power on 24V fans individually

Hi everyone I have five 24V 0.38A Fans and would like to turn them on or off individually (start with one, hits the next threshold turn on another and so on until all five are running if needed) depending on a certain temperature threshold. I will be using a DHT11 for the temperature and humidity sensor and will also be using light sensors but it has nothing to do with the fans. Can anyone point me in the right direction? I have done lots of research but have not come across anything about turning them on individually .... if anyone can help in anyway it would be greatly appreciated. Thank you in advance for any replies.


I would suggest an ordinary npn transistor, such as BC337, to switch each fan. These should be "low-side" switches, ie. between the fan and 0V. BC337 can handle 800mA, I think, so more than enough. But some npn can only handle 100mA, so check the data sheet.

If you can arrange that the transistors are all in the air-flow, that will help keep them cool, as will ensuring they are fully switched on ("saturated") by a decent base current, perhaps 10mA, so a resistor of around 500R between the base and an Arduino output.

As fans are reactive loads, you should be sure to have protective diodes such as IN1004 in parrallel with the fan's connectors to short-circuit any reverse voltage created by the fan when switching on/off.


See attached schematic:

digitalWrite(12,HIGH); will turn on the top fan.
digitalWrite(12,LOW); will turn off the top fan.


Having used the DHT11 sensor before and one thing to consider is the airflow over the sensor itself. In still air the sensor is quite slow to adjust to temperature changes. This could allow the temp of the air to rise quite dramatically before it is detected. With even the slightest breeze the response time shrinks to only a couple of seconds. Take this into account when placing the sensor and deciding on a minimum fan level.

Hi Larry,

I though about ULN2803 also, but was concerned that with all 5 fans running, the chip could be having to sink around 2A, which is close to the 2.5A limit, plus the 0.38A per fan is close to the 500mA limit per pin. If the current draw is higher at startup (e.g. the 5th fan staring up) it might be a little too close for comfort?


I am confident this would work.
It would be interesting to measure the running current of a fan and compare it to the 380ma.

Total substrate-terminal current -2.5 amps.

Thats exactly what I was thinking I should be able to do but was not sure if it would work, I am still a noob at actual electronics (always been a tinkerer) … thank you

The DHT will not be in direct line of the fans and actually will be either in an upper compartment in which some obstacles will be in the way or close to the bottom which would be under the fans direct flow … but thank you

in the end I am hoping to have 5 fans, 1 dht11, (possibly another temp sensor only), 6 Photo Resistors, an LCD and possibly a SD Module or an ethernet module if I can ever get to a point where I can save data to mysql with the mighty1284 + enc28j60 or even an uno with the enc28j60, oh and an RTC using a ds1307