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 Transformers, power and audio, coils, chokes

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fifties



Join date : 2011-02-14

PostSubject: Re: Transformers, power and audio, coils, chokes   2011-02-14, 11:38 pm

I recently restored a Philco farm radio, and in the course of repair, discovered that the audio output transformer had an open primary.

(An easy way to validate the audio section of a receiver, BTW, is to connect an audio source -I use a Transistor radio, with a plug inserted into the earphone jack, and it's two wires (earphone removed) going to the ground and center pin on the volume control of the receiver- and see if audio comes out, and in particular amplified audio, with the source at a low volume.

Anyways, when I got no audio output, I connected the source directly to the speaker, and cleared it. I then measured the resistance on each side of the audio output transformer, and discovered the open (which I have found from experience is not all that uncommon on older tube sets).

The print called for 1100 ohms on the primary, and the secondary measured a little over one ohm. Not having an audio out transformer with more than about 400 ohms on the primary, I measured a 117 volt/12 volt center tapped power transformer, and discovered it had a 1000 ohm primary, and about a 3 ohm secondary.

Worked like a charm, and actually better using the center tap on the output side.
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PostSubject: Re: Transformers, power and audio, coils, chokes   2011-02-15, 12:27 pm

Good suggestion! Well explained. Some users have set up a speaker with a universal transformer with switching and connectors to use as a test bed, Using your method.

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PostSubject: Testing output transformers   2011-02-24, 8:31 pm

This may end up being a long topic.
I will go off line and compose the topic and return and post it when finished.
-------------------------

This is a lecture on using Test equipment to Find out How to measure transformers Using different methods . One problem is transformers don't always have the impedance marked on them.

One method is to use a filament transformer plugged into the wall, for your 60 HZ Voltage source. The transformer winding On the primary Should of course be 115 V . The secondary Winding Is Six Or twelve volts out. This is to be our source voltage, measure it to make sure.

That concept of measuring Impedance indirectly is this shortcut method used when you don't have an impedance bridge, this is accomplished By using a variable resistor In series with an unknown transformer or coil and this in turn is hooked up to the 6 or 12 volt output of your voltage source. An AC voltage is impressed on both the series variable resistor and coil, when doing this, a voltmeter is first placed across either the resistor or the coil and the voltage drop measured. The test leads are then moved to the other component. Its measurement is also noted.

The variable resistor is adjusted till both of the voltage drops are equal. Disconnect the circuit and measure the resistance of the variable resistor. That resistance is the equivalent of the impedance of the coil.

A resistance value of 20,000 ohms would probably be sufficient.

The next method used is involving some math.

One rule of thumb to use when checking the impedance of a speaker is using a DC ohm meter. The rule is the resistance measured of the voice coil, is multiplied by 1.25 to arrive at the close approximation of the impedance of the speaker.

So a measured resistance of 6.5 ohms would be 6.5 times 1.25 = 8.1 ohms
or if you know the voice coil impedance you can reverse the method by dividing the known impedance of 16 ohms impedance into the value of 1.25 = 16 ohms / 1.25 = 12.5 ohms DC
Now to determine the ratio of transformer windings, of input to output.

We this time use a variac transformer as the voltage source, A variac transformer is used on the primary side of the unknown transformer. This time a AC voltmeter is used to measure the output voltage of the secondary. The variac is adjusted to make the output voltage read 1 volt. This will be our reference standard.

Now after setting the Variac to get the one volt standard, measure the variac output voltage used on the primary side of the unknown transformer. This will give you the ratio in voltage. So if you measure the input voltage as 75 volts, you then know the ratio is 75:1
(Hence-75 volts into primary and 1 volt out on the secondary)

Finding impedance for the primary of your transformer.

You need first to get the voltage (turns) ratio.
So you first divide the input voltage by the output voltage
This gives you the turns on the primary to 1 turn on the secondary.
(Input voltage / output voltage = VTR)
Example: 115 vac/ 1 vac = 115
Ratio: → 115:1 = (VTR) One Hundred fifteen to one.

Now you need to get the impedance ratio.
Take the voltage (turns) ratio and square it = (VTR x VTR).
Volts (turn) ratio x Volts (turns) ratio = Impedance ratio.
Example: → 115volts X 115volts = 13225 = Impedance Ratio.

Finally take the impedance ratio and multiply it by the secondary load resistance.
Impedance ratio x Secondary load = Impedance ratio.
Example: → 13225 x 8 ohms = 105800


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PostSubject: Tip on Testing transformer Load capacity   2011-06-06, 11:03 pm

You will need a thermometer and a load resistor.
The load resistor will be determined by either the transformer specifications data, or trial and error experimentation.

Hook up a power plug to the transformer primary leads and connect to The AC power.
Let the transformer run for about 60 minutes then measure the temperature.

Then apply your load resistor to the secondary
There is a rule of thumb that I learned for smaller and larger transformers.

On the smaller transformers after applying the load for another hour you should (normally) see a rise in temperature above room temperature of no more than 68 degrees.
On larger transformers after applying the load for another hour you should (normally) see a rise in temperature above room temperature of no more than 86 degrees.
This is due to Hysteresis.

If the temperature is very hot or rises above the maximum stated temperature rise you may have a shorted winding

If you need a safe load use the older filament style light bulbs, either in series to reduce the load or paralleled to increase the load. I also used this method when I worked on Navy teletype power supplies to check for voltage regulation..

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fifties



Join date : 2011-02-14

PostSubject: Re: Transformers, power and audio, coils, chokes   2011-06-07, 12:37 am

A pair of power transformers can be used to make an isolation transformer.

If you have two 117 volt to 12 volt transformers (or 6 volt secondaries...The important thing is that the output voltages are the same), you can hook up their outputs together, so the 117 volt side of one goes to the AC line, and the 117 volt side of the second is your isolated output.

If you are using wall warts, of course, I am not sure if the conversion to 12 volts DC, and then back to AC would work, so it might be best to take out the rectifier circuits of each, thus allowing just AC to pass through the entire circuit.
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ve1arn
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PostSubject: Re: Transformers, power and audio, coils, chokes   2011-06-07, 3:49 am

Great tips guys.Very Happy

I've copied them and put them in my little notebook I keep on my hard drive for future reference.

Bob
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PostSubject: Re: Transformers, power and audio, coils, chokes   2011-06-07, 10:45 am

fifties wrote:
A pair of power transformers can be used to make an isolation transformer.

If you have two 117 volt to 12 volt transformers (or 6 volt secondaries...The important thing is that the output voltages are the same), you can hook up their outputs together, so the 117 volt side of one goes to the AC line, and the 117 volt side of the second is your isolated output.

If you are using wall warts, of course, I am not sure if the conversion to 12 volts DC, and then back to AC would work, so it might be best to take out the rectifier circuits of each, thus allowing just AC to pass through the entire circuit.

The use of wall warts may or may not work, some that are designed only convert the AC as a step down to a lower AC voltage, Then you have those that convert to DC lower voltage, and yes you would have to remove the rectifier(s), then there are those that use switching transformers which use a higher frequency to transfer voltage-which would be of little use for Power transfer, by just using the transformer itself. Switching power supplies use a higher frequency to allow a smaller transformers to be used and get a substantial increase in low voltage DC current. This type is used a lot in the computer industry for laptops and desktops.

I haven't looked at switching power supplies admittedly, but I am assuming the transformer inputs are quite a bit different. I will have to look into those.

The current provided from wall warts can be any where from 50 milliamps to 3 or 4 amps, so you would have to know the power requirements of your isolation transformer design, and also consider the losses. That is a good Idea of using wall warts for small power usage projects though.

I went- I looked- Rolling Eyes at switchers, my brain couldn't take it.

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FrankB
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PostSubject: Emergency impedence matching transformer   2011-07-19, 11:19 pm

I needed a matching transformer for my old EH Scott SLR-F. 500 ohms to 8 ohms.
None being readily available here I used a 6.3 V filament transformer, connecting the primary to the receiver and the secondary tot he headphones. It worked quite well tillI could get to a hamfair & find the correct transformer.

In another pinch I needed an output transformer for a AA5 radio. I used a 70.7 V line transformer, and it actually worked quite well.
FrankB
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PostSubject: Re: Transformers, power and audio, coils, chokes   2011-07-25, 12:11 pm

This kind of information is always nice to disseminate.

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PostSubject: Link to rebuilding Transformers   2012-06-01, 3:13 pm

I took a look at this web page and found it quite useful. This was mentioned from ARF

[You must be registered and logged in to see this link.]

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PostSubject: Saturable reactor for GE Colorama Models   2012-06-05, 10:52 pm

One of the friends on ARF is having some windings custom made, He says He will order 10 to cut costs.
He said they may run $60 but if enough orders are given they may be less. So I am going to jump-in and take a chance. I will follow up on this when I get the radio working.

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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-01-23, 9:46 pm

Don't feel bad, some don't understand a SPDT switch No
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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-01-23, 11:24 pm

What's that? Suspect lol!

Oh! I get it, it is a South Pole Digital Transformer study cheers

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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-01-24, 8:16 pm

How did you know that ????
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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-01-24, 10:15 pm

As homer Simpson said "I are smart, S-a-m-r-t!" DOOOH!

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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-02-14, 9:11 pm

fifties wrote:
A pair of power transformers can be used to make an isolation transformer.

If you have two 117 volt to 12 volt transformers (or 6 volt secondaries...The important thing is that the output voltages are the same), you can hook up their outputs together, so the 117 volt side of one goes to the AC line, and the 117 volt side of the second is your isolated output.

If you are using wall warts, of course, I am not sure if the conversion to 12 volts DC, and then back to AC would work, so it might be best to take out the rectifier circuits of each, thus allowing just AC to pass through the entire circuit.
Yup, I made one years ago and still use it. I also wired in a light bulb that will indicate a short in the radio ( filter caps etc ). It helps to keep the radio from frying itself. A short will make the lightbulb bright and if normal, the bulb will light dimly. I also have a variack to bring up the voltage slowly. Isolation transformers !! Thats why I am still here !! LOL Oh! I get it, it is a South Pole Digital Transformer
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PostSubject: testing transformers   2013-07-20, 10:00 pm

This is a simple solution to find out if a transformer is shorted or open. First make sure all the transformer leads are disconnected and not connect to any other component.

Take a battery 1.5-6 Volts DC, a 100 kohm resistor, a neon light, and connect momentarily in series with the suspected transformer to see if the bulb flashes.(of course do this on both the primary and secondary windings in turn) If the transformer has a short it will not allow the light to flash as the short acts as a load. With a multimeter you will not notice if the short is only one turn of the winding as the resistance reading is to low for the multimeter to register.

Also you can use this method to check for open windings.

No Now don't shock yourself by induction currents. And don't use this method on plugged in radios. No

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Last edited by Cliff Jones on 2013-07-27, 10:45 pm; edited 1 time in total
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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-07-27, 9:59 pm

This quote is from the book by  Markus and Levy,  Elements of Radio Servicing 1947

How to check the power transformer.  The best check for normal operation of the power transformer is to use a watt meter,, or AC amp-meter connected in the primary circuit.  The servicemen's multitester, however, rarely includes scales in ranges that are suitable for this purpose. A check with inexpensive equipment can be made as follows:

One.  Remove all tubes from the radio.
Two.  Plugthe radio into an outlet that contains an ordinary 25 or 40 W lamp in series with the line....
Three.  A good transformer will cause the lamp just to glow.
Four.  Any short that is present will cause the land to glow brighty.
Five.  If a short is present, remove the transformer secondary leads from their connection points, one winding at a time, to determine whether the short is internal or external; in the latter case, to determine which circuit contains the short.

Two interpret the above checks, it might be well at this point to give some more transformer theory.  With all the tubes removed, the secondaries are not drawing current and consequently, the primary should not be drawing current.  This would be true if the transformer were 100% efficient.  Since this is not so, the primaries will draw a small amount of current to overcome, the hysteresis   eddy-current losses in the iron core.  With the average radio power transformer, this a small amount of current is sufficient to cause the series 25 watt lamp just to glow.  This is the test for a good transformer.

Now, assume some shorted turns, or in short in the six volt amplifier filament wiring.  The primary must furnish the power that this short consumes.  The added primary drain causes more current to flow through the series 25 watt lamp, and the lamp glows more brightly.
Now, suppose that we disconnect the six volt transformer leads.  If the lamp brightness drops to just a glow, we must inspect the receiver filament circuit for the short.  If the lamp filament continues to glow brightly, even after all circuits have been opened, the short is within the transformer.

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PostSubject: Power Transformer color code AND TESTING   2013-07-27, 11:19 pm

Radio Manufacturers Association (R.M.A.)
Power Transformer color code:

Primary winding:
1. both windings are black if it has no primary tap.
2. Otherwise :
A1 Black and red.
A2. Black and yellow (tapped).
A3. Black.
---------------
Secondary windings as follows.

Rectifier Filament:
1. Yellow
2. yellow and blue (tapped)
3. Yellow

High Voltage Windings:
1. Red
2. Red and Yellow (tapped)
3. Red

Amplifier Filament windings 1
1. Green
2. Green and Yellow (tapped)
3. Green


Amplifier Filament windings 2

1. Brown
2. Brown and Yellow
3. Brown
---------------------------
If you have a transformer that isn't color coded you can still Identify the leads.

Using an ohmmeter and a voltmeter:
First you need to pair up leads. You do this by resistance checks.
First connect one ohmmeter lead to one wire and the other lead to each of the other wires in turn. write down each resistance read.
The lead that shows continuity is the other end of that winding. (may be a total of three leads if the winding has a tap).
Pair those wires up and move on to a new lead and proceed in the same fashion, till all the wires are paired together.
Note you may have a wire that reads open, that may be a shield for the transformer.

A. The primary leads should read 5-15 ohms generally.
B. The secondary high voltage leads should show 200-400 ohms
c. the filament windings should show less than 1 ohm.
-------------
Next connect the primary to the AC line, (I would connect a dim bulb first to the primary to check for shorts)
Using the Voltmeter check the voltages of all paired secondaries and write down those voltages.

BE SAFE YOU WILL ENCOUNTER HIGH :cherry:VOLTAGES.

This is a very simple way to test power transformers. Have fun and again be safe.

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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-07-28, 10:58 pm

And NEVER ohm the low voltage leads while touching the HV windings.
(Think car ignition coil).
Don't ask....................
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PostSubject: Re: Transformers, power and audio, coils, chokes   2013-07-29, 8:35 am

Ohhhhhhhh! I see, that's that old induction trick, I wondered why my arm did a kneejerk response as a kid. LOL

Had a model T coil back then that I disassembled, sorriest mistake I made.Sad

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PostSubject: Re: Transformers, power and audio, coils, chokes   2014-10-30, 3:17 pm

FrankB wrote:
And NEVER ohm the low voltage leads while touching the HV windings.
(Think car ignition coil).
Don't ask....................
Dang!!! You too? clown Embarassed

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