A little confused why fan isn't spinning at max rpm - Page 1
Forum section: electronics / beginners

author: Discotech
author_rank: Regular Contributor
timestamp: August 19, 2018, 07:26:44 pm
content: So I've built a variable fan control using LM317, 10k rotary pot a 510ohm resistor a 1uf cap and .22uf cap (I had no .1's) following this videoThe fan is rated for 12v @ 0.18amps which is 2.16watts, I've got my bench power supply set to 12v and 0.3amps but the fan is only drawing 0.08amps on the PSU which obviously means it's nowhere max RPM, I've tried running just the fan connected to the PSU and again it only draws that same amount of amps, only when I turn the voltage up on the PSU do the RPM & current draw start increasing to what it should beI'm obviously doing something wrong but I don't know what ?

author: Ranayna
author_rank: Super Contributor
timestamp: August 19, 2018, 07:40:59 pm
content: If you have a multimeter, you should measure the voltage across your fan.You will see that the fan will be getting less that the 12 volts you are feeding the regulator. A voltage regulator like the LM317 needs a couple volts of headroom, thias i called the dropout voltage.If you can, increase the input voltage of your circuit to between 13 and 14 volt, and you then should be able to give the fan full 12 volts.

author: glarsson
author_rank: Frequent Contributor
timestamp: August 19, 2018, 07:49:54 pm
content: The max rpm and max current does not necessarily happen at the same time. Max rpm is most likely in free air and max current is most likely when mounted in an enclosure with air flow restrictions. Max current can also be when completely stalled (0 rpm).

author: Discotech
author_rank: Regular Contributor
timestamp: August 19, 2018, 07:56:46 pm
content: Quote from: Ranayna on August 19, 2018, 07:40:59 pmI knew I wouldn't be getting full voltage, it's just over 10v btwWhat I'm confused about is the fan will run at max RPM on a PC PSU 12v railWhy will it not run max RPM when I connect it to my bench PSU set at 12v and enough amps ? (I'm talking directly connected, ignoring the LM317 circuit)What am I missing ? Why is it not drawing all the amps I'm providing it ?

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2018, 08:02:07 pm
content: Quote from: Discotech on August 19, 2018, 07:56:46 pmThe fan is a motorized device that consumes power. How much power it consumes depends on how hard it is working. If the fan is just spinning in free air it is not working very hard, so it will consume less power and therefore draw less current. If you load up the fan, for example by putting it in an enclosure with resistance to air flow the power it consumes may go up.

author: Monkeh
author_rank: Super Contributor
timestamp: August 19, 2018, 08:03:31 pm
content: Quote from: Discotech on August 19, 2018, 07:26:44 pmNot so. Maximum current and running current in free air are two very different things.You measure rotation with a tachometer, not an ammeter.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2018, 08:04:40 pm
content: Quote from: Discotech on August 19, 2018, 07:56:46 pmBy the way, how do you know whether it is or is not running at max RPM? Are you measuring the speed with a tachometer? Otherwise you cannot really know what speed it is running at.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2018, 08:07:00 pm
content: You should watch some of Matthias Wandel's fan videos on YouTube. He has a lot of good illustrations about fan speed and power consumption.