I think this is going to be a newb question, but I have to ask it because maybe I'm wrong. When you hook up fans to motherboards I thought all of them are variable speed controlled by the motherboard. If that is the case then why would you need or want a fan that is variable speed fan when it is hooked up to a motherboard or fan controller?
Not all motherboards have variable speed, but it's very unlikely most modern boards don't have it for system and/or case fans. Fans can be controlled by either pulse-width modulation (PWM) or voltage regulation.
If the motherboard method of control is PWM, then a PWM fan will be needed. The first fan is one of these and it's model number is DFS122512L-PWM. Notice the PWM at the end and the specs are 310(±200) - 1200 ±10% RPM. The slowest it will run is 310, but with +/- 200 it could be as low as 110rpm or as high as 510rpm at miminum PWM output.
PWM output keeps the voltage the same (say 12V), put the pulses of 12V are varied in duration over a given time, so that 12V is on, the off, then on, etc. The longer the ON time for the pulse, the quicker the fan will run. The shorter the ON time, the slower it will run.
Take the given time as 1 second and 10 pules per second. When each pulse is 0.1 second long, this is at 100%, 10 pulses x 0.1 pulse length = 1 second of total pulse time during a 1 second time frame. The fan will run at it's rated maximum rpm for the voltage of the pulse.
Now reduce the pulse time to 0.05, so this is a 50% PWM setting, 10 pulses x 0.05 pulse length = 0.5 second of total pulse time duing a 1 second time frame. Pulses are still at maximum voltage, but a PWM fan will run approximately 50% of it's rated maximum rpm.
and so on...
If you plug a standard fan onto a board using PWM, it will run at the speed of the voltage it's given, no variation in rpm over the PWM range. The second fan is one of these, specs are 1900 ±10% RPM. If this is a 12V fan the only way to make it run slower is to drop the voltage. If given 12V it will run at 1900rpm. Less volts means less rpm.
However, if the board has both voltage and PWM as a control method, you would select "PWM" for the first fan and "voltage" for the second fan. Assuming the sensor is set at the same temp for both, they will both operate as a variable fan controlled by the motherboard.
Alternatively, you could set either fan for "manual" and they will operate at 1200rpm (+/-10%) and 1900rpm (+/-10%), respectively.
If in doubt, check your BIOS settings or motherboard manual.
Wow really enjoyed the read thank you very much for the information. That helps a lot and makes sense for a lot of builds I have. Is PWM or Voltage a typical default setting for boards or is it random based on the manufacturer?
I don't have anything on hand to indicate which any manufacturer prefer. It seems to vary not only with the manufacturer, but with the models, chipsets, etc.
If you're looking to buy a motherboard, download the user manual and read through the BIOS section. The Hardware Monitor or similar chapter will indicate the setting options for cpu fan, system fan, case fan, etc.
I recall the Asrock AOD790GX/128M motherboard and the cpu fans settings: Auto, Manual, PWM and Voltage.
If left on Auto and the temp set to, say 45C, it doen't matter which type of fan is installed becuase the board will detect it. Most cpu coolers are PWM, but if not they must be controlled by voltage regulation.
Most system fan and case fan outputs on motherboards I've come across are PWM, if they are variable, otherwise they are fixed voltage output with no pulses. The rest are usually voltage regulation and a few are a combination of both, like the Asrock board above, but on the system or case fan and not the cpu fan.
Some boards have Manual as the default setting on all fans (J&W G13-MKII springs to mind), including cpu. You know the type, install everyting, boot up, and it's all roaring away until you go in BIOS and changes setting, determine temperature thresholds and min/max rpms (via percentages).