Thing-O-Matic: Dummy Load Fan Replacement

Dummy load fan replacement

Dummy load fan replacement

The fan on the dummy load that consumes the required minimum current to keep the ATX power supply happy wasn’t starting up reliably. That’s not surprising: I connected it to 5 V rather than the rated 12 V, because the load heatsink needs just a whisper of air flow to stay barely above room temperature, so it’s barely turning over and has no spare torque at all.

It turns out the heatsink really doesn’t need any forced air flow, despite having the fins oriented crosswise. Without the fan, it stabilizes just above comfortable-to-the-touch, a bit hotter than I’d prefer.

While I had the hood up for the HBP rebuild, though, I swapped in another fan and the heatsink is now cool to the touch. I did clean that dust off the fins, too.

If this one also fails at +5 V, I’ll fiddle the wiring to put it across the +12 V and +5 V supplies, where it’ll see 7 V. That should improve its disposition…

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  1. #1 by hexley ball on 18-January-2012 - 15:29

    >>If this one also fails at +5 V, I’ll fiddle the wiring to put it across the +12 V and +5 V supplies, where it’ll see 7 V. That should improve its disposition…

    Sure, you could solve it elegantly like that. But I have a couple of suggestions from the Department of Needlessly Complex Designs :-)

    1. I wonder if you could jump start the fan using something along the lines of the relay speed-up circuit that was developed by K1KP for antenna relays…

    (see K6XX’s site http://www.k6xx.com/radio/fastrely.html , or check out G3SEK’s site http://www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#speedup).

    2. Or you could drive the fan with a PWM chopper that starts out at full voltage (12V), then modulates down to 5 or 6 volts after a few hundred mS — probably an 8-pin PIC and an FET would do the trick.

    • #2 by Ed on 18-January-2012 - 18:19

      jump start the fan

      The thing goes on when the power comes up, so there’s no way to pre-charge the capacitor. Of course, one could certainly stir a 555 in there somewhere…

      probably an 8-pin PIC and an FET would do the trick.

      Aaaaaaaahhhhhhh!

  2. #3 by Bill Rutiser on 19-January-2012 - 12:50

    How Rube Goldberg are you? Connect the fan thru diodes to both 5 and 12 v. Arrange a microswitch to open the 12v circuit when the fan’s air presses on a wind sensor.

    • #4 by Ed on 19-January-2012 - 15:05

      How Rube Goldberg are you?

      You must be new around here… [grin]

      • #5 by david on 20-January-2012 - 04:52

        well, at that point, throw another one of your incandescent bulbs a la your heater-resistor-warning circuit in series with the fan on the 12V line. High inrush current to spin up, then lower maintain current once the bulb gets hot. At least, it seems like it ought to work, right? :)

        • #6 by Ed on 20-January-2012 - 06:42

          At least, it seems like it ought to work, right? :)

          OK, OK, OK! Next time it gives up, I’ll do something appropriate… [wince]