Ok my basic circuit schematic is:
Using 10k for R1, 100k trim pot configured as a voltage divider for R2, 1N4148 diodes, and a 451K rated ceramic disc capacitor. No scope, but according to my DMM the circuit is oscillating between ~21-24K hertz based upon duty cycle. Duty cycle control appears to be working when I slow down the timer by using a larger capacitor and connect my output to drive a LED, although I am not exactly for sure how many % I can vary my Ton/Toff times with my current resistor values.
As of right now I'm using the 555 timer to directly drive the gate of a IRF540 mosfet to which I have attached a 12v ~0.5 amp rated PC fan as a test load. The circuit does indeed work, the fan gets slow and faster as I turn the pot, but again it defiantly needs a bit of tinkering with my resistor values. Also I have noticed that without any sort of capacitance between the FET's source pin and the fan, the fan seems to run weakly no matter how I have things set, and it makes a crapload of squealing noises if i try and run the circuit slower than 20Khz, plus my circuit seems to go a bit on the unstable side. I dunno whats up with that.. pin 5 is bypassed to ground, and pin 8 has got a 1000uf electrolytic on it and all that good stuff...
Also, should I be placing any gate/pulldown resistors between my 555 timer and my FET? What about a "freewheeling" diode or whatever? What about shrinking R1 down a few thousand ohms and upping R2 to 250k along with bumping up the capacitance to say a 102J mono cap... think that might help? Or should I just try and go back down in into the 1Khz range?
Anyhoos, even though it appears there are quite a few bugs that need to be worked out, on the upside I must say that my FET barely gets warm... I would almost say it could handle the fan without a heatsink.
*Sigh* there is soooo much to learn about all this crap... its enough to make your head swim!
Using 10k for R1, 100k trim pot configured as a voltage divider for R2, 1N4148 diodes, and a 451K rated ceramic disc capacitor. No scope, but according to my DMM the circuit is oscillating between ~21-24K hertz based upon duty cycle. Duty cycle control appears to be working when I slow down the timer by using a larger capacitor and connect my output to drive a LED, although I am not exactly for sure how many % I can vary my Ton/Toff times with my current resistor values.
As of right now I'm using the 555 timer to directly drive the gate of a IRF540 mosfet to which I have attached a 12v ~0.5 amp rated PC fan as a test load. The circuit does indeed work, the fan gets slow and faster as I turn the pot, but again it defiantly needs a bit of tinkering with my resistor values. Also I have noticed that without any sort of capacitance between the FET's source pin and the fan, the fan seems to run weakly no matter how I have things set, and it makes a crapload of squealing noises if i try and run the circuit slower than 20Khz, plus my circuit seems to go a bit on the unstable side. I dunno whats up with that.. pin 5 is bypassed to ground, and pin 8 has got a 1000uf electrolytic on it and all that good stuff...
Also, should I be placing any gate/pulldown resistors between my 555 timer and my FET? What about a "freewheeling" diode or whatever? What about shrinking R1 down a few thousand ohms and upping R2 to 250k along with bumping up the capacitance to say a 102J mono cap... think that might help? Or should I just try and go back down in into the 1Khz range?
Anyhoos, even though it appears there are quite a few bugs that need to be worked out, on the upside I must say that my FET barely gets warm... I would almost say it could handle the fan without a heatsink.
*Sigh* there is soooo much to learn about all this crap... its enough to make your head swim!
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