Plum

@plumpan

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Plum @plumpan5/24/2024, 7:33 PM

Dear EE peoples

What is the easiest way to double a fan tachometer frequency signal by 2x?

If the fan is spinning 3000rpm, I want it to report 6000rpm.

I do not know if that would be easier than trying to get the code off of these Zilog Z8Fs, modify it, and load it back on. Probably not since they're little surface mount packages already soldered onto a board.

EDIT: Yeah messing with the microcontroller is not an option lol. I'm sure there's SOME way to flash it from the rest of the system but I'm not figuring that out unless I know someone who worked at Xyratex.

#electronics

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in reply to @plumpan's post:

@kyogi5/25/2024, 12:01 AM

I haven't tried it, but there's an application note from Analog Devices that gives a simple circuit consisting of an opamp, xor, and a couple resistors and caps that should do the job. (https://www.analog.com/en/resources/design-notes/simple-circuit-doubles-input-frequency.html)

I've also seen a version with just an XOR feeding back its output through a series resistor to the second input with a cap to ground. (Input 1 would be the original signal.) Good luck!

Plum @plumpan5/25/2024, 2:22 AM

So it turns out I have some LME49720 Dual High Performance, High Fidelity Audio Operational Amplifier kicking around from an unfinished project that accept a wide enough voltage range to use! Wonderful!

The problem is I'm basically trying to cheat a fan controller. It use the RPM sense to set the output voltage, and hopefully achieve a target fan speed. This means I can't just stick a resistor on the output line, since it will just pump more volts (or freak out when it can't hit target speed), and I can't do any PWM magic since there's no PWM.

Now, if it's possible I'd really like for this to be inline, that is to say it uses that voltage that the fan controller is putting out to run any circuitry required. The opamps I have take 2.5-17v, so they don't care, but I don't have any XOR gates on hand. I don't know if they come in wide input voltage ranges. I could source a 5V from nearby (spoilers: this is actually inside a PSU, so it should have 5 and 12V lines just chillin'), but I'd really rather not if I can avoid it. I'm also not sure where I'd get just a couple of gates for dirt cheap...

For what it's worth, I do actually have two fans inline in these applications. I don't think I could magically make them send the signal in phase enough to track the fan controller (which also has a separate input for each fan), plus the second fan tends to run a couple hundred RPM faster on account of having another fan blasting into it.

If I find some XOR gates maybe I'll try the basic one first.

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