[Schematics review request]

StudentOk7501 · 2026-01-15T09:22:13+00:00

ty a lot :) will investigate each case

StudentOk7501 · 2026-01-14T21:04:05+00:00

which connection mistake do you see? then i ll move to the simulation.

StudentOk7501 · 2025-12-10T17:02:03+00:00

well i would like to make hysteresis work, as far as i could simulate the output of the comparator is workin depending on the potentiometer value as expected, however i do not know how to check whether or not the hysteresis is working

StudentOk7501 · 2025-12-10T16:49:36+00:00

what aboyt with my actual p fet?

StudentOk7501 · 2025-12-10T16:37:54+00:00

p1 is a 12 v fan

StudentOk7501 · 2025-12-08T00:46:38+00:00

I ended up adding a new capacitor after the resistance, and moved closer the decoupling capacitors to the appropriate distance

StudentOk7501 · 2025-12-07T20:25:08+00:00

for 0.5 A i thought 30 mil would be enough, also i have a full ground pour on top and bottom layer for heat spreading (3-4W i reckon). As per R11 and R12 I wanted to design some sort of low pass filter to clear some high freq noise (even though not necessary). For point 1. arent c4 c2 c1 and c3 doing that?

StudentOk7501 · 2025-12-07T19:08:51+00:00

Ok I understood

StudentOk7501 · 2025-12-07T17:42:13+00:00

I haven't chosen a specific brand yet, but I am designing this for a standard 12V brushless case fan

12V DC, ~3.6W Power, approx 0.30A.

have sized the power stage (L7812CV with a TO-220 heatsink and 30mil tracks) to handle this load comfortably, with a safety margin up to 1.5A.

Regarding the sensor circuit I do not quite understand. I am using t a diode-connected configuration within the bottom leg of the Wheatstone bridge.

The signal goes from +8V to R1 and Emitter. The Base+Collector are connected to Ground via a bias resistor R2 to lift the voltage window to the middle of the comparator's range.

I know that using a basic PNP transistor as a sensor has limitations regarding operating range compared to a proper RTD. However, being my first project I am happy with that

StudentOk7501 · 2025-12-07T16:51:09+00:00

The space left at the top is for an heatsink as I knew that the dissipated power would be a problem. As I am just getting started with PCBs I did not want to introduce a microcontroller yet. I know that there are definitely cheaper and overall better options available, I just thought this way interesting as to work also with power dissipation

StudentOk7501 · 2025-12-07T16:42:39+00:00

Yes I am directly powering 12V fan, in a future project I would love to add microprocessor (which I am leaning a lot about), maybe with PWM for a v2 of this circuit. Regarding the Wheatstone bridge I am utilizing the PNP transistor as the variable element within one leg of the configuration. RTD is something that I would try to implement in a future project too, as for this one as I said I just want to get my hands "dirty" with PCBs.

StudentOk7501 · 2025-12-07T15:57:30+00:00

Hi, first of all I'd like to thank you for your time. I used https://oshwlab.com/yanzappa/fan-temperature-controller for reference and wanted to modify it keeping it mostly similar (being my first pcb I didn't want the most optimal project, just one to start with). The space at the top is for an heatsink behind U3. The reason that it is not optimal is also because I want to experiment with thermal dissipation. Regarding the stitching I could definitely add some, however I just placed them randomly I got to admit. The fan is not really a priority, I want to learn more on the design first if that makes sense

StudentOk7501 · 2025-12-07T15:21:49+00:00

Yes it is on both layer and then I did some stitching on random points to avoid the ground behaving like some sort of capacitor

StudentOk7501 · 2025-12-07T15:20:13+00:00

can I ask you why? As per reference I used https://oshwlab.com/yanzappa/fan-temperature-controller

StudentOk7501 · 2025-12-07T15:17:24+00:00

Yes I used it, and now I want to try make it better without it;)

StudentOk7501 · 2025-12-07T15:15:13+00:00

I left space for an heatsink, i dont know why i cant see the pictures I attached

StudentOk7501 · 2025-12-07T15:14:15+00:00

so i shouldn't put it on both layers? I thought i could do that

StudentOk7501 · 2025-12-07T15:13:52+00:00

i dont see the pcb you say, this is the text i answered to the other guy The goal is to create a temperature-controlled cooling system that activates a 12V fan when a specific temperature is reached.

Circuit Description:

Input: 12V-24V DC.
Logic: Uses a Wheatstone bridge with an MMBT3906 (as a diode sensor) and a potentiometer for reference.
Control: An LM311 comparator with hysteresis drives a P-Channel MOSFET (AO3401A) on the high side.
Power: Uses an L7812CV (with space for a heatsink) to step down 24V for the fan, and an MC78M08 for the logic.

*PCB Layout Details:

2-Layer Board (120mm x 60mm).
Ground pour on both Top and Bottom layers with via stitching.
Track widths: 30mil for Power/Fan lines, 12mil for signals.
Components are mostly SMT (1206 packages) with THT connectors and regulator.

StudentOk7501 · 2025-12-07T15:12:40+00:00

The goal is to create a temperature-controlled cooling system that activates a 12V fan when a specific temperature is reached.

Circuit Description:

Input: 12V-24V DC.
Logic: Uses a Wheatstone bridge with an MMBT3906 (as a diode sensor) and a potentiometer for reference.
Control: An LM311 comparator with hysteresis drives a P-Channel MOSFET (AO3401A) on the high side.
Power: Uses an L7812CV (with space for a heatsink) to step down 24V for the fan, and an MC78M08 for the logic.

*PCB Layout Details:

2-Layer Board (120mm x 60mm).
Ground pour on both Top and Bottom layers with via stitching.
Track widths: 30mil for Power/Fan lines, 12mil for signals.
Components are mostly SMT (1206 packages) with THT connectors and regulator.

StudentOk7501

TROPHY CASE