LA-3A inductance autotransformer and resonance problems

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miquel

Active member
Joined
Nov 6, 2009
Messages
39
Location
Barcelona
Hello

I am experimenting with LA-3A variations. I built the Electroluminescent Panel driver with TDA2050 and substituted the autotransformer B11184 with a cheap and conventional 24:230 V transformer (1:10 relation more or less).

In principle works fine, but experimenting the frequency response it results that shows resonance problems between the secondary transformer inductance and the (high of course) Panel capacitance.

I substituted the transformer with others and the frequency response is different, but every time bad. In really, when you are in the resonance frequency, the panel bright with intensity, and at different frequencies, the bright is attenuate

I have substituted the autotransformer with real audio transformer 1:5 (big output line transformer) with the same problems.

I have tried to put the realimentation R37 (220K) and C13 (10n) but the effect is negligible, wit other resistors is not better.

In really, if you look the waveform with the oscilloscope (channel 1 = input, channel 2 = Panel signal) looks the phase change typical in the resonance… a physics lesson.

Has any body any experience?

Thanks

Miquel

 
> when you are in the resonance frequency

What is the resonance frequency?

Try series resistance between amp and transformer. 10 ohms may be a good start.
 
Thank you

The resonance between Panel capacitance and secondary transformer inductance.

(Note: When I build audio transformers, I can measure the inductance using the resonance point method, but now I use the voltage drop in a series resistance, its faster).

I think the real problem is the high capacitance load. In really I was wrong because I used a complete piece of sheet Panel (I buy panel at Farnell) and they are, in origin, 6 x 6 cm (36 square cm) and the measured capacitance is …. 14 nF…! (near 400 pF / cm2).

If the transformer secondary has 1H inductance, the resonance frequency is 1.300 Hz more or less. The secondary transformer and the Panel are a resonant circuit, in really, using the BF generator looks easy the (I think) resonance point.

In really, when I work with a “real” DIY Panel (like a T4b) I cut the Panel at 4 x 3,5 cm (14 cm2) and the capacitance is (high of course) but only 5n6. I results better, but I don’t like the result.

I think the autotransformer output has a very high inductance to drive a high capacitance dispositive (like an Electroluminescent panel), but… of course, LA-3A works fine.

This weekend I try to built a different Panel driver, wit a MOSFET and high voltage (like a vacuum tube LA-2A), I think the MOSFET output is not inductive and I will not have these problems.

Miquel

 
¿No one has measured the T4B Electroluminescent panel capacity and its influence jointly the output impedance of LA-2A tube, or LA-3A autotransformer?.

 

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