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How to make an BJT astable multivibrator run faster than 30Khz that works? Answered

I am working on a dc to dc converter and it works but it seems to me that when i go higher than that it doesn't work anymore. I am using two C945 transistors. The resistor values for R1 and R4 are 1K and for R2 and R3 are 15k . For the capacitors C1 is 2.2nf and 1nf for C1. i have tried putting in smaler capacitors and increasing the two base resistor values. BTW its for a dc to dc converter that im working on. I also have no idea how much inductance on the inductor is. I am try to step up 6V dc to 15 V. It works put its low on power thats what i want.

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josh1001 (author)2009-09-02
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josh1001 (author)2009-09-01

not really i broke my last mosfet's pins but im sure ill get it working

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josh1001 (author)2009-08-30

No i pulled it out of a old CRT monitor and i believe u have answered my question. Thanks!

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josh1001 (author)josh10012009-08-30

Actually with the 12 V .7A fan i got 15.65V, and the power is about 3 watts under a 70 ohm load

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josh1001 (author)2009-08-29

Im not using a fly back converter. its a boost converter circuit and at one point i put in a particular inductor and it flew straight to 15V under a bedini fan rated at 12V 0.7A. The only problem is that i cant find the inductor now. The supply is a 6V 4A battery . I think the lack of power is the voltage coming from the astable multivibrator into the mosfet Im using, so can i use a simple audio amplifier to boost the signal ? I used the basic circuit shown with some modifications, excluded D1, D3 changed Q3 to a mosfet and the resistor values as you know.http://www.aaroncake.net/circuits/6-12conv.asp

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josh1001 (author)2009-08-29

i put some more details in the question but i think it may not be that the multivibrator isn't going fast enough its that it the mosfet cant handle the speed or the inductor is reducing the power because im working on a dc to dc converter. BTW im 17 and im not professional

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VIRON (author)2009-08-27

30Khz? How do you know it isn't working, and how do you change it's speed? Got a O-scope? Frequency counter? AM radio? Can your meter measure high frequency voltage? If you could hear sound from it before, see if a dog notices when you turn it on because you can't hear it anymore. The circuit I'm familiar with has 2 capacitors, use smaller ones and it goes faster. 30Mhz is possible, 100Khz is easy. To test with an AM radio, put it behind the radio, and see if you get totally quiet frequencies 30Khz apart something like 1000, 1030, 1060, 1090 when it's on, maybe add a long wire for a test transmit antenna to the transistor that isn't powering the transformer or voltage doubler. If both transistors power the transformer then the problem maybe using a 50-60 Hz transformer which doesn't work at 30khz.

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josh1001 (author)VIRON2009-08-29

Im using two C945 transistors and R1& R4 are 1k, R2 &R3; are 15K. the capacitor C is 2.2nf and C2 is 1nf to give me a 68% duty cycle but when i put in smaller capacitors it doesn't seem to work better. BTW im working on a dc to dc converter i don't have an oscilloscope and its the inductive type. sorry i don't have a schematic to show u.

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steveastrouk (author)2009-08-28

Also, if you use a crappy socketed breadboard, you will struggle with high speed analogue circuits.

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steveastrouk (author)2009-08-28

like much more patient Steve suggested

Do I detect a hint of sarcasm ???? ;-)

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The Lightning Stalker (author)2009-08-28

Try a Royer oscillator like the one on my site.

http://kickme.to/lightningstalker

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steveastrouk (author)2009-08-27

circuit ? Component values ? Transistor type ?

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