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I'm a French student (please forgive my bad English spelling) who works on a system of portable echograph (you can find the information of the project on the wiki of echopen). Could you help me please?

I would like to generate a tension of 100V. First, I took a transformer from a system which is used to transform 230v to 12v. I tested it with a frequency generator to determine the inductance of the coil and the transformation coefficient:

L1 = 11.13mH
L2 = 31.89 mH

I saw that at 13kHz the voltage was higher. I transformed 15Vpp to 110V and I wanted to integrate it into a system which used 10v. So, I used a LM555 component to have a oscillating circuit.

The circuit I used: Oscillating circuit

My observation and problem:

  • I want to use a MOSFET to control the 10v but the voltage is dropping
  • The generating voltage from the transformer is not enough maybe I need two elevation stages
  • When I plug the transformer, the square wave signal is distorted
  • I can't have more than 50V from the transformer (I have more using the frequency generator)

You can find my work on this page

Edit: I forgot to tell, I can't have more than 50V from the transformer using the oscillating circuit, with the frequency generator I have 100V from 15Vpp (the problem is the tension dropping with a load resistance I suppose the frequency generator don't have power enough)

Edit 2 (02/31/2015) : this circuit works thanks a lot to everybody and especially "jp314", I have 130V from 12v!

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    \$\begingroup\$ Does C5 really connect from Vcc (pin 8) to the collector of Q1? totally wrong. \$\endgroup\$
    – JIm Dearden
    Dec 30, 2015 at 17:34
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    \$\begingroup\$ Just a quick thought: Your transformer's core is designed for use with 50 Hz, using it on 13 kHz will cause a large eddy current loss. Increase all the relevant capacitors and drop the conversion frequency. Also better utilization of primary DC bus voltage can be achieved by the use of "H" bridge driver. I agree with Jim on C5, once it charges, no more power can flow to the output stage. \$\endgroup\$
    – Dario Dentes
    Dec 30, 2015 at 18:00
  • \$\begingroup\$ Thank you for your comment, I am going to update the circuit, could you tell me what is the characteristic of the transformer which determine that it is designed for use with 50Hz? I found 13KHz as the resonnance frequency using the gbf to "check" the tension with the frequency variation, Why could it cause current loss? Why is it better to use a "H" bridge? \$\endgroup\$
    – uFarad
    Dec 31, 2015 at 9:03

3 Answers 3

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You need to determine how much power you need to transfer. From your wiki, I think you need plenty of peak power, and this circuit won't do that.

You don't actually need a transformer -- a flyback converter (boost DC/DC converter) will work, although you will need a high duty cycle to get 230 V from 12 V at high power. At low power, you can use discontinuous conduction, and simpler control (maybe even a 555).

To get started, use a single inductor between supply (12V) and a switch transistor (rated at > 230 V). drive the transistor on with the 555 for a time so that Ipeak = Vsupply*T_on/L, and Ipeak reaches the peak you need (related to the power you need). Then turn off the transistor, and the collector voltage will spike up (because of the inductance) to a high value -- use your D1 & C7 to rectify that. If you limit may duty cycle from the 555 to < 90 %, your circuit won't run away. Regulated the output by masking pulses when VOUT > 230 V.

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  • \$\begingroup\$ Thank you very much for your explanation. In fact, I don't have a lot of component and no flyback converter either... I'm trying to achieve the analog part of the project with everything I can have easily. For the rest of your comment, I will try and post the result! Thank you again! \$\endgroup\$
    – uFarad
    Dec 31, 2015 at 9:07
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Throw away that transformer if you want to work at 13 kHz - the laminates will be acting as partial short circuits (due to induced eddy currents) and your inductance measurements are totally invalid at 13 kHz because of this. Use a ferrite transformer and wind your own in the correct turns ratio. If you need further help just ask but don't use something intended to work at 50/60 Hz at a frequency that is over 200x higher.

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  • \$\begingroup\$ Thank you for your answer, How can I mesure the inductance if it's invalid? I have a ferrite transformer (I took it from the same kind of system than the first transformer) why is it better than a toric transformer? Which characteristic of the transformer tell that it is intended to work at 50Hz (I thought calcul the inductance with a known capacitor and the "resonnance frequency" worked)? \$\endgroup\$
    – uFarad
    Dec 31, 2015 at 9:12
  • \$\begingroup\$ Ferrite means no eddy currents in the laminations. It doesn't have laminations. Lamination eddy current means high losses and partial shorted turns at 13 kHz which give a false reading of inductance and poor working as a transformer. Google for more detail. \$\endgroup\$
    – Andy aka
    Dec 31, 2015 at 10:40
  • \$\begingroup\$ Your inductance measurements may be correct, but they don't take into account the effective series resistance (ESR) that represent the losses that the core generates. These losses increase with frequency. Thus, at 13 kHz, it might mean that a large fraction of the power is wasted in this resistance. You cannot measure this resistance with an ohmmeter -- you can measure by looking at the 'Q' of a resonant circuit, or the phase shift between voltage and current in the inductor. \$\endgroup\$
    – jp314
    Dec 31, 2015 at 15:44
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Your transformer is nowhere near optimum and should be changed.You are trying to draw DC from the transformer which the transformer is not set up to do because it does not have a defined air gap .Why dont you full wave rectify the output ? Be sure to use fast diodes .Your circuit it not galvanicly isolated hopefully because it doesnt need to be .If this is the case you will be better to run a boost convertor into a diode pump .Then you have no transformer but just a cheap small easy to buy inducter.

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  • \$\begingroup\$ Thank you for your answer! The fact is I don't have a lot of component and just 2 transformer (a toric and a ferrite)... How could I full wave rectify the output and how it could be helped? I will check how to build a boost convertor and tell if it worked! \$\endgroup\$
    – uFarad
    Dec 31, 2015 at 9:14

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