Hi folks I am trying to build a circuit which will govern the speed of the fans in my solar cabinet based off the temperature inside.


https://www.circuits-diy.com/temperature-controlled-fan-using-555/


I built this circuit hoping it would work with 6x 24v / 0.08A PC fans (40mm) I have 3 of these either side (laminar flow)
I want the fans to idle at 10% duty cycle at room temp, and max out at 18v @ ~40C

As is, It won't provide adequate voltage to even turn them on (8.6v output @ about 26c with fixed resistors in diagram)

What would be a simple way to redesign this circuit with minimal modifications so that it will output 18v instead of 12v?
Can I just swap the SD buck converter (27v in from SCC) from the 75-12v 3A to a 32v-3v 5A Step down "buck" converter and then run the circuit at 18v?
(18v is optimal for these fans, plenty of airflow, very little noise undervolted)

Are all the components included rated to 18v?
Or would it be better to use a 2k rotary pot instead of fixed 1K resistor on the BJT base allowing more current @ 12v?
(This seems ideal in order to finetune its performance once installed @ 18v, too)

Perhaps someone has a more suitable diagram in mind that I can copy? (save from reinventing this particular circuit)

Electronics is not my area of expertise so I appreciate any helpful feedback.
Many thanks in advance for such!

PS. Thanks to mods for relocating to a more suitable subforum

 

I want the fans to idle at 10% duty cycle at room temp, and max out at 18v @ ~40C

That 555 circuit will only give a small change in PWM duty-cycle for the expected change in thermistor resistance (10k to 5kΩ), not give your desired range of duty-cycle.
Would you consider a circuit using a LM339/393 comparator circuit instead, which can go from 0% to 100% duty-cycle?

Then we can talk about voltage.

 

I took a different approach with an almost identical circuit.
I put the thermistor where the 0.1uF capacitor is, and then two fixed resistors above it. That gives you a standard hysteresis control, so the fan is either on or off.
Brushless DC fans don't take kindly to a low supply voltage, they don't rotate, but the internal circuit takes current and generates heat in the motor, which is effectively stalled.
If you choose the "fan on" temperature and the "fan off" temperature, you can calculate the two resistor values.
If you have 24V fans, you need a 24V supply. You aren't going to get 18V from a 12V supply (easily).
Is that a Victron MPPT? The colour is unmistakable!

 

That 555 circuit will only give a small change in PWM duty-cycle for the expected change in thermistor resistance (10k to 5kΩ), not give your desired range of duty-cycle.
Would you consider a circuit using a LM339/393 comparator circuit instead, which can go from 0% to 100% duty-cycle?

Then we can talk about voltage.

I'll build the suggested circuit next then, thanks for the suggestion.
Hands on projects like these are the best way to learn IMO
Didn't know that the NE555 was limited in such a way - I do now!

 

I'll build the suggested circuit next then, thanks for the suggestion.
Hands on projects like these are the best way to learn IMO
Didn't know that the NE555 was limited in such a way - I do now!

I wouldn't recommend it - for a fan with a brushless motor, it won't start until the PWM gets to about 50% and below that it will sit the with the motor effectively stalled. Use a standard hysteresis controller, unless you can figure out a way that the PWM never goes below 50%.

 

I took a different approach with an almost identical circuit.
I put the thermistor where the 0.1uF capacitor is, and then two fixed resistors above it. That gives you a standard hysteresis control, so the fan is either on or off.
Brushless DC fans don't take kindly to a low supply voltage, they don't rotate, but the internal circuit takes current and generates heat in the motor, which is effectively stalled.
If you choose the "fan on" temperature and the "fan off" temperature, you can calculate the two resistor values.
If you have 24V fans, you need a 24V supply. You aren't going to get 18V from a 12V supply (easily).
Is that a Victron MPPT? The colour is unmistakable!

Neat idea - I would like this to be linear with the temp if possible, mostly for the aesthetics of the RPM coinciding with given temp that day + the load as it varies etc, although I suppose On/Off would work fine too.

Noted RE BLDC Fans.. They still work OK thankfully.
Trouble is the system runs between 26.8 and 27.6v - too high for these fans! they'll burn out.
You can get small load boost converters that will boost 12 up to 80v ish or anywhere in between (adjustable V / I)
But I was referring to stepping 27v down to 18v (run them directly off the SCC Output)

It's a Victron 100/50 with 4x 270w/24v panels above.
Gets toasty with these fans out of order! But that doesn't seem to affect anything tbh
Cooler is always better though iinm

Would strongly recommend this brand of solar equipment - It just works!

 

I wouldn't recommend it - for a fan with a brushless motor, it won't start until the PWM gets to about 50% and below that it will sit the with the motor effectively stalled. Use a standard hysteresis controller, unless you can figure out a way that the PWM never goes below 50%.

How about using a rotary potentiometer to set the minimum required 51% whilst the sensor is "on ice" and then knowing temps will never go past 60c (they have yet to anyway) set 100% current allowance there (using infra red probe bit of steel and a lighter?

Not sure on the specifics / equations involved and I don't own an oscilloscope etc unfortunately
But surely some variation thereof could be viable?
It's a little beyond me atm hence the post .. lol

 

Given that you have 6 fans, if the LM3914 still existed, I'd use its outputs to switch on one fan at a time.
In the absence the LM3914, I'd use a LM339 to switch fans on at different temperatures, so you can have from zero to six fans running depending on the temperature. I suspect would be rather more efficient than attempting to speed-control the fans.
By the way, you should blow cold air into the enclosure, rather than blowing hot air out. The fans run cooler that way and last longer.
No point at all in running the fans at night!
Another thought - if you switched the fans according to the current not the temperature, you would preempt the heating - you would start cooling before the heat from the buck regulator made its way through the heatsink.

 

Given that you have 6 fans, if the LM3914 still existed, I'd use its outputs to switch on one fan at a time.
In the absence the LM3914, I'd use a LM339 to switch fans on at different temperatures, so you can have from zero to six fans running depending on the temperature. I suspect would be rather more efficient than attempting to speed-control the fans.
By the way, you should blow cold air into the enclosure, rather than blowing hot air out. The fans run cooler that way and last longer.
No point at all in running the fans at night!
Another thought - if you switched the fans according to the current not the temperature, you would preempt the heating - you would start cooling before the heat from the buck regulator made its way through the heatsink.

Will consider multiple stage operation, that's a good idea I had not considered.
Efficiency is no problem here, primarily want to extend longevity of hardware.
The 6 fans maxed out @24v/8ma. could only ever pull 11.52w
If I shut the solar off and ran them independently (theoretically) I would get somewhere in the neighbourhood of 130 hours / 5 days of runtime off a charged battery - suffice to say probably double that If I ran them at 18v.

the 3 fans in the image are intakes, the other 3 are exhausts.
They hit the SCC and the 3 exhaust fans take the hot air from scc and the inverter, it goes left to right. takes about 3 seconds for e cig vapour to pass through (did a test)
Victron recommend 100mm space around the inverter however, this active cooling system should circumvent the need for it. But it is definitely necessary unless I prop the door open, if more than a few 00 watts is being used.

I'm surprised such a circuit doesn't exist prebuilt, have looked but to no avail.
Perhaps I am not looking hard enough?
I'm determined to get this working as I want it to but didn't think it'd be this tricky.

RE preemptive cooling: also not a bad idea, if I took the reference inline with the 27v supply
I will consider that too. Thanks for the ideas

 

Will consider multiple stage operation, that's a good idea I had not considered.
Efficiency is no problem here, primarily want to extend longevity of hardware.
The 6 fans maxed out @24v/8ma. could only ever pull 11.52w
If I shut the solar off and ran them independently (theoretically) I would get somewhere in the neighbourhood of 130 hours / 5 days of runtime off a charged battery - suffice to say probably double that If I ran them at 18v.

the 3 fans in the image are intakes, the other 3 are exhausts.
They hit the SCC and the 3 exhaust fans take the hot air from scc and the inverter, it's goes left to right. takes about 3 seconds for e cig vapour to pass through (did a test)
Victron recommend 100mm space around the inverter however, this active cooling system should circumvent the need for it. But it is definitely necessary unless I prop the door open, if more than a few 00 watts is being used.

I'm surprised such a circuit doesn't exist prebuilt, have looked but to no avail.
Perhaps I am not looking hard enough?
I'm determined to get this working as I want it to but didn't think it'd be this tricky.

I would suggest putting all the fans on the intakes, and just some vents on the exhausts.
You would get much longer life for a fan that is not powered up!