Combined LM2941 Manual Fan Regulator & LM3914 Voltage Monitor


This project is based on my LM2941 Manual Fanregulator schematic and my LM3914 Voltage Monitor schematic, combined together in one project. I needed this project to control the speed of my Panaflo L1A radiator fan and at the same time have a visual indication of the applied fan voltage. I only use 6 of the 10 LED outputs of the LM3914 because that way the display is 7 to 12V (1V / LED). Also, since I use rectangular 5x2.5mm LEDs in a vertical pattern (6x5=30mm) and since the height of a 5.25" Lian Li bezel is limited to approximatively 42mm I cannot put the 10 LEDs.

I like to use the LM2941 regulator (about 5 Euro here in Belgium) instead of a cheap LM317 because of the low voltage drop over the regulator, typically 0.2-0.3V (max 0.5V at 1A) as opposed to the 1.5-2.5V dropout voltage of the LM317. With the 12 to 12.xV on a common PSU rail you get the full or almost the full 12V on your fans. The formulas for resistor calculation are the same as for a LM317, but the LM2941 has 5 pins as opposed to the 3 of the LM317 (the adjust pin is separate and a "ON/OFF" pin is also provided to remotely shutdown the regulator. The LM2941 is rated for 1A max. with some cooling. The values of R1, P1 and P2 in the schematic have been calculated for an output range from about 7 to 12V and a 10K linear potentiometer.

The LM3914 voltage monitor is also straight forward. As told I use only 6 of the 10 led outputs. A voltage divider on the input (R1-R2) decreases the 7-12V input to about 1.96-3.36V (x0.28). The P1-R4 combination sets the high reference voltage (12/12 of 3.36V, level for 12V readout) and P2-R5 sets the low reference voltage (7/12 of 3.36V, level for 7V readout).

Calibration is very easy: Set trimpot P2 on the fan regulator to obtain an output voltage going from about 7 to 12V for a complete turn of potmeter P1. For minimum speed setting of the fan regulator, set trimpot P2 on the voltage monitor to lit only the lowest led. Then, for maximum speed setting of the fan regulator, set trimpot P1 on the voltage monitor to lit all 6 leds. Test all positions of potmeter P1 of the fanregualtor again and finetune if needed.

Schematic of the LM2941 fanregulator
Schematic of the LM3914 Voltage Monitor

Download Schematic in Circuitmaker2000 format

Download Schematic in Circuitmaker2000 format

Display PCB layout
Fanregulator & Voltage Monitor PCB layout

Download PCB layout in Circad'98 format

Download PCB layout in Circad'98 format

I made two different versions of the display PCB layout because I didn't know if I could obtain a potentiometer with a 4mm shaft, so a layout for a 6mm shaft potentiometer is also provided.

The part list and component layout will follow as soon as I can find some time to do it...

Solder side of the Fanregulator & Voltage Monitor
Component side of the Fanregulator & Voltage Monitor


Solder side of the Monitor Display
Component side of the Monitor Display


Mounted PCB's
Idem in the right position for the bezel

The two PCB's are soldered together by means of the solder pads at the edge of the main PCB, and the pads in the middle of the display PCB.

Front view
Test with a Nidec Gamma28 blower


Minimum speed (7V)
Intermediate speed (9V)
Full speed (12V)

In the next pictures the fan reglator & voltage monitor is mounted into a Lian Li bezel, together with my Seetron LCD (not powered on)

Rear view
Other view


In action...
Rear view
Other view

Mounted in the front panel of my PC-70...


General views of the front panel...