A lot here depends on what your 15V supply is REALLY putting out.  If
it keeps outputting 15V even at full load, you're probably in good
shape.  If it's completely unregulated it probably puts out more than
15V at low loads, and less than 15V at high loads, in which case you
could have problems depending on how close YOUR top load is to what
the power supply thinks is full load.  (OTOH, if we're talking FANS,
I don't think you have much to worry about.)

Have you measured what the voltage looks like at the input to the
regulator at all?  A simple voltmeter should be sufficient.

BillW

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