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I am trying to learn astable multivibrator circuits and I found one in an EL wire driver.

I have noticed it has a 100nF ceramic capacitor connected between the transistor bases and additional 22R resistors. I have never seen these before in the standard astable multivibrator schematics.

Could somebody please explain the purpose of these components? The EL wire does not illuminate without the 100nF cap.

https://i.stack.imgur.com/kvzRX.jpg

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2 Answers 2

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This a tuned, relaxation oscillator called Royer, or Baxandall. Variations of the configuration are commonly used in DC to AC converters and inductive heating transmitters.

An important characteristic of this type of circuit is the tuning of the primary coils. In this case, the tuning capacitance is the series combination of 4u7, 100n and 4u7.

In many Royer-type oscillators the feedback to the base of the BJTs (or to the gates of FETs) is provided by an extra winding in the transformer. In this ingenious circuit, the feedback is by means of a capacitance divider (a bit like the resistor equivalent) so an extra winding is not needed.

The two 22 Ohm resistors limit the base current at switching.

There is no protection against reverse Vbe on the BJTs. However, the value of primary inductance is needed in order to simulate how close the BJTs are to destruction!!

Answers to 3 questions in comments

Would the transformer primary inductance be part of the tuning requirements to make this oscillator work?

Yes.

The usual method of oscillation makes use of transformer saturation. At saturation, the collapsing magnetic field in one primary winding triggers positive feedback to switch the current to the other primary winding. Thus, the primary might not have a constant inductance during operation. EL wires are operated at a frequency of about 1 kHz, I think.

The transformer steps from 3V to 120V. Is this due to the resonance rather than turns ratio?

Voltage step up is due to turns ratio only.

120 VAC is high enough to be lethal - be VERY careful.

The resonance helps with power efficiency: there are smoother signals and fewer wasteful high frequencies. The approximately 1 kHz frequency i) allows you to make a smaller transformer (than, say, for 50 or 60 Hz); ii) eliminates flicker of the light.

Would an improvement be needed or anything added to this circuit for the issue of no reverse Vbe protection for safety?

"Safety" around Vbe is only for the transistor, but see the previous note about 120 VAC.

This type of low cost circuit is finely balanced in terms of good performance from a few components. If anything is changed the balance will be upset. The good news is that the supply voltage is low and (I think) the current on the secondary side will be low - so, overall, low power. Hopefully, the BJTs will survive. If your BJTs fail frequently, excess reverse Vbe might be the reason. Try: either diodes in series with the bases, or, diodes anti-parallel with the bases. You might have to make other adjustments to make it work again...

This is tricky analogue circuit, Good Luck!

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  • \$\begingroup\$ Would the transformer primary inductance be part of the tuning requirements to make this oscillator work? The transformer steps from 3V to 120V. Is this due to the resonance rather than turns ratio? Would an improvement be needed or anything added to this circuit for the issue of no reverse Vbe protection for safety? \$\endgroup\$
    – CDW
    Nov 20, 2021 at 16:03
  • \$\begingroup\$ I have replied with an edit to my answer - too much text for a comment. \$\endgroup\$
    – GeBJT
    Nov 20, 2021 at 17:14
  • \$\begingroup\$ I really appreciate the information, thank you! I have just 2 final questions - which combination of components are used for the frequency calculation? I measured it to be 2.5kHz. Previously I was using the standard multivibrator single RC (1.5k and 4u7) combination and couldn’t work out why it was only something like 150Hz. I was beginning to question if transformer inductance was part of it. If a BJT fails short, is that a short circuit/fire risk in this application without overcurrent protection? \$\endgroup\$
    – CDW
    Nov 20, 2021 at 18:09
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This circuit have one "problem" if it's not designed carefully it doesn't begin/stops oscillate and end up with both transistor conducting resulting with high current flowing through coils. The 100n cap supports the oscillation.

Imagine, you force to open both transistor pulling bases to gnd with two external wires(with some resistance of course) so both BJTs are conducting. If you remove this wire simultaneously the transistors stay conducting because base currents would flow through 1k5.

But 100n cap can overcome the BJTs saturation catching and "collecting" even small noise from Vcc and bring the circuit back to oscillation.

The oscillation capability of this circuit is very dependent on BJTs type (Beta, fastnest).

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  • \$\begingroup\$ Thanks for the info. Are you able to explain how the capacitor arrangement is used for feedback to switch the conducting transistor off and switch the other one on using feedback from the transformer magnetic field after breakdown? Is it 2 capacitors that set a base voltage? Which directions do the current flow as the both sides are connected through the 100n giving 2 paths out of each base \$\endgroup\$
    – CDW
    Nov 21, 2021 at 20:52

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