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| Trying to build a PWM fan controller |
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| spec:
Attached below is your modified schematic with component values included. The first thing I happened to notice is that 1N4148 diodes are called up in the components list, but schottky diode symbols are shown on the schematic. Schottky diodes (BAT48) would be much better because they have a lower forward drop (Vf) than ordinary diodes. This is especially relevant with this circuit when running from a low supply rail, like 5V. The second thing is that there appears to be an error on the schematic- will describe in next post. I will have a look at the 556 circuit further. |
| spec:
Another schematic attached showing a correction in red for the 'error' I mentioned. On reflection, the original circuit would always work, but the circuit, as I have shown, is much cleaner and better practice. AFAIK, the fan PW control input is expecting an open collector/drain drive with pull up resistor, but the present 556 circuit is providing source and sink drive. Not sure how happy the fan circuitry would be with that. |
| spec:
Yet another schematic attached. :) This schematic only uses the first section of the 556 and provides an open drain drive for the PW input to the fan. Not sure if a 10K pull up resistor is low enough, but that is easily fixed. |
| Ian.M:
The 556 or dual 555 circuits from overclockers.com are pointless, and only showcase the limited electronic design skillbase at that site. Instead of driving the fan from the push-pull signal on the 555's Out in, simply drive the fan PWM input from the 555's Discharge pin, which is open drain if you use a CMOS 555 (and open collector for a bipolar one), so is closer to the Intel 4 wire fan specification drive requirements, and also removes any need for a second 555 or 1/2 556 for buffering. I'm rather suspicious of the 100K pot and 680pF capacitor as I think the frequency will be outside the 21KHz to 28KHz range called for in the Intel spec. near the ends of pot's travel. I've attached a LTspice sim of a simplified single 555, single 12V supply circuit, using a potential divider to pull up the Discharge pin to an effective 5V level, with a 50K pot and a 1nF timing cap which should meet the 21KHz to 28KHz range requirement, or at least be far closer to it. Edit: it looks like Spec's been working on something similar while I've been typing this. |
| spec:
--- Quote from: Ian.M on December 04, 2018, 10:37:57 am ---The 556 or dual 555 circuits from overclockers.com are pointless, and only showcase the limited electronic design skillbase at that site. --- End quote --- That applies to many circuits you see on the net and to a lesser degree in books ::) --- Quote from: Ian.M on December 04, 2018, 10:37:57 am ---Instead of driving the fan from the push-pull signal on the 555's Out in, simply drive the fan PWM input from the 555's Discharge pin, which is open drain if you use a CMOS 555 (and open collector for a bipolar one), so is closer to the Intel 4 wire fan specification drive requirements, and also removes any need for a second 555 or 1/2 556 for buffering. --- End quote --- Great minds think alike :) The OP is using a CMOS 556 --- Quote from: Ian.M on December 04, 2018, 10:37:57 am ---I'm rather suspicious of the 100K pot and 680pF capacitor as I think the frequency will be outside the 21KHz to 28KHz range called for in the Intel spec. near the ends of pot's travel. --- End quote --- That was my concern too. --- Quote from: Ian.M on December 04, 2018, 10:37:57 am ---I've attached a LTspice sim of a simplified single 555, single 12V supply circuit, using a potential divider to pull up the Discharge pin to an effective 5V level, with a 50K pot and a 1nF timing cap which should meet the 21KHz to 28KHz range requirement, or at least be far closer to it. --- End quote --- Good move. Handy info about the Intel spec frequency range. --- Quote from: Ian.M on December 04, 2018, 10:37:57 am ---Edit: it looks like Spec's been working on something similar while I've been typing this. --- End quote --- Happens to me all the time ;D |
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