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In many digital lab PSU designs, designers usually use a thermistor to control the cooling fan speed based on the main regulator actual temperature.

When we have both current and voltage values, is there a reason why they don't control the cooling fan speed based on how much power (in watts) is being drawn from PSU?

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  • \$\begingroup\$ Probably because: why overcomplicate things? \$\endgroup\$ – user253751 Nov 29 '19 at 13:05
  • \$\begingroup\$ @user253751 calibrating the thermistor isn't complicated? \$\endgroup\$ – ElectronSurf Nov 29 '19 at 13:06
  • \$\begingroup\$ Does it have to be very accurately calibrated? \$\endgroup\$ – user253751 Nov 29 '19 at 13:08
  • \$\begingroup\$ @user253751 no it doesn't, but still i feel like it's easier to do based on watts. I actually asked this question to know if there's a down-side or something to use this method... \$\endgroup\$ – ElectronSurf Nov 29 '19 at 13:17
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    \$\begingroup\$ Probably just the usual downsides of added complexity as well as the usual downsides of estimating something you could measure. This is a guess, not an answer. For example, it won't run the fans faster in hot weather when there's less passive cooling. \$\endgroup\$ – user253751 Nov 29 '19 at 13:22
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As the goal is to not have the heatsinks too hot, it's far easier to measure the temperature of the heatsinks, than to estimate their temperature by measuring the line input voltage, the output current, the air inlet temperature, the mass air flow through the heatsink, and compute a non-linear function for heat transfer versus flow rate.

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In addition to the accurate answer by Neil_UK, I don't know what type of linear PSU design you have seen, but some of them do employ some smart schemes where the wasted power can be optimized by methods that are easy to implement, but would complicate your proposed scheme of estimating heat generation.

My linear PSU has a multitap mains transformer, and a relay (relays?) to choose the tap that is closest to the desired output voltage, so the pass element has to drop the minimal voltage needed. Of course, the heat dissipated is still (Vtap - Vout)×Iout, but it's not easy to measure Vtap - it has some AC ripple on top of it (I guess it is straight after the bridge rectifier and filter caps), and it will also depend on mains power variation (210-250VAC). My linear PSU, as it is now, is designed so that you don't care how much Vtap is (as long as it is within some limits of course), but with your scheme you'd have to measure, filter it, etc. Thus your proposed scheme doesn't necessarily simplify things.

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