Trouble with high frequency astable multivibrator

[phyzguy]

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.

 

[Exidor]

Wouldn’t that damage the pot?

 

[phyzguy]

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?

 

[Exidor]

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.

 

[phyzguy]

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?

 

[Exidor]

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.

 

[darth boozer]

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

 

[Exidor]

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.

 

[rbelli1]

It is breadboarded.

Can you post a photo of the actual circuit?

BoB

 

[Exidor]

 

[Exidor]

 

[Tom.G]

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We expect to see some photos of the arcs out of that thing fairly soon.

 

[Exidor]

Here's hoping.

 

[Jp Tremblay]

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 ?

 

[Jp Tremblay]