555 Timer PWM Not Changing DC Fans Speed

[ashybuuny]

No the fan keeps spinning no matter what position. I verified it now by moving the pot and stopping the fan with my hand and letting go of it. I left the pot at each position about 5 seconds.

[ashybuuny]

EDIT: correction, you said OL, which probably means open circuit in this case.  You had the PCB powered?  If so, then check if you have connectivity from 555 output (pin 3) to the MOSFET base. It should be 1K via R1.

I did not measure when the PCB was powered.

[ashybuuny]

EDIT: correction, you said OL, which probably means open circuit in this case.  You had the PCB powered?  If so, then check if you have connectivity from 555 output (pin 3) to the MOSFET base. It should be 1K via R1.

I did not measure when the PCB was powered.

I measure when powered on and I am not sure if I did it correct but I had a lot of OL at different points. What is MOSFET base mean ? Does it refer to the gate ?

[pqass]

EDIT: correction, you said OL, which probably means open circuit in this case.  You had the PCB powered?  If so, then check if you have connectivity from 555 output (pin 3) to the MOSFET base. It should be 1K via R1.

I did not measure when the PCB was powered.

(With PCB un-powered) check resistance between 555 output (pin 3) and MOSFET gate.  It should be 1K.

If I said "base" I meant gate.

[ashybuuny]

EDIT: correction, you said OL, which probably means open circuit in this case.  You had the PCB powered?  If so, then check if you have connectivity from 555 output (pin 3) to the MOSFET base. It should be 1K via R1.

I did not measure when the PCB was powered.

(With PCB un-powered) check resistance between 555 output (pin 3) and MOSFET gate.  It should be 1K.

If I said "base" I meant gate.

I got 500 Ohms which is right since R1 and R2 is parallel.

[pqass]

EDIT: correction, you said OL, which probably means open circuit in this case.  You had the PCB powered?  If so, then check if you have connectivity from 555 output (pin 3) to the MOSFET base. It should be 1K via R1.

I did not measure when the PCB was powered.

(With PCB un-powered) check resistance between 555 output (pin 3) and MOSFET gate.  It should be 1K.

If I said "base" I meant gate.

I got 500 Ohms which is right since R1 and R2 is parallel.

They aren't really in parallel; they both start from same 555 pin 3 but one goes to the gate the other to GND.
But what you said was interesting.  It would look like two parallel 1Ks IF the MOSFET is damaged.

Again, with the PCB un-powered, check the resistance between MOSFET
GATE and SOURCE,  and between DRAIN and SOURCE (while you short GATE and SOURCE).

[ashybuuny]

Again, with the PCB un-powered, check the resistance between MOSFET gate and source,  and between DRAIN and source.

Red on Gate black on source I get 0ohm, Red on drain and black on source I get 230ohm.

[pqass]

Again, with the PCB un-powered, check the resistance between MOSFET gate and source,  and between DRAIN and source.

Red on Gate black on source I get 0ohm, Red on drain and black on source I get 230ohm.

Sorry, I was to quick to post.   
Can you retry the DRAIN and SOURCE resistance when GATE is shorted to SOURCE.
Yes, BLACK lead on source, RED on DRAIN.

[ashybuuny]

if I did it correctly it's 220Ohms, but I wonder why is C1 and C2 connected when on the schematics it wasn't? Am I just confused because they lead to ground?

[pqass]

if I did it correctly it's 220Ohms, but I wonder why is C1 and C2 connected when on the schematics it wasn't? Am I just confused because they lead to ground?

No.  That's not what I meant.  You can remove that jumper from R1 to C2.

What I mean is just to measure the resistance (with your MM) from the DRAIN (RED lead) to the SOURCE (BLACK lead) pins on the MOSFET while, with a flat-blade screwdriver, just touch the GATE and SOURCE pins together.

The idea, is make sure the MOSFET is completely off (by shorting GATE and SOURCE) and if there is a low resistance between DRAIN and SOURCE, then the MOSFET is broken.

[ashybuuny]

No.  That's not what I meant.  You can remove that jumper from R1 to C2.

What I mean is just to measure the resistance (with your MM) from the DRAIN (RED lead) to the SOURCE (BLACK lead) pins on the MOSFET while, with a flat-blade screwdriver, just touch the GATE and SOURCE pins together.

The idea, is make sure the MOSFET is completely off (by shorting GATE and SOURCE) and if there is a low resistance between DRAIN and SOURCE, then the MOSFET is broken.

Its still 220 ohms

[pqass]

Actually, I see what you did there.  Yes this does short GATE and SOURCE (GND).

As such, you've measured 220ohms, which means the MOSFET is permanently ON when the gate should be off.

Replace with a new MOSFET.   

FYI: MOSFETs can be turned-on just by touching the GATE; they're very sensitive.  They can also be damaged by static electric charge.

If you don't have a ground strap, before handling any static sensitive components, ground yourself by touching a large metal object like a desk lamp.

[ashybuuny]

Actually, I see what you did there.  Yes this does short GATE and SOURCE (GND).

As such, you've measured 220ohms, which means the MOSFET is permanently ON when the gate should be off.

Replace with a new MOSFET.   

FYI: MOSFETs can be turned-on just by touching the GATE; they're very sensitive.  They can also be damaged by static electric charge.

If you don't have a ground strap, before handling any static sensitive components, ground yourself by touching a large metal object like a desk lamp.

New one is replaced and it should work now right?

[pqass]

New one is replaced and it should work now right?

It should.

[ashybuuny]

IT WORKS! Although It screeches like a siren. I thought it was the fan blades but when I stopped it with my hand it kept going. It was definitely coming from the PC fan and I am not sure why. The screeching only occurs if about 85% or lower than the pot turns to what I believe is CCW. So if the pot is above what I think is like the 85% turn threshold it runs silently and fast.

[pqass]

IT WORKS! Although It screeches like a siren. I thought it was the fan blades but when I stopped it with my hand it kept going. It was definitely coming from the PC fan and I am not sure why. The screeching only occurs if about 85% or lower than the pot turns to what I believe is CCW. So if the pot is above what I think is like the 85% turn threshold it runs silently and fast.

GREAT!

You probably have to increase the PWM frequency.  As it's now only 1.2KHz.

Change C2 to 1nF which should boost the frequency to 12Khz.  That's easier than changing the pot to 10K.

[ashybuuny]

Is that all I have to change to fix that? I don't have it so I will probably order 1 when I am doing my next project. Will this change also make the change of speed smoother.

THANK YOU so much!

[pqass]

Is that all I have to change to fix that? I don't have it so I will probably order 1 when I am doing my next project. Will this change also make the change of speed smoother.

THANK YOU so much!

Low PWM frequencies can cause audible resonances.   Boosting it to high or above hearing range makes them go away usually.

The "smoother" issue may be related to the fact that you can't dial a motor down to a very low duty cycle without stalling it.  To remedy this, we put in a default low resistance on the D2 (cathode) leg of the pot; ie. to not go lower than a minimum speed.  You may choose to add another resistor on the D3 (anode) leg of the pot too if you find there isn't a visible speed change on the upper end of the dial.  This way, the pot range will be tuned to have the most effect on the fan speed.