Hooking up Output Transformer Crystal Radio Plans Help!! Help!!!

[[email protected]]

Greetings,

Need a little help here. I am building the crystal radio plans here:

http://hibp.ecse.rpi.edu/~john/xtal.html

It calls for an Output Transformer. I 'Think' I have one (Caltronics
2510). The problem is that I don't have the docs for it. The
secondary terminals are labeled in Ohms and it has a 4 Ohm terminal
with Common just like the plans call for which is fine but the
Primary Terminals are labeled like this

10W, 5W, 2.5W, 1.25W, 0.62W and Common

First question: Is this the type of Transformer the plans are calling
for?

Second question: Which of the terminals would be the 10,000 ohm input?
(or is there one??)

Third question: How would a person measure the impedance of a
Transformer like this?

Fourth question: Would it be possible to wind your own and how would
you figure the turns ratio for a transformer of this type?

Fifth question: how is this different from a ordinary power
transformer in design?

Sixth Question: Why is mine labeled in Watts? if the W is for Watts?

Thanks in advance for your help!

 

[Spehro Pefhany]

On Wed, 31 Dec 2003 05:27:30 GMT, the renowned

Greetings,

Need a little help here. I am building the crystal radio plans here:

http://hibp.ecse.rpi.edu/~john/xtal.html

It calls for an Output Transformer. I 'Think' I have one (Caltronics
2510). The problem is that I don't have the docs for it. The
secondary terminals are labeled in Ohms and it has a 4 Ohm terminal
with Common just like the plans call for which is fine but the
Primary Terminals are labeled like this

10W, 5W, 2.5W, 1.25W, 0.62W and Common

First question: Is this the type of Transformer the plans are calling
for?

I don't think so. That sounds like a PA distribution transformer. Some
of the audio guys will be able to clarify further.

Second question: Which of the terminals would be the 10,000 ohm input?
(or is there one??)

Third question: How would a person measure the impedance of a
Transformer like this?

Fourth question: Would it be possible to wind your own and how would
you figure the turns ratio for a transformer of this type?

Fifth question: how is this different from a ordinary power
transformer in design?

Sixth Question: Why is mine labeled in Watts? if the W is for Watts?

Thanks in advance for your help!

Best regards,
Spehro Pefhany

 

[Michael A. Terrell]

Greetings,

Need a little help here. I am building the crystal radio plans here:

http://hibp.ecse.rpi.edu/~john/xtal.html

It calls for an Output Transformer. I 'Think' I have one (Caltronics
2510). The problem is that I don't have the docs for it. The
secondary terminals are labeled in Ohms and it has a 4 Ohm terminal
with Common just like the plans call for which is fine but the
Primary Terminals are labeled like this

10W, 5W, 2.5W, 1.25W, 0.62W and Common

First question: Is this the type of Transformer the plans are calling
for?

Second question: Which of the terminals would be the 10,000 ohm input?
(or is there one??)

Third question: How would a person measure the impedance of a
Transformer like this?

Fourth question: Would it be possible to wind your own and how would
you figure the turns ratio for a transformer of this type?

Fifth question: how is this different from a ordinary power
transformer in design?

Sixth Question: Why is mine labeled in Watts? if the W is for Watts?

Thanks in advance for your help!

That is a "Line transformer" The idea behind marking it in watts is
for quick calculations for PA and large intercom systems. The two common
systems in the US are 25 and 70 volt.

If it is a 70 volt line transformer the taps = 5,000 Ohms/Watt

10 watts = 500 Ohms
5 watts = 1,000 Ohms
2.5 watts = 2,000 Ohms
1.25 watts = 4,000 Ohms
..625 watts = 8,000 Ohms

If it is a 25 volt line transformer the taps = 625 Ohms/Watt

10 watts = 62.5 Ohms
5 watts = 125 Ohms
2.5 watts = 250 Ohms
1.25 watts = 500 Ohms
..625 watts = 1,000 Ohms

There are 100 volt line transformers in some countries.


If it is a 100 volt line transformer the taps = 10,000 Ohms/Watt

10 watts = 1000 Ohms
5 watts = 2000 Ohms
2.5 watts = 4000 Ohms
1.25 watts = 8000 Ohms
..625 watts = 16000 Ohms


--
Merry Christmas!

Take care, and God bless.
Michael A. Terrell
Central Florida

 

[Spehro Pefhany]

I don't think so. That sounds like a PA distribution transformer. Some
of the audio guys will be able to clarify further.

Oops, this post escaped early. Make that line matching transformer.

Here's the closest I could find with a quick search:

http://www.mouser.com/catalog/specsheets/164278.pdf

1K2 to 4 ohms, 400mW $2.19 ea.

Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

systems in the US are 25 and 70 volt.

If it is a 70 volt line transformer the taps = 5,000 Ohms/Watt

10 watts = 500 Ohms
5 watts = 1,000 Ohms
2.5 watts = 2,000 Ohms
1.25 watts = 4,000 Ohms
.625 watts = 8,000 Ohms

If it is a 25 volt line transformer the taps = 625 Ohms/Watt

10 watts = 62.5 Ohms
5 watts = 125 Ohms
2.5 watts = 250 Ohms
1.25 watts = 500 Ohms
.625 watts = 1,000 Ohms

There are 100 volt line transformers in some countries.


If it is a 100 volt line transformer the taps = 10,000 Ohms/Watt

10 watts = 1000 Ohms
5 watts = 2000 Ohms
2.5 watts = 4000 Ohms
1.25 watts = 8000 Ohms
.625 watts = 16000 Ohms

So, it should work quite well if it's the (most common, IME) 70V type.

Best regards,
Spehro Pefhany

 

[John Popelish]

Greetings,

Need a little help here. I am building the crystal radio plans here:

http://hibp.ecse.rpi.edu/~john/xtal.html

It calls for an Output Transformer. I 'Think' I have one (Caltronics
2510). The problem is that I don't have the docs for it. The
secondary terminals are labeled in Ohms and it has a 4 Ohm terminal
with Common just like the plans call for which is fine but the
Primary Terminals are labeled like this

10W, 5W, 2.5W, 1.25W, 0.62W and Common

First question: Is this the type of Transformer the plans are calling
for?

Probably not. It is likely a lot bigger and lower input impedance but
you might get it to work a bit.

Second question: Which of the terminals would be the 10,000 ohm input?
(or is there one??)

I doubt it. It is a public address distribution transformer (for amps
that have lots of speakers of different wattages and desired sound
levels connected). You adjust the power out (assuming some maximum
amplifier output voltage) by picking the appropriate wattage tap.

There are two standard distribution voltages, 25 volts and 70 volts
RMS. There may be some indication on the transformer which kind you
have, or if it handles both voltages. The input impedance can be
calculated by the formula for power of P=V^2/R dividing the maximum
voltage squared rating by the power (on the tap) gives the approximate
input impedance. So if it is a 25 volt transformer, the highest input
impedance would be the 0.62 watt tap, with an impedance of
25*25/0.62=202 ohms. However, if it is a 70 volt transformer, the
input impedance would be 70*70/0.62=7900 ohms, which is within
spitting distance of your required 10k.

Third question: How would a person measure the impedance of a
Transformer like this?

You could put a 4 ohm resistor across the output an test it with an
impedance bridge excited at 1000 Hz.

Fourth question: Would it be possible to wind your own and how would
you figure the turns ratio for a transformer of this type?

Impedance goes as the square of turns ratio, so a 4 ohm to 10,000 ohm
transformer would have to have a turns ratio of the square root of
10,000/4 or 50 to 1. That's the easy part. Picking the best actual
turns count depends on the core size and what the frequency response
needs to be. Big subject.

Fifth question: how is this different from a ordinary power
transformer in design?

Not very different. Possibly thinner laminations, less insulation
between primary and secondary, or actually an auto transformer
(primary and secondary are just taps on a single coil.) Good audio
transformers often have the primary split, with half wound on each
side of the secondary or other tricks to get good magnetic coupling to
extend the high frequency response.

Sixth Question: Why is mine labeled in Watts? if the W is for Watts?

Hope I covered that.

Thanks in advance for your help!

You are welcome.

Since a crystal set delivers a very low power to the audio, you could
easily get by with a 50 milliwatt transformer instead of your 10 watt
version. But those miniature buggers can be quite spendy.

E.G.
check out type 144p
http://rocky.digikey.com/WebLib/Hammond/Web Data/140,143,144,145,146.pdf

That little gem will put you out about $13 plus shipping and minimum
order charge from Digikey. But I think this is the sort of thing the
crystal set designers have in mind.

 

[[email protected]]

Wow, Thank You!!! The dark fog is starting to lift (I think)

Am I correct in saying "Impedance = AC Resistance?"
Because a coil/inductor resists the flow of AC??!!

Is that right?

BTW, this one is a 25V but I have a 70V too so I am in business...this
transformer thing has been a major obstacle in me building that radio.
It looks really neat with the two detector diodes added together.
Dunno how well it will work but we will see. I am also trying to
understand the concept of matching impedance How do you measure or
calculate AC resistance? I have a 10V AC supply and a multimeter to
work with. I would like to measure the impedance of the transformer I
have and also figure out the turns ratio if possible. Can I do that
by feeding AC into it and measuring the output voltage or is this
transformer different somehow?

 

[William Sommerwerck]

Am I correct in saying "impedance = AC resistance?"

Because a coil/inductor resists the flow of AC??!!

Very close. Reactance is AC-only part -- the part contributed by the coil's
inductance. Impedance is the combination of reactance plus resistance (the DC
part).

 

[John Woodgate]

I read in sci.electronics.design that brokepokerplayerREMOVE@yahooJUNKTO
REPLY.com wrote (in <[email protected]>) about
'Hooking up Output Transformer Crystal Radio Plans Help!! Help!!!', on
Wed, 31 Dec 2003:

Greetings,

Need a little help here. I am building the crystal radio plans here:

http://hibp.ecse.rpi.edu/~john/xtal.html

It calls for an Output Transformer. I 'Think' I have one (Caltronics
2510). The problem is that I don't have the docs for it. The
secondary terminals are labeled in Ohms and it has a 4 Ohm terminal
with Common just like the plans call for which is fine but the
Primary Terminals are labeled like this

10W, 5W, 2.5W, 1.25W, 0.62W and Common

First question: Is this the type of Transformer the plans are calling
for?

Not really; what you have is a line transformer for a public address
loudspeaker. But read on.

Second question: Which of the terminals would be the 10,000 ohm input?
(or is there one??)

You have a tapped-primary transformer. The highest impedance you can get
is by using the Common and 0.62 W tap. If you are in UK, those watt
values are the powers delivered to the transformer (and passed on almost
unaltered to the loudspeaker) when the input voltage is 100 V. For the
Americas, the voltage is probably 70 V.

To get 0.63 W from 100 V, the impedance must be V^2/W = 15873 ohms.
The next tap up gives you 8000 ohms, which is within the range specified
by the designer.

Third question: How would a person measure the impedance of a
Transformer like this?

The transformer itself doesn't have an impedance in the sense you mean.
It transforms the impedance of the load on the secondary winding to a
higher impedance on the primary side, and that impedance depends on
which tap you use.

You can measure the impedance by connecting a 4 ohm resistor to the
secondary winding and an audio signal generator with a reasonably high
output voltage (say 10 V) to the primary winding through a 10 ohm
resistor. By measuring the voltage across the 10 ohms, you can calculate
the current into the transformer, and very nearly all the 10 V appears
across it. Knowing the voltage and current, you can calculate the
impedance.

Fourth question: Would it be possible to wind your own

It has been done, but you need to wind thousands of turns of very fine
wire. So, first build a coil-winder.

and how would
you figure the turns ratio for a transformer of this type?

Turns ratio = square root of impedance ratio. So here, sqrt(3000/4) =
27.4 and sqrt(10000/4) = 50.

Fifth question: how is this different from a ordinary power
transformer in design?

Too complex to answer in a newsgroup article. Basic principles are the
same, but many additional theoretical and practical issues differ.

Sixth Question: Why is mine labeled in Watts? if the W is for Watts?

See above.

 

[John Woodgate]

I read in sci.electronics.design that [email protected]
om wrote (in <[email protected]>) about 'Hooking
up Output Transformer Crystal Radio Plans Help!! Help!!!', on Wed, 31
Dec 2003:

Can I do that
by feeding AC into it and measuring the output voltage or is this
transformer different somehow?

If you do it at 400 Hz and measure carefully both the input and output
voltages, you will get a reasonably reliable figure for turns ratio. But
check your meter specification to see if it measures up to at least 400
Hz. Using 60 Hz you may well get a misleading figure.

 

[John Woodgate]

I read in sci.electronics.design that John Popelish <[email protected]>

with an impedance of
25*25/0.62=202 ohms.

Calculator frolic. 992 ohms.

 

[Active8]

Wow, Thank You!!! The dark fog is starting to lift (I think)

Am I correct in saying "Impedance = AC Resistance?"
Because a coil/inductor resists the flow of AC??!!

Is that right?

Impedance is the vector sum of resistance and reactance. Try
googling on Ian Purdie tutorial

I think you'll probably end up becoming totally addicted to
electronics if you read through his stuff.

BTW, this one is a 25V but I have a 70V too so I am in business...this
transformer thing has been a major obstacle in me building that radio.
It looks really neat with the two detector diodes added together.
Dunno how well it will work but we will see. I am also trying to
understand the concept of matching impedance How do you measure or
calculate AC resistance? I have a 10V AC supply and a multimeter to
work with. I would like to measure the impedance of the transformer I
have and also figure out the turns ratio if possible. Can I do that
by feeding AC into it and measuring the output voltage or is this
transformer different somehow?

What you can do is connect a signal generator set at 1kHz to the
input of the transformer or whatever thru a variable resistance
thusly:

o--------o
| |
| | a ___ b o-----o
| AC o------|___| +-------o| |
| | A | | | Load
| | | | o--o--o
o---o----o +-----+ |
| |
| |
| |
| |
+-------------------------------+

Here's your equivalent circuit

a ___Rv b
+-----|___|----+
| |
| |
| |
.-. .-. XL
| | Rs | | XL Vb = Va* ----------
| | | | Rv + XL
'-' '-'
| |
| |
.-. |
( V ) |
'-' |
| |
+--------------+
created by Andy´s ASCII-Circuit v1.22.310103 Beta www.tech-chat.de

So if XL = Rv, then

Rv
Vb = Va*---- = .5*Rv
2*Rv

You measure the voltage at a and b while you adjust the pot and
when the voltage at Vb = 0.5*Va, you know the resistance is
approximately equal to the load impedance at 1kHz, a standard audio
test frequency. Measure the R from the pot's wiper to it's input
and you're done.

<snip>

HTH

 

[Active8]

Oops, this post escaped early. Make that line matching transformer.

Here's the closest I could find with a quick search:

http://www.mouser.com/catalog/specsheets/164278.pdf

1K2 to 4 ohms, 400mW $2.19 ea.

Best regards,
Spehro Pefhany

That's the same one they sent me free. The gal on the phone found
the part from the number I gave her. I was really satisfied with
the efficiency of service and that's one reason I use Mouser to
this day. They're still on the ball.

 

[Active8]

Impedance is the vector sum of resistance and reactance. Try
googling on Ian Purdie tutorial

I think you'll probably end up becoming totally addicted to
electronics if you read through his stuff.

Whoops! I forgot. With a transformer, it needs a load on the output
for this to work, so use whatever load is called for to do this
test.

 

[John Popelish]

I read in sci.electronics.design that John Popelish <[email protected]>


Calculator frolic. 992 ohms.

Dang! I remember entering 125 /.62 into the calculator (doing the
25*25 in my head). I'm getting old. )-;

 

[fjdvgnkjfndkjvgn knmdslkfgnekldf]

Your calculator frolics differently than mine. I get:

25*25=625 625/0.62=1008.+

Why the difference?

Harley L. Miller

////////////////////////

 

[YD]

I read in sci.electronics.design that brokepokerplayerREMOVE@yahooJUNKTO
REPLY.com wrote (in <[email protected]>) about
'Hooking up Output Transformer Crystal Radio Plans Help!! Help!!!', on
Wed, 31 Dec 2003:

Not really; what you have is a line transformer for a public address
loudspeaker. But read on.

You have a tapped-primary transformer. The highest impedance you can get
is by using the Common and 0.62 W tap. If you are in UK, those watt
values are the powers delivered to the transformer (and passed on almost
unaltered to the loudspeaker) when the input voltage is 100 V. For the
Americas, the voltage is probably 70 V.

To get 0.63 W from 100 V, the impedance must be V^2/W = 15873 ohms.
The next tap up gives you 8000 ohms, which is within the range specified
by the designer.

As an aside, the crystal radio is unlikely to have enough punch to
drive a speaker, a pair of phones is better for this. As they can be
found with 32 ohms impedance, the primary impedance will be
correspondingly higher too. Likely to improve the performance, crystal
receivers tend to be high impedance devices.

Dang, I miss my old 2k telegraph phones, lost while moving an
uncountable number of years ago. And crystal earplug phones don't seem
to exist anymore.

- YD.

 

[John Woodgate]

I read in sci.electronics.design that fjdvgnkjfndkjvgn knmdslkfgnekldf
kc.rr.com>) about 'Hooking up Output Transformer Crystal Radio Plans
Help!! Help!!!', on Wed, 31 Dec 2003:

Your calculator frolics differently than mine. I get:

25*25=625 625/0.62=1008.+

It was 0.63 not 0.62. Of course, it was probably 0.625 plus or minus 1
turn on the winding.

 

[John Woodgate]

(in <[email protected]>) about 'Hooking up
Output Transformer Crystal Radio Plans Help!! Help!!!', on Wed, 31 Dec
2003:

Dang, I miss my old 2k telegraph phones, lost while moving an
uncountable number of years ago. And crystal earplug phones don't seem
to exist anymore.

If you do a Google for 'crystal set' you will find suppliers of 2 kohm
headphones after a bit of surfing. I was surprised, too, but luckily I
came across a pair without having to buy new.

 

[Spehro Pefhany]

(in <[email protected]>) about 'Hooking up
Output Transformer Crystal Radio Plans Help!! Help!!!', on Wed, 31 Dec
2003:

If you do a Google for 'crystal set' you will find suppliers of 2 kohm
headphones after a bit of surfing. I was surprised, too, but luckily I
came across a pair without having to buy new.

Ignoring the impedance matching issue, would they be as efficient as a
modern rare-earth magnet "Wallkman" type earbud?

Best regards,
Spehro Pefhany