Simulating RPM sensor for low speed fans

Control: management of fans, temp/rpm monitoring via soft/hardware

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MoJo
Posts: 773
Joined: Mon Jan 13, 2003 9:20 am
Location: UK

Simulating RPM sensor for low speed fans

Post by MoJo » Wed Dec 23, 2009 3:42 pm

I am trying to quieten a broadcast monitor which has three fans inside. They are all 2000RPM models running at 12V. The problem is that the control board shuts the monitor off if it doesn't read the fan speed as over a certain limit, which has been set fairly high.

I tried using just slower rated fans but even at 1500RPM the monitor shuts down. What I need to do is generate a 2000RPM signal to make it think everything is okay.

Before you panic, there will be no problem with cooling if I do this. The monitor, an NEC XM29 Plus, is designed for broadcast environments where it will be surrounded by lots of other equipment. Inside it's actually pretty similar to a normal CRT and would probably be fine passively cooled, but if I can just replace all three fans with quiet ones that will be enough for me.

The only thing I can think of doing at the moment is to use a 555 timer or an AVR to generate a 4000Hz signal and use a transistor to interface it to the open collector speed sense wire. 4000Hz is required because most fans put out two pulses per revolution. I'll try and test that but in the mean time I'm open to good ideas.
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MoJo
Posts: 773
Joined: Mon Jan 13, 2003 9:20 am
Location: UK

Post by MoJo » Sat Dec 26, 2009 11:38 am

I fixed it.

I discovered that although the monitor does not seem to like any fans I connect other than the original ones, it will accept the originals running at 5V instead of 12V. The problem is that the originals don't start at 5V and even at 5V are not that quiet.

My solution was to use the quietest of the three original fans at 5V. I made a little LM317 based kickstart circuit to run it at 10V for 2 seconds at power on to get it started. Then it runs at 5V and is pretty quiet.

For the other two fans I replaced them with Panaflos running at 5V (serial zeners). Because the XM29 does not seem to like their RPM sensors I just attached their sense lines to the first fan. It doesn't seem to cause a problem having them commoned.

Now the monitor is virtually silent. It's quieter than my slim PS2. It buzzes for a few minutes after being turned on (coil whine) which is much louder than the fans until it clears up, and it's more than adequate for watching TV via a silent Freeview box from a few meters away.

Oh, and of course it's probably the best CRT picture you can get :D
http://world3.net

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