Replacing fans in a benchtop instrument - troubleshooting - Page 1
Forum section: electronics / projects

author: Aiy
author_rank: Regular Contributor
timestamp: April 22, 2025, 08:51:47 pm
content: Hi All,I recently attempted to replace two fans in an Hittite T2220 which are worn out in the bearings.The original fans are Everflow R124028BL - 40x40x28mm, 12V, 6000rpm, 3pin,https://www.everflowtech.com/upload/files/2019%E7%94%A2%E5%93%81%E7%9B%AE%E9%8C%84.pdfI attempted to replace them with similar airflow fans from digikeyhttps://www.digikey.se/sv/products/detail/same-sky-formerly-cui-devices/CFM-4015C-165-256-20/22259230But the instrument detects the fans as failed.What could be the reason for this?The fan speed differs by 500rpm, 6000 vs. 6500 rpm. Could the RPM target be that narrow?The other reason I could come up with is that the tachometer signal is different between the two, but the original Everflow datasheet does not provide any info regarding the tachometer signal.I probed the tachometer signal on the replacement fan and its a square pulse train with 0/5V leading me to believe that the fan is open collector with a pullup somewhere on the board.The original fan tachometer signal is something else, hovering around 0V, which I'm having trouble understanding, a STALL signal? Or is it something else?Best regards,

author: Langdon
author_rank: Frequent Contributor
timestamp: April 22, 2025, 09:45:00 pm
content: I wouldn't buy from Digi-Key in Sweden.The shipping must be expensive!Would you consider connecting the original, broken fans to make it shut up, then externally power the working fans?

author: Benta
author_rank: Super Contributor
timestamp: April 22, 2025, 11:29:34 pm
content: Quote from: Langdon on April 22, 2025, 09:45:00 pmHuh?Sweden is EU.

author: tunk
author_rank: Super Contributor
timestamp: April 23, 2025, 01:52:17 pm
content: I think I would start with finding out what the third cabledoes, and if the new fans are the same. You could alsotry to replace the ball bearings in the old fans.

author: eutectique
author_rank: Frequent Contributor
timestamp: April 23, 2025, 04:58:14 pm
content: There could be a current sense in the fan supply circuit. You are replacing a 150 mA fan with an 80 mA one.Add a 171 ohm resistor parallel to the new fan.

author: Langdon
author_rank: Frequent Contributor
timestamp: April 24, 2025, 04:04:45 pm
content: Quote from: Benta on April 22, 2025, 11:29:34 pmDigi-Key is US.

author: Aiy
author_rank: Regular Contributor
timestamp: May 20, 2025, 08:33:26 pm
content: Hi All,I tried running the original fans with the third pin unplugged, and it gets detected as fan failure.It might be an rotation detection / alarm signal instead as illustrated in this PDF page 9 - application notes:https://www.sameskydevices.com/product/resource/cfm-40v.pdfI might try it by pulling the input to ground as a checkBest regards,

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 21, 2025, 09:55:38 pm
content: 6000rpm versus 6500rpm is quite a small difference, but if it has a hall sensor for feedback, it's unlikely another method would be used too.Maybe the new fan needs more time to get to a high enough RPM?Maybe the feedback pin does not work properly?One thing you can experiment with is adding a vacuum cleaner to boost the fan a bit, or up the voltage to the fan. ( or lab power supply)Or create a fake feedback signal from a function generator.If it works you can create a more permanent solution. add a small step-up SMPS, or maybe tap some extra voltage from a negative power supply (or unregulated higher input voltage.

author: Someone
author_rank: Super Contributor
timestamp: May 21, 2025, 11:31:46 pm
content: Quote from: Doctorandus_P on May 21, 2025, 09:55:38 pmNot all fans have the same number of pulses per revolution!

author: Langdon
author_rank: Frequent Contributor
timestamp: May 22, 2025, 01:14:14 am
content: Perhaps an astable 555 timer that makes decoy pulses would bew a good idea?Avoiding the problem might be easier than a "true" solution.