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thebob

Rewinding transformers.(Electrical boffin question)

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thebob

Many appliances are imported from 110V countries, and the usual mode of operation is to use a "step down" to operate them.

 

Often this goes pearshaped when they are accidentally plugged into the mains here.

 

My observation is that this does not damage the internal transformer, for appliances with EL's or Toroid's. But switching PSU's are toast!

 

I need a few big transformers for an audio project.

 

If I strip out the secondary windings, I can rewind these transformers for my own purposes.

 

For example a 110V +18V-0-18V 300VA when plugged into the 230v mains gives an output of about +36-0-36V. I'm assuming that the voltage has doubled and the current has remained the same. So this is now a 600VA transformer?

 

My intention is to rewind the secondary coils with wire double the cross sectional area, and with half the number of turns, to bring the output back to +18-0-18V.

 

Is this prudent? Is there something I have missed? Should I just remove half of the secondary windings, to keep the current the same?

Edited by thebob

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Headshot

So...you're putting a big sound system in your little nipa hut? Cool. Just buy the transformers that you need to get the voltage you want and don't try messing with stuff that is internal to the equipment. If you are afraid that stuff will be connected directly into the wall, just securely tape the cords into the outlets on the step-down transformer you are using, so that nobody can disconnect them. Problem solved.

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thebob

I live in a very "big" nippa hut I'll have you know! And my system is far too big to fit inside!

 

I'm building amplifiers from scratch, but the cost of transformers is more than half the cost of the projects so I'm recycling components from other equipment, not necessarily audio.

 

And whats wrong with modding hi-fi? Upgrading power supplies is often one of the first things you need to do, but I'm not interested in commercial equipment for my application.

 

My question is about calculating parameters for existing primary windings. There are a few "sparkies" on here that know the answers, I'm sure.

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Fender rules

 

My my, you are a glutton for punishment. Why not just put non standard plug on the power cord for the amp and a socket that will accept the non standard plug, then no accidents with plugging in the amp directly into the wall outlet. You could also put a diode in series with the sec outs, i believe this will half the AC out from each half. OR, just leave the trans as is and make up a voltage regulator that will give you the output voltage you require. Lots of circuits inline.

How would go about rewinding a tranny? ..... do you have a machine? I have seen a lot of places that do rewinding... they rewind fan motors as a rule, I guess they could do your rewind without any problems :)

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Alan S

Just taking a normal transformer, if you apply the wrong voltage to the primary, wont do any good.

 

Too low (input voltage) is fine, albeit inefficient, but too high and the winding will soon burn out.

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thebob

A transformer is just 2 coils of wire, wound inside a laminated steel frame. One coil takes either 110V or 230V input and the other side outputs a voltage depending on the ratio of the turns of the two coils.

 

For example if there are 110 turns on the input side, and 12 turns on the other, the transformer will output 12v. 1V per turn. Voltage is the easy part!

 

Transformers can take lots of voltage, it's current, Amps that kill them!

 

A transformer designed for a particular purpose at 110V that outputs 30V will output 60V when it is connected to 230V mains. That kills any 30V rated components that are connected to, but it doesn't hurt the transformer.

 

Transformers are rated in Volt/Amps, we usually call this Watts. The VA of a transformer is always the same, it is a function of the window area of the metal laminations of the "frame". The frame is made of thin plates of enameled steel, that contain the magnetic flux.

 

So when you double the Volts going into a transformer, you halve the Amps coming out of it!

 

Now the thickness of the wire is what protects it from Amps. So the output coil, called the secondary is, designed thick enough to withstand the current (Amps) flowing through it when it is doing a certain amount of work (Watts).

 

Transformers dismantle quite easily. The many steel plates, shaped like "E's" and "I's" can all be pulled out, and you end up with a "bobbin" with 2 coils on it.

 

If you replace the secondary coil with a known amount of turns of wire, reassemble and test the output voltage you can calculate the "turns per volt" and rewind a secondary for the voltage that you require.

 

My problem is calculating the current that this new secondary coil can safely flow.

 

An assembled 500W mono amplifier module costs about 1,300 Peso. The problem is that the transformer for it costs 7,000 peso!

 

There are discarded 110V transformers all over this country. I just want to recycle some of them.

 

Using a step down on something like an amp, is quite silly. You have to oversize the step-down to deal with the "inrush" current. The internal transformer is usually about 90% efficient, and the step down usually about 80% efficient and it adds audio noise! So you pay extra money, to have a worse sounding amp that cost you more money to run. Replacing the transformer in an existing 110V amp is cheaper and better than a step-down, and it is usually only a case of connecting 5 wires.

 

If you think any of this is dangerous, you should see home made arc welders made from microwave oven transformers. Now that is dangerous!

 

Beware if you don't know what you are doing, this stuff can kill you.

Edited by thebob

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Woolf

the 500 watt mono amplifier need, what is the specified voltage input?

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AB2000

I don't know anything about transformers, but I would think the secondary is seeing the same voltage and current as the widget you are powering hasn't changed, you would actually want to change the primary, since the input is what is being changed. Double the primary windings, but use a smaller wire since the current is less (but not sure how much smaller you could go).

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thebob

the 500 watt mono amplifier need, what is the specified voltage input?

 

+50V/0/-50V DC so you need a 36V-0V-36V transformer.

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JSL-USMC

One rule I try to keep in mind is the KISS rule. Plug it into a transformer tape it so it can't be removed and voila, way problema. Oh, that's what Headshot said.

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thebob

But when the cost of a large stepdown is almost the same as an amplifier, that doesn't make much sense.

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Topper

You're right about the turns/ratio bob, but the current will determine the size of the conductors used. When you step down a voltage you increase the current. They are inversely proportional. Voltage up, current down. That is how they transmit high voltages over long distances and why we use AC instead of DC. That being said, I would not try to rewind my own transformers. The wire must be sized for the current and properly insulated to handle the voltage. One mistake and POOF! Just too much stuff to do and watch out for. Even though the cost is high, compared to the safety factor, I would tend to agree with the others and just buy the transformer you need for your sound system. Don't go cheap on safety. BTW, considering the size of your sound system, is it pay back time for the neighbors?

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