BLDC fan on unfiltered DC?

[c4757p]

I'm still working on repairing that HP 3437A - the fan is pretty much completely shot. Time for a replacement.

The original HP fan had three windings which they ran using a three-phase driver circuit on a rectified but unfiltered (not just no regulator - no capacitor) transformer tap. This sort of fan is pretty hard to find now (and expensive when you do find it), so I'd rather just put in a modern brushless (probably this one). Would it be unhappy without a filter capacitor? That's a pretty big cap (probably at least 1000uF) and I'd rather not have to find a place to stuff it.

[sync]

It doesn't work.
Btw: Is that 12V unreg. the RMS value? Then a cap alone would produce to much voltage.

[c4757p]

The peak is just over 12V - it could easily average 12V with either a series resistor to drop a bit, or a slightly undersized capacitor. I just chose a 2200uF cap for it, so I'll go with the series resistor.

I just noticed that the service manual has a memo attached addressing this - the original fan was discontinued and they describe replacing it with a DC fan: direct replacement, disabling the three-phase oscillator, for some serial numbers, and adding a 22 ohm resistor in series for others. Nothing about a capacitor; they must have selected a replacement fan which doesn't need it.

[sync]

No need for the resistor. 12V fans can usually operate at 15V.