Trouble with high frequency astable multivibrator

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Discussion Overview

The discussion revolves around challenges faced in achieving high-frequency oscillation with an astable multivibrator circuit, specifically targeting a frequency of 816.5 KHz. Participants explore various aspects including component selection, circuit design, and potential modifications to improve performance.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant reports difficulty in getting an astable multivibrator to oscillate above 480 KHz, despite improvements from a "baker clamp."
  • Another participant suggests that parasitic resistances and capacitances might be larger than anticipated, recommending measurements of these components.
  • There is a discussion about the appropriateness of using a multivibrator circuit versus other oscillator topologies, with one participant expressing a preference for avoiding inductors.
  • Concerns are raised regarding the speed of the transistors used, with one participant calculating that the current transistors may limit the maximum frequency to around 770 KHz.
  • Suggestions for alternative power transistors that can switch effectively at frequencies below 1 MHz are provided, along with links to potential options.
  • One participant mentions the need for a two-phase square wave for driving a Tesla coil, which influences their design choices.
  • There are discussions about the limitations of using a 555 timer IC for high-frequency applications, with one participant noting the need for tunability.
  • Another participant suggests using Schmitt trigger ICs as an alternative, citing their capability to operate at higher frequencies.
  • Concerns about the vulnerability of certain ICs to high voltage are raised, with suggestions for over-voltage suppression methods to protect components.
  • One participant notes that below 2 nF capacitance, the astable multivibrator does not oscillate with the current transistors, indicating a limitation in their setup.

Areas of Agreement / Disagreement

Participants express various viewpoints on the best approach to achieve the desired frequency, with no consensus on a single solution. Disagreements exist regarding the suitability of different components and circuit designs, and the discussion remains unresolved.

Contextual Notes

Limitations include the absence of a schematic for clearer assessment, uncertainty about the load on the multivibrator, and the need for specific component values that some participants do not have on hand.

  • #31
Exidor said:
Below 2 nF the AMV won't oscillate with the current transistors. It is breadboarded. I don't have the necessary power resistors on hand to try.

Can you just short out the 1 KOhm resistors and rely on the variable resistors only? This would test out whether reducing the RC would speed up the oscillator.
 
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  • #32
Wouldn’t that damage the pot?
 
  • #33
Exidor said:
Wouldn’t that damage the pot?

Why should it? Before you have 1 KOhm plus whatever the pot is set for, which ranges from 0-1KOhm. You want to try reducing the RC time constant. You said you can't go smaller than 2nF on the capacitor. If you set the pots at the highest setting, you will have 1 KOhm, which is the value you had before with the pots turned to minimum. Then you can try slowly reducing the pot setting and see if the frequency increases. According to your schematic, there is only about 20 mA flowing down that leg. The pot should be able to handle this, even if the current increases. Do you know the power rating on the pot?
 
  • #34
I could give that a try. It’s a 10k pot @ 1 watt. The schematic editor wouldn’t allow me to change the value of the pot.
 
  • #35
Exidor said:
I could give that a try. It’s a 10k pot @ 1 watt. The schematic editor wouldn’t allow me to change the value of the pot.

It's a 10K pot? Your schematic shows it at 1K. What's the resistance when turned to the lowest setting?
 
  • #36
On second thought, I am pretty sure it would damage the pot. At 150 ohms, it would draw around 200 mA and there would be more than 1 watt across it.
 
  • #37
As Vortector said, use a CMOS clock generator circuit. This can give you a tuneable frequency as well as easily providing the inverted and non-inverted signals to drive the power transistors. See "Clock Waveform Generators", about 1/3 down the page on the linked site.

https://www.electronics-tutorials.ws/waveforms/generators.html
 
  • #38
I have already ordered 2n3904's which should work. I am still not sure what power transistors would work well switching at 816.5 KHz. Wouldn't the 2n3904's hold up better than a CMOS chip in the vicinity of the Tesla secondary? Also, additional circuitry would be needed for the chip since I am running at 30V.
 
  • #39
Exidor said:
It is breadboarded.

Can you post a photo of the actual circuit?

BoB
 
  • #40
IMG_0050.JPG
 

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  • #41
IMG_0051.JPG
 

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  • #42
/\
/\
:eek:

We expect to see some photos of the arcs out of that thing fairly soon.
 
  • #43
Here's hoping.
 
  • #44
Exidor said:
I wanted to avoid inductors. I need a two phase square wave at 816.5 KHz. There would be high voltage around so I wanted to go with transistors instead of IC's. The output signals drive a transistor amplifier, which would feed the primary of a Tesla coil. I could use a CD4046 and an inverter, but the chips might get burnt out.
Hi maybe i missed something, but a regular push pull, using 2n2222a and it s conterpart 2n2907a can give good results at 1Mhz, as long you keep input resistor around 220 ohm or below if possible. In order to do so, you have to use 2 schotky diodes insteed, which greatly speeds up response time, installed between bases and collectors ( blocking sides toward + nodes, of course) such as MBR150. It drives fets satisfactory. Rise and fall time are about 25ns each (to 90%). Try this out ?
 
  • #45
20220111_215524.jpg
 

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