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I have built a Wien bridge oscillator which outputs 5V at 1.5 kHz. When I hook up this output to the primary coil of a transformer I can't measure any voltage in the secondary coil.

Schematic:

schematic

simulate this circuit – Schematic created using CircuitLab

I thought that the problem might be a lack of current and therefore power in the primary coil. (How would I change that?)

How can I get a voltage induced in the secondary coil with this oscillator (or possibly another oscillator design?)

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  • \$\begingroup\$ Could you add a schematic of what you did? There is a schematic editing tool available in the tool bar when you edit your question. \$\endgroup\$ Jan 16 at 15:46
  • \$\begingroup\$ and what do you measure across the primary? Does this transformer have a specification? Is the oscillator even running with the transformer as a load? \$\endgroup\$ Jan 16 at 15:51
  • \$\begingroup\$ I measure the output voltage and frequency of the oscillator which gave me 5V 5kHz. No, it does not have a specification since it just consists of an iron core with wires wound around it. However the secondary coil has about 2 times as many windings. \$\endgroup\$
    – ht332932
    Jan 16 at 16:09
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    \$\begingroup\$ You can't just add any old transformer to an op-amp and expect anything to work. Brian specifically asked this very pertinent question: Is the oscillator even running with the transformer as a load?. Show a picture of your transformer. \$\endgroup\$
    – Andy aka
    Jan 16 at 16:12
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    \$\begingroup\$ Have you measured the output of the oscillator with the transformer connected, or only before? \$\endgroup\$
    – Simon B
    Jan 16 at 16:49
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The problem with your circuit is that the oscillator is not starting reliably when your load is your transformer. When the oscillator does not start, the output voltage is in the uV range.

One design goal of a Wien bridge oscillator is that it start reliably. Often there is another design goal which is that the amplifying element should not saturate. These competing design goals are often met by adding circuitry that stabilizes the amplitude of the oscillations.

When the oscillations are weak, the amplitude stabilization circuitry provides high positive feedback to increase the amplitude. When the oscillations are at the desired level, the positive feedback is reduced so that the overall closed loop gain is 1. If the oscillations are above the desired level, the positive feedback is reduced further, so that the oscillations dampen.

Components that are used for amplitude stabilization include small incandescent light bulbs, thermistors, back to back diodes, and JFETs.

Incandescent light bulbs provide amplitude stabilization through the varying resistance they have with temperature. If oscillation is weak, the filament is cold, and the resistance is low. If the oscillation is strong, the current heats the filament, which in turn increases its resistance.

One possible solution to your oscillator not starting when loaded with the transformer is to increase the value of R2 (or alternatively decrease the value of R1). You may double it, or even more. The down side is that this will surely cause the op amp to saturate if oscillations start. This will cause the waveform to deviate considerably from a pure sine wave. Saturation may also alter the frequency of oscillation somewhat. You may or may not care about these effects.

Another possible solution is to increase the number of turns on your transformer primary. This may not work without also increasing the value of R2 (or decreasing that of R1). When a transformer secondary only has a volt-meter across it, the transformer acts essentially like an inductor. The inductance seen at the primary is too low, which causes the op-amp to be heavily loaded, and this decreases the feedback. That in turn causes the oscillator not to start. Increasing the number of turns on your transformer will increase the primary inductance, and that is probably a good thing even if it is insufficient to start the oscillator.

Another solution is to use back to back diodes to achieve amplitude stabilization. I have provided a circuit here:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for the helpful answer! I have some questions regarding it (please note that I don't know too much about all of this): Why does the closed loop gain have to be exactly 1? What other components could I use instead of a light bulb as non-linear resistance? How can I adjust R2 accordingly (according to which criteria)? \$\endgroup\$
    – ht332932
    Jan 16 at 18:20
  • \$\begingroup\$ I have added an explanation of why stability requires a gain of 1 at some particular amplitude. As for adjusting R2, that depends upon the resistance of the light bulb at design amplitude for the oscillations. \$\endgroup\$ Jan 16 at 23:52
  • \$\begingroup\$ Thanks for the explanation of the closed loop gain. Could I also use a diode as a nonlinear resistance instead of a light bulb? Will I need a potentiometer as R2 to adjust it properly? Do you know why a LED connected in the place of the transformer will glow but the transformer does not work? And why can I measure a voltage of 5V if there is no transformer connected but can't measure it if I connect a coil or transformer? \$\endgroup\$
    – ht332932
    Jan 17 at 11:57
  • \$\begingroup\$ Can I use any standard household lightbulb? Does it have to be an incandescent one? \$\endgroup\$
    – ht332932
    Jan 17 at 12:51
  • \$\begingroup\$ I have modified the answer in a way that hopefully answers your questions. The LED worked because it didn't load the op amp to the point where feedback suffered too much. The transformer loads the op amp too much and feedback is insufficient to start the oscilllations. With lightbulb method, it must be incandescent, and low power. \$\endgroup\$ Jan 17 at 17:37
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If you are not getting a voltage out of the secondary(I am assuming your have measured the current in the primary and it is good) then most likely it would be the transformer. If you are using a 60Hz/50Hz(laminated steel) transformer then your high(ish) frequency signal is not getting through because the transformer is attenuating it. Go for a ferrite transformer instead. Steel transformers don't allow(you know what I mean) AC signals through much over a couple of hundred hertz because of hysteresis.

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