Transformers

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Transformers

Post by Rickey on Tue Oct 09, 2012 10:13 am

I read a lot about transfomers, in antique radios ,being safer than an aa5 radio. If a radio has a transformer, does that automatically mean it is isolated from the mains?
I don't fully understand isolation transformers?
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Re: Transformers

Post by Guest on Tue Oct 09, 2012 10:22 am

Line current is applied to the primary winding and as the AC cycles toward zero volts, the magnetic field collapses, inducing a current into the secondary winding. There is no physical connection between the circuitry and line current except for the power switch.

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Re: Transformers

Post by Bill Cahill on Tue Oct 09, 2012 11:31 am

Not quite so. Alot of radios have line bypass caps to keep rf frequencies from getting into line, and, radio. A block from things like car alternators, medical devices, etc...
Personally, I remove those caps, and, if there, resistors on transformer operated sets.
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Re: Transformers

Post by Guest on Tue Oct 09, 2012 1:46 pm

I was thinking of the transformer secondary connected to, for example; tube heaters and isolation of line current shock to persons. Are you refering to the many ciruit connections to chassis ground? Maybe I need to understand better as my formal electronic education ended in 1970.....LOL Maybe I was also tying to over simplify my answer, but I'll bet I could stand a better understanding! Embarassed

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Re: Transformers

Post by Leigh on Tue Oct 09, 2012 4:00 pm

One side of the AC line at your wall outlet is connected to earth ground; the other is not.
If you touch both sides of the line you'll get a serious shock.

However, if you touch the ungrounded line but do not touch anything that's grounded, you'll not get a shock.
This is why birds can sit on high-voltage power lines without being shocked. They're not touching ground.

All metal appliances in your house are grounded, including your test equipment.
If you touch the ungrounded power line and touch your test equipment you'll kill yourself.

On transformerless radios (AA5s and such), one side of the AC line may be connected to the chassis.
You don't know which side since they usually have unpolarized plugs. This presents a very obvious shock hazard with the test gear.

The isolation transformer removes this danger by providing a source of 120 volts AC that is not connected to ground at all.
It's completely isolated from ground, hence the name.

By using an isolation transformer you increase your chances of surviving your repair efforts by a large margin.

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Re: Transformers

Post by Guest on Tue Oct 09, 2012 5:19 pm

Leigh, I agree will all you said, but I'm trying to understand what Bill is refering too.

I think this may be the offending sentence;
There is no physical connection between the circuitry and line current except for the power switch.

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Re: Transformers

Post by Bill Cahill on Tue Oct 09, 2012 5:28 pm

Not sure what that means either. But, alot or radios, not all, though, have at least one capacitor going from one side of the main line to dhassis ground on transformer sets.
I remove that cap to make set a little safer.
Mind, you, there are other factors. With age, sometimes from past dampness, certainly age on at least some, leakage happens in the transformer. Sometimes, very little. other times, alot.
I check for voltage on the chassis from main line. The more I get, the more nervous I get.
No, the six volta windings, usually one end grounded to chassis, is not a worrty to me. That's isolated.
But, I agree with Leigh touching the wrong things at the same time will get you electricuted.
I always ere on the side of over cautious, rather than make fatal mistakes.
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Re: Transformers

Post by Guest on Tue Oct 09, 2012 5:51 pm

I know too well what you are saying, I had a friend loose his son to electrocution while working on a television set. I think about that many times while working on live equipment.

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Re: Transformers

Post by Guest on Tue Oct 09, 2012 9:10 pm

At the barest minimum, when there is potentially lethal voltage on the equipment, I invoke the one hand rule.

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Re: Transformers

Post by Dr. Radio on Tue Oct 09, 2012 9:35 pm

As far as radios that use power transformers, I always state you need to look at how the radio is wired to see if it is "safe" or not. Too many people make the assumption that because a radio uses a transformer, there is no risk of the "hot chassis" situation found on AC/DC sets. There ARE plenty of transformer sets out there that have cap(s) going from one side or both of the incoming ac line to chassis before the transformer.
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Re: Transformers

Post by Leigh on Wed Oct 10, 2012 12:01 am

Bill Cahill wrote:Alot of radios have line bypass caps to keep rf frequencies from getting into line, and, radio.
Common misconception.

The capacitors from chassis to the power line provide a ground for the antenna circuit through the power line ground.
They were present from both sides of the AC line because there was no way to predict which side would be grounded.

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Re: Transformers

Post by willy3486 on Wed Oct 10, 2012 8:59 am

If anyone is looking for a good isolation transformer find you a sencore Model PR57 transformer. I have had mine close to 20 years and it has been well worth it. If you find one get one of the adapter plugs that plug into a wall plug and give you three outlets. The reason is not so you can use the three outlets at once but so that you plug in stuff to the adapter and not the original plug on the transformer. The plug will wear out from constant use but you can toss the adapter and get a new one. The adapters I use are like this, I have replaced it a couple of times from it wearing out.

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Re: Transformers

Post by Resistance is Futile on Wed Oct 10, 2012 2:17 pm

I have seen in some radios a tapped auto transformer for obtaining various voltages. Just because it looks like a transformer in the radio but does not always make it a true two isolated windings transformer. This is unusual but does occur and still has the hot chassis issue.
An Isolation transformer is best if you are in doubt or it's questionable as to safety.

There are two types of transformers, the predominant one in use has two windings (or more) of wire, that are insulated from each other. The Iron core transfers the alternating current by magnetic fields (Induction) from the primary winding to the secondary winding.
Depending on the ratio of wire turns it causes Higher, equal, or lower voltages or in the case of multiple windings, multiple voltages. It still is AC voltage though.
The Higher voltages above 12 volts are usually rectified over to DC voltages for use. The lower voltages are used usually to power the filaments. But they can also be used to charge batteries by rectification to DC or supply computer dc voltages usually 3.3vdc, 5vdc and 12vdc.
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On the other hand Auto transformers usually only have one winding but still operate using magnetic induction but use wire taps for different voltages and are not isolated, This type is usually in the form of variacs, but not always. Variacs use a brush that slides over exposed windings to adjust to the desired voltage.


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Now on AA5 radios there is no power transformer, so both the tube filaments and the radio it self uses un-isolated direct power from a wall socket. If you have a piece of test equipment that also has no transformer then when you hook up the ground of the test equipment to the radio the chassis may be hot and you will either get an electrical shock in the process of hooking the connection up or you will blow a fuse, circuit breaker, or worse yet have sparks fly from the plug or wires that burn out with a bright white light. NOT GOOD for you or the equipment, in the simplest terms or significantly an electrocution of your personal body. scratch
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Re: Transformers

Post by Bill Cahill on Wed Oct 10, 2012 4:30 pm

Juist as a note of interest here.
I once had a fourties Silvertone table top radio phono sadly missing the record player. It had a power transformer with a higher voltage secondary that ran everything, except the motor.
The motor was standard 110v.

That radio had series wired tubes. About 8 of them, I believe. It had P P audio outputs, two rectifiers, an extra audio tube, and, the rest was standard. I don't remember the tube line up, but, I think total filament set up ran on about 200 volts. Also, it put out a higher ac voltage to the rectifier plates to get higher B+ voltage to operate the set.
That was one strange radio.

Then, I remember a console radio-phono Zenith made that had an auto former that made higher ac voltage for the plates of the rectifier tube so set could put out higher B+ voltage.
That one had a weird circuit design.
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Re: Transformers

Post by Rickey on Wed Oct 10, 2012 4:32 pm

Thanks guys, this has been a great thread!
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