Could use a little advice on my 555 timer mosfet driver circuit.

[Evil Lurker]

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!

[Th3_uN1Qu3]

Re: Could use a little advice on my 555 timer mosfet driver circuit.

The 555 has trouble with getting output "off" voltage low enough, especially at high frequencies. Try a 1k resistor from 555 output to ground. Also put a 10 ohm resistor in series with the mosfet gate, right next to it, if the 555 is a significant distance away from the mosfet.

The value of the trimpot doesn't really matter. It only sets frequency. Only if it goes crazy when you touch it, in that case you should be using a lower, not higher value pot. In fact i think that's part of your problem - use smaller resistor and pot (somewhere around 10k) and increase the timing cap to compensate.

And what do you mean by capacitance between source and fan? The mosfet source is always wired to ground, the load is connected between drain and 12v. I don't understand where you put this capacitor. If you have put it between the fan wire that is connected to the mosfet and ground, that's very bad news for the cap...

Also that circuit is lacking something essential: Pin 4 should be connected to pin 8 on the 555. It is true that leaving it floating will bias it high and thus turn on the 555, but the current available is very little and it will cause intermittent operation.

[Evil Lurker]

Re: Could use a little advice on my 555 timer mosfet driver circuit.

The 555 has trouble with getting output "off" voltage low enough, especially at high frequencies. Try a 1k resistor from 555 output to ground. Also put a 10 ohm resistor in series with the mosfet gate, right next to it, if the 555 is a significant distance away from the mosfet.

The value of the trimpot doesn't really matter. It only sets frequency. Only if it goes crazy when you touch it, in that case you should be using a lower, not higher value pot. In fact i think that's part of your problem - use smaller resistor and pot (somewhere around 10k) and increase the timing cap to compensate.

And what do you mean by capacitance between source and fan? The mosfet source is always wired to ground, the load is connected between drain and 12v. I don't understand where you put this capacitor. If you have put it between the fan wire that is connected to the mosfet and ground, that's very bad news for the cap...

Also that circuit is lacking something essential: Pin 4 should be connected to pin 8 on the 555. It is true that leaving it floating will bias it high and thus turn on the 555, but the current available is very little and it will cause intermittent operation.

Whoops. That would explain that... I put the cart before the proverbial horse.

[Evil Lurker]

Re: Could use a little advice on my 555 timer mosfet driver circuit.

That fixed it. Using the same caps and resistors as before, I'm now running my fan at ~23-24Khz and have nearly a full duty cycle to choose from and it isn't making any more stinkin squealing sounds. My FET isn't even warming now. I'm happy.

[Th3_uN1Qu3]

Re: Could use a little advice on my 555 timer mosfet driver circuit.

Good job. 20+ kHz is kinda lossy for a fan tho, you'll want somewhere around 100-200Hz. Yes that means it might buzz slightly at minimum speed but i doubt it'll be audible. Compare the fan speed that you get by setting PWM to maximum to the speed it has when wired straight to 12v, if it is much slower with the PWM you need a lower frequency. And as anything in the midrange spectrum of the human ear would cause an annoying whine, the low register is where you would want it to run. CCFL dimmers in monitors also run at ~200Hz.