AF-transformer 1:10, 20W

[Boncuk]

Hello friends,

I would like to build an electronic muscle stimulator for a person who is partly paralyzed. For that purpose I need a transformer as specified in the title. If possible the transformer should a printed circuit version.

I tried several suppliers but unfortunately I couldn't find the proper device.

Any hint appreciated.

Remark: The stimulator is available at hospitals for rent, but very much to my disappointment not affordable for average people.

Regards

 

[Nigel Goodwin]

Your specification is far too vague - and it's almost certain that the transformers are custom made.

 

[Boncuk]

Your specification is far too vague -

Hi Nigel,

what's too vague Transformers like this are commonly used for 100V-technique AF-amplifiers. They are normally produced for 4-, 8- and 16 Ohms output impedance.

The ones I found so far have a mimimum of 50W and they are wired. That wouldn't match my imagination for a mobile device.

 

[Nigel Goodwin]

Hi Nigel,

what's too vague Transformers like this are commonly used for 100V-technique AF-amplifiers. They are normally produced for 4-, 8- and 16 Ohms output impedance.

You've now completely altered your question, you're now saying you want a 100V line transformer with 4, 8, and 16 ohm taps - this bears no resemblance to your previous question?.

 

[audioguru]

I think a 100V speaker distribution voltage is used for PA systems in Japan.
It is 70.7V in North America but is 25V in schools and in hospitals.

Speakers on the ceiling of stores have little 2W transformers. 20W transformers might be used on speakers in arenas. They are pretty big and heavy, certainly not pcb mount.

 

[Boncuk]

you're now saying you want a 100V line transformer with 4, 8, and 16 ohm taps - this bears no resemblance to your previous question?.

I didn't say I want a transformer with 4, 8 and 16 ohm taps. I said there are normally made that way!

Speaking of 20W it is certainly not a transformer for preamps.

 

[Boncuk]

Hi audioguru,

100V systems are used in many countries for events like airport car races etc.

The device I need it for produces bursts of pulses of 65microseconds pulse length with a break between bursts depending on the program lasting from 1 second to 4 seconds.

A main transformer with that ratio wouldn't do the job.

 

[Nigel Goodwin]

Hi audioguru,

100V systems are used in many countries for events like airport car races etc.

Yes, 100V line systems are common in the UK - a 25V system sounds rather counter productive?, the idea is to minimise loses in the cabling.

The device I need it for produces bursts of pulses of 65microseconds pulse length with a break between bursts depending on the program lasting from 1 second to 4 seconds.

A main transformer with that ratio wouldn't do the job.

Would a 100V line transformer do it?, are the impedances correct? - like I said originally, just the ratio isn't anywhere near enough information.

 

[Boncuk]

Hi Nigel,

thank you so far for your kind support. May be they use the 25V lines at schools because of the danger involved with 100V lines. The losses would be too much applying hundreds of yards of cable like it is done in open air festivals. I also know that a special cable is used like it is used for military purposes concerning field telephones.

The transformer I am looking for just needs a 4ohms tap and 16ohms total. The 4ohm tap will be the "center" tap. It will be used reverse and the normally secondary winding will be the primary, transforming low voltage (max 5V) pulses to higher voltage up to 39V - at least limited to that value by appropriate means.

The ratio of 1:10 would theroretically mean 50V output, but it will be limited to 39V anyway because of the danger of burns on the skin of the patient. To elimate uncomfortness because of a voltage too high the output level will be made variable for individual use.

A normal 50 or 60Hz transformer would be too big around and wouldn't give the desired results due to a different frequency. About 15KHz is something an AF-transformer can handle.

Regards

Boncuk

 

[audioguru]

A max speaker line voltage of only 25V is used in schools in North America to avoid zapping the little kids who are "running around inside the ceilings".
Some of our more powerful amplifiers use step-down transformers for this application.

Most 20W audio transformers are made with a response from 30Hz to 15kHz so they are as large or are larger than a 20W mains transformer.

 

[Nigel Goodwin]

A max speaker line voltage of only 25V is used in schools in North America to avoid zapping the little kids who are "running around inside the ceilings".
Some of our more powerful amplifiers use step-down transformers for this application.

No wonder America has a terrible record on global warming issues!

Most 20W audio transformers are made with a response from 30Hz to 15kHz so they are as large or are larger than a 20W mains transformer.

Exactly, a 100V line transformer rated at 20W is a fairly large device.

However, is 20W required? - I don't see as skin is going to pass any where near that amount of power, at least at the low voltages suggested. I also wouldn't like to think of the damage done by passing 20W of power through the body either?.

 

[audioguru]

A 20W transformer for a 100V line has a step-down voltage ratio of 8.9 times to 4 ohms. The requirement is a little less than 10 times so the transformer can be used in reverse and will stepup a 5V signal to 39V.

A 2W transformer for a 100V line has a ratio of 35 times and would stepup the 5V to about 175V. ZAP!

A little 2W transformer for a 25V line would be perfect if one can be found with 4 ohm and 16 ohm taps (very unlikely).

 

[Nigel Goodwin]

A 2W transformer for a 100V line has a ratio of 35 times and would stepup the 5V to about 175V. ZAP!

A simple potential divider would cure that!.

 

[Boncuk]

Hi audioguru,

since you are an AF-expert here is another question. I found a small AF-transformer with exactly 1:4 ratio and 4-6ohms output impedance and PCB connectors. They only thing missing is a center tap.

How about using two of them in series and construct a center tap?

It's not the power which is decisive, its just the ratio and the operating frequency range. With a ratio of 2X1:4 the output voltage should be very close to the desired 39V at 5V input voltage.

Thank you

Boncuk

 

[audioguru]

Two separate transformers in series don't create one with a center tap.

Each transformer could be capacitor-coupled to a 5V audio amplifier's output and the secondaries could be connected in series. But an audio amplifier with a 5V supply has an output of only about 3V p-p. Then the voltage output is only 24V p-p.

Use an audio amplifier with a 12V supply and capacitor-couple it to a single 1:4 transformer for a 40V p-p output.

 

[Boncuk]

Here is again what the transformer is supposed to do:

Step up from VCC (+5V) to about 40V by means of a MosFet driver circuit being connected to each outer tap with VCC at the center tap continuously. The MosFets will only switch ground alternating at a frequency of exactly 15.385KHz.

The only thing of concern is the frequency and of course gaining high frequency and voltage DC out of 5VDC. Using a normal line transformer will fail due to the high frequency.

(I just call it high freq. DC since it doesn't produce negative voltage) You might as well call it fast chopped high voltage DC.

 

[audioguru]

A center-tapped transformer winding with a Mosfet driving each end has AC in the primary and AC in the secondary.

The waveform is a square-wave with many odd-numbered harmonics that are suppressed by the transformer..

 

[Boncuk]

Sorry, I was a little mistaken there, but that correction didn't answer my question: What about connecting two transformers in series and use the low impedance side as a primary winding with the equivalent taps treated as a center tap? For the output also series connection will apply using the outer taps for output voltage connection.

 

[k7elp60]

I think on Boncuk is wanting is what is known as a Tens or Tenz unit. In the USA they are regulated by the FDA(Federal Drug Adminstration). In simple terms they produce short pulses that varies in frequency and the amplitude is limited. They used conductive pads that are attached to the human body to transfer the pulses between parts of the body.

 

[Boncuk]

The device is generally known as Low Impact Muscle Stimulator(LIMS).