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I have a "6V 4.5W" solar panel (Generic) and a 12V PC cooling fan (Evercool PWR-1225H12B) that consumes 450mA at 12V full speed. PC Fansolar panel

Running the fan directly from the solar panel in full sun works decently; enough to push air at an acceptable rate. It is a "permanent" setup, with the fan and solar panel always being connected.

My question is: Would there be a premature aging issue to either fan or panel if there is only partial sun?

There is a bit of hysteresis because it of course takes more power to start the fan than to keep it running. So if it stalls, it can stay stalled for quite some time before it gets going again (if at all before next sun-up).

I think the panel will be fine, as I've read that short-circuits are not inherently bad for them, so the increased stall current should be no issue.
However, I'm more concerned about the fan circuitry. Since they are designed to run from a regulated 12V supply, I wonder if the drivers will have an issue running with only partial power and a stalled fan.

I realize that it can depend on the specifics of the fan I'm using, and that the innards are not disclosed. I imagine the fans are typically similar though, so anyone who has an understanding of how a typical PC fan works may be able to shed some wisdom as to why it would or would not be an issue to stall a PC fan for extended periods.

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  • \$\begingroup\$ Evercool PWR-1225H12B product page says 0.8A @12V. \$\endgroup\$ – Turbo J May 11 '18 at 19:38
  • \$\begingroup\$ can you measure the voltage and current when it's stalled due to low voltage - that will tell you how much heat it's making \$\endgroup\$ – Jasen May 11 '18 at 21:26
  • \$\begingroup\$ Don't be afraid to experiment with things like this. Just make sure you take some precautions so that nothing catastrophic happens. Make sure you can de-energize the device safely even if it catches on fire. Make sure it can catch on fire without spreading the fire elsewhere. And make sure you can put out the fire if it does spread despite your precautions. And don't leave it unattended. If it gets really hot or starts to smoke or melt, de-energize the device. \$\endgroup\$ – mkeith May 12 '18 at 20:04
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Should not be a problem for either panel or fan. Note, that "stall" does not mean "short circuit", not even in brushed motors, and PC fans are brushless.

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  • \$\begingroup\$ Yes, I just meant that conceptually, if shorting a panel is okay, then the higher-current from a stalled motor should be okay. \$\endgroup\$ – Bort May 11 '18 at 19:26
  • \$\begingroup\$ How familiar are you with the internals of a PC fan? I'm not sure how they are typically driven. \$\endgroup\$ – Bort May 11 '18 at 19:29
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    \$\begingroup\$ I've disassembled one or two out of curiosity before :). They usually have hall-effect position sensors and amplifier chip driving the coils. If the current through the winding is not enough to turn the fan it should not be enough to overheat it or damage the circuit. \$\endgroup\$ – Maple May 11 '18 at 19:39
  • \$\begingroup\$ BLDC with electronically switched coils. These should still switch on and off when the rotor is stalled until the voltage gets very low. \$\endgroup\$ – Turbo J May 11 '18 at 19:40
  • \$\begingroup\$ “Should” makes a leap of faith on low cost fan designs that satisfy my criteria.(see my answer) -1 \$\endgroup\$ – Sunnyskyguy EE75 May 12 '18 at 20:05
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You are stall safe with all PC fan implementations.

Most PC fans are in fact 2 phase motors. A good reference chip to understand is the DIODES AH2985, this chip works from 2.5 to 15 VDC and is very common in both the 5 Volt and 12 Volt fans.

enter image description here

The driver chips have inbuilt stall detection and restart logic, so won't overheat under any circumstances.

If you are concerned enough you can simply test the fans you use with an adjustable power supply. Bring the voltage slowly up from zero and measure the current, it will soon be apparent if you have a voltage/power/start problem.

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  • \$\begingroup\$ That is not my experience on Nidec fans for stall detection. They only have to pass UL’s locked rotor” fire test” and don’t care about inherent temp rise of drivers. They don’t detect current. Only phase with RPM maybe is detected. \$\endgroup\$ – Sunnyskyguy EE75 May 12 '18 at 20:07
  • \$\begingroup\$ Not all fans use this cct, so we cannot generalize all fans are reliable on start/ stall threshold so -1 yet this is a better design than those which do not use this 2015 chip which validated that it is a potential MTBF issue for other cheap fans. \$\endgroup\$ – Sunnyskyguy EE75 May 12 '18 at 20:13
  • \$\begingroup\$ @TonyStewartolderthandirt Yes we can generalize for the class of PC fans we are talking about....I'll bet you can't find much variation ...though some have speed controls and tachometer outputs they are all 2 phase drivers. Since they are 2 phase they need start logic, which is the same as the stall logic. \$\endgroup\$ – Jack Creasey May 12 '18 at 21:03
  • \$\begingroup\$ Again this is not my experience with PC fans with many brands failing to start with even full voltage due to lack of stall logic and misaligned Hall phase detectors relative to magnetic vs rotor start positions Failure rates up to 1% in 1of4 start positions have been discovered by me on many brands. \$\endgroup\$ – Sunnyskyguy EE75 May 12 '18 at 22:57
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There will be heat loss generated by stalled fans perhaps at 15% of the applied voltage and thus the switching transistors will be warm and heat add it to the base but this should not affect the reliability of the fan if the stall starting voltage is < 1/3Vcc

The BLDC drivers may depend on air flow for cooling and Pd is dumped on these devices in stall mode while they are buried inside fan so you cannot sense Tjcn so my values above are based on this. This may be only a problem if RdsOn or Rce of drivers is not much less <10% of coil DCR . Designers should remember this ratio in future BLDC design of drivers. Thus if so, the coil absorbs the power and with no air flow may be safe.

Since fan load increases with RPM and DC motors draw less current with RPM the motor current is somewhat constant or slightly linear with RPM above stall speed.

Variations depend on Fan design.

Without detailed fan specs this accurately describes the heat and thus reliability risk with fans at stall threshold on starting.

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