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I need to create a 100-120V 50mA output from a transformer.

I do not need continuous steady output (no need DC 120V a 120V pulsating signal on output is enough.)

  • N1/N2=1/12.2
  • secondary impedance 1.2kOhm
  • primary impedance 8Ohm
  • The primary current is limited to 0.75A. -The frequency response of transformer 10Hz to 10kHz.
  • PWM signal is from STM32F103C8 microcontroller 3.3V DC (PWM frequency 20Hz-1kHz)

I have the following doubts:

  • Will a 12V DC PWM signal create 120V and 50mA on secondary winding?

  • Following is my schematic. Is it possible to workout?

  • Back EMF from the transformer primary winding - do I need a diode across the winding?

Schematic

The main purpose of this circuit is built a transcutaneous electrical nerve stimulation machine as shown in http://chemelec.com/Projects/Tens/Tens-2-Rev-4.png

Turns Ratio N3 2-3: 0.11 gage 380 turns

N2 1-2: 0.11 gage 380 turns

and N1 4-5: 0.21 gage 60 turns

schematic-transformer

I am using 1-3 (output 120V generation) and 4-5 (input 12V)here.

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    \$\begingroup\$ It looks to me like the primary current will be closer to 1A than to 0.75mA. Secondary current would then be more like 100mA. Potentially deadly at 120V. \$\endgroup\$
    – JRE
    Nov 3, 2019 at 12:12
  • \$\begingroup\$ @JRE I control primary current from MOSFET gate voltage. using a multiturn potentiometer. but with eddy losses, it is not possible to generate 100mA. 100mA is the ideal condition, isn't it? also, the primary current is limited by winding impedance and the 1W resistor 12V/16.2=0.75A \$\endgroup\$
    – oppo
    Nov 3, 2019 at 12:15
  • \$\begingroup\$ Yes, 100mA is something like ideal. \$\endgroup\$
    – JRE
    Nov 3, 2019 at 12:17
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    \$\begingroup\$ THIS MACHINE KILLS || I max is VERY POORLY defined - it's ABOUT (VPWM- Vgs) / 8.2 - which could be anything between about 300 mA and zero. If the transformer dotting matches what was originally shown my original comments stand. If reversed it stands with polarities reversed. || AS I SAID - the original circuit probably uses BOTH polarities - and inductor stored energy is probably a major factor. || Look at the many many many many circuits in the reference I provided and see what they do. These are data points. Most TENS "designers" have no clue. | THIS MACHINE KILLS \$\endgroup\$
    – Russell McMahon ♦
    Nov 3, 2019 at 23:17
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    \$\begingroup\$ @Roland this is not for the brain just for arm or legs \$\endgroup\$
    – oppo
    Nov 4, 2019 at 14:19

1 Answer 1

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- THIS MACHINE COULD KILL

The circuit shown will produce a positive going output voltage during the FET on period.

BUT

It will produce a negative going negative amplitude signal that may be VERY large when the FET is tuned off.

On Q16 turnoff the drain will "ring" positive relative to +12V so the input dotted winding end is most negative, so the output dotted winding end - shown as +120V will ring to some negative value. An output diode will isolate this negative spike BUT you still need to deal with it.

As the original circuit does not show a means of eliminating the output negative pulse it may well be the main output being experienced. Do you know the turns ratio and inductance of their transformer - or the inductance of yours?

The use of R1 to vary the amplitude is not a good one. It will work "after a fashion" but depends on the FET's characteristics slightly above Vgs = Vgsth.

While the positive Vout peak is dependant on the turns ratio, the negative one is dependant on energy stored in the transformer - this could be larger than you'd want. Or MUCH larger.

here are a few zillion other TENS circuits for research purposes.

Note that it is uncertain whether TENS machines accomplish what is claimed for them. (I've played with them long ago).If you don't allow them to kill you they probably do no harm (although set too high they can be extremely unpleasant. The "jury is still out" on them, and has been for decades.

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  • \$\begingroup\$ I am going to put Back EMF diode as well for primary wining, how will this affect output will the negative charge as u mentioned go high? \$\endgroup\$
    – oppo
    Nov 3, 2019 at 12:06
  • \$\begingroup\$ i have updated the dot notation and turns ratio of the transformer, what if I use voltage suppression 120V diodes on output side of the transformer? \$\endgroup\$
    – oppo
    Nov 3, 2019 at 12:22

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