Quieter fan for ET5410A+

[HwAoRrDk]

I recently got myself an East Tester ET5410A+ electronic load, but I'm finding the loudness of the fan a bit wearying. Also, mine seems to have a slight intermittent rattling sound. Has anyone replaced the fan with something quieter?

I can't see what kind of fan it is without disassembling some stuff (label is not visible), but it seems to be an 80mm fan - is that right? But as for ratings, etc. I'm not sure what's required. It's a two-pin connection on the fan, so no tacho output required at least. I found some reverse-engineered schematics in another thread on here which indicate it is 12V and speed is controlled by PWM, but there also appears to be some kind of fan monitoring circuit. If I'm interpreting the schematic (see attached) right, it's some kind of current-sensing circuit? I guess I would need to find a fan that has similar current draw?

BTW, my review of it so far is that it's a decent piece of equipment, with my only complaint so far being the fan noise and that the front panel buttons have an occasional responsiveness issue - sometimes you'll press a button and the unit will beep, but the button press isn't recognised. It's almost like the beep is triggered by an edge on the button signal, but the software has a de-bounce that's a little too over-zealous and which requires buttons to be pressed for a certain minimum period.

I also had to chuckle at the as-supplied fuse situation. It came with a proper UK mains cable (which I wasn't expecting), but the plug had a 13A fuse! I swapped it for a 3A one. And then you get two spare fuses for the fused IEC connector on the back, but it's confusing because these fuses (and the one already fitted) are 0.5A but the IEC receptacle has "Use only with 2A 250V fuse" written on it.

[HwAoRrDk]

I simulated the fan control and monitoring circuit and it's using the PWM to control linear regulation of the voltage supplied to the fan (hence the heatsink on Q6, which is probably dissipating >0.5W at 100% fan speed). The lower part of the circuit is indeed doing current sensing, but translating that into a logic low/high indicating basically whether the fan is running. The threshold seems to be about 45mA, below which point output FAN_OK will go high (so it's actually active-low for 'OK').

I don't understand what the zener diode Z4 is doing, but it seems to have the effect of enforcing a minimum regulated voltage to the fan. And why they are using three 1.5k pull-up resistors instead of a single 500 ohm (or maybe 470) I don't know.

Given that, I can probably use any 12V fan and the control circuitry will be happy.

Not sure about whether the fan may need to be tailored towards higher static pressure, as the fins of the heatsink and outlet grille of the fan on the rear panel are quite restrictive.