage in mind. Applying excessive voltage can cause an excess of magnetic flux. I applied 120v AC into the secondary of a 24v transformer one time. Immediately, the transformer vibrated excessively and the core heated up, so I disconnected it.

Peter's advice is sound, but know that some transformers are over-engineered and will therefore withstand applying higher voltage to the primary, where others will easily burn up. Use caution when playing around with an unknown transformer.

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PostPosted: 03 Mar 2017, 11:03 
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The winding of a transformer is designed with a certain voltage in mind. Applying excessive voltage can cause an excess of magnetic flux. I applied 120v AC into the secondary of a 24v transformer one time. Immediately, the transformer vibrated excessively and the core heated up, so I disconnected it.

At a 5:1 increase, that is no surprise. The OP is suggesting a 2:1, so not necessarily as much of a strain on things. Worth a try with the proper cautions. I have two bi-directional 2:1 transformers from my days working overseas - but they were designed specifically for the use.


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PostPosted: 03 May 2017, 17:33 
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Joined: 29 Apr 2017, 15:14
Posts: 43
If a 110v xfmr is used on 240v, that sucker will fry, even more so, the frequency
is more likely to be 50hz if it is 240v


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