fans in series

There are fans mounted on GPU boards and inside laptops that would fit in the space. Probably 5 V though. Why not 48V > 5V and make life easy picking the fan?

snipped-for-privacy@highlandsniptechnology.com wrote: ================================

1. Load current ( average value ) goes up by 70 to 100% when stalled.

2. Current is drawn in steady pulses of about 90% duty cycle, frequency related to RPM and number of poles.

1. During the "off" time current draw swings briefly negative.

1. All fans are quiet, some damn quiet.

IMO all are nice, easy to use items.

Go with the 4 x 12V zener idea - using at least 3W types.

...... Phil

These beasts are deep (possible design constraint), redundant and last forever. Pretty loud at full blast too.

Not sure what reliability you want, but small thin fans suck, I think some sort of hokey laptop cooler fan in mentioned in the thread. It might help dissipate low tens of watts on a good day. Everone has seen how laptops die. They overheat more and more often and then the thermal stresses finally break connections on system board. That's no accident.

I'd like to dump at least 100 watts on the dummy load board. The little 35 mm fans are good for maybe 7 cfm each unrestricted. Three might manage 15 cfm in real life, maybe less. 100 watts into 15 cfm heats the air roughly 10c, not bad. The heat sink and mosfet temps will of course be higher.

So, how hot might we allow the heat sink to get? 100c? With a sticker

WARNING HOT SURFACE

The giant fets wouldn't get a lot hotter than the sink.

From a reliability standpoint, I wouldn't do it. One fan opens up and the others quit functioning.

It might be simpler to use a 120V fan in a big box, with a thermostat fan control. Regulated voltages are unnecessary, dependence on the 48V source is unnecessary, fitting into the thin box is unnecessary, and a thermostat allows complete independence of any logic signals. When the fan needs lube, the guy in the field doesn't need to open a computer at a static-safe station and recalibrate a precision instrument.

Think we can dump 100 watts? Maybe we should dump the little fans... they would be a nuisance to install and wire up. With a really big heat sink, the overall box air flow might work as well.

We need to try it.

"One experiment is worth a thousand expert opinions."

I think Werner Von Braun said that.

Well, he was a very flexible fellow. ;)

Phil Hobbs

The supreme leader would like a word with you.

So make sure that at least one fan is running to git rid of the smoke.

I ordered one just for fun. I'd have to stick my four giant mosfets on the bottom somehow.

lørdag den 29. januar 2022 kl. 23.01.28 UTC+1 skrev snipped-for-privacy@highlandsniptechnology.com:

flip the whole pcb uupside down and place them around a square cutout in the middle of the pcb?

mount them tap up and squeeze them between the pcb and heatsink with the X bracket

Spehro has been helping us with the thermals of our dummy load module. He found this heat sink

Well, the pcb is mounted on the heat sink more than vice versa.

If we put two giant fans on the front of the rack box, and make their speed adaptive on heat sink temps, they will deafen only a minority of our customers.

We can bolt that bottom heat sink fin to the bottom of the rackmount enclosure. That will improve cooling some, and keep things from ripping apart when some UPS guy throws it off the back of a truck.