EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: PedroDaGr8 on September 27, 2016, 04:48:40 pm
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Trying to repair a Beckman Coulter Microfuge 18 centrifuge. One of the solenoids for the lid gummed up and wouldn't open. We cleaned up the gummed up solenoid and it still didn't work. Found this blown NTC, trying to figure out the replacement. The logo is a triangle with S*M inside of it.
(http://uploads.tapatalk-cdn.com/20160927/0ed2e6ac5b5cdcf293438204451264d2.jpg)
(http://uploads.tapatalk-cdn.com/20160927/b956887a9551932d67e8b975e16e3de7.jpg)
(http://uploads.tapatalk-cdn.com/20160927/5ebb14235fafe6bc73b85a30a7a18ae7.jpg)
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Looks to be "NTC 15"?
Luckily for you, NTCs are all about the same thing, so you'll probably want to shop for a mains inrush current limiter type part, with the same resistance (15 ohms at 25C) and dimensions as your part there.
Is it actually connected to AC mains traces, or might it be something more interesting?
Tim
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Looks to be "NTC 15"?
Luckily for you, NTCs are all about the same thing, so you'll probably want to shop for a mains inrush current limiter type part, with the same resistance (15 ohms at 25C) and dimensions as your part there.
Is it actually connected to AC mains traces, or might it be something more interesting?
Tim
Thanks, it is connected between the mains transformer and the solenoids. I am not sure how a gummed up solenoid which is held closed would affect the current draw but this NTC is very clearly toast. Just ordered some replacements from Mouser, we will see how it goes.
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Guessing the solenoid got stuck at short-circuit current, then burned out and failed shorted. Which toasted the NTC, and hopefully there was a fuse that opened as well, but I guess too late to save the NTC.
The way an AC solenoid works, the current draw is very large when open, and less when closed. The extra current draw means the force vs. distance curve isn't as dramatically skewed as bare magnets (which go something like the inverse cube of distance). It's more of a constant force, plus some extra grab when the armature seats fully. But if it never closes, well...
And if there isn't a fuse... melting NTCs isn't a good idea... :-BROKE :scared: you should strongly consider adding one! (Anywhere is good, inline with the solenoid for instance. Any old inline fuse holder, rated for AC mains insulation, will do the job. Ideally, it should be upstream, towards the mains input side of the NTC, or whatever's switching it (relay contacts?), on the HOT wire. And if this is inside a bit of industrial kit, maybe there's opportunity to add a fuse block to a panel or DIN rail?)
Tim
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Guessing the solenoid got stuck at short-circuit current, then burned out and failed shorted. Which toasted the NTC, and hopefully there was a fuse that opened as well, but I guess too late to save the NTC.
The way an AC solenoid works, the current draw is very large when open, and less when closed. The extra current draw means the force vs. distance curve isn't as dramatically skewed as bare magnets (which go something like the inverse cube of distance). It's more of a constant force, plus some extra grab when the armature seats fully. But if it never closes, well...
And if there isn't a fuse... melting NTCs isn't a good idea... :-BROKE :scared: you should strongly consider adding one! (Anywhere is good, inline with the solenoid for instance. Any old inline fuse holder, rated for AC mains insulation, will do the job. Ideally, it should be upstream, towards the mains input side of the NTC, or whatever's switching it (relay contacts?), on the HOT wire. And if this is inside a bit of industrial kit, maybe there's opportunity to add a fuse block to a panel or DIN rail?)
Tim
Actually, I think that is exactly what happened. This is on a small benchtop centrifuge used in a biological lab, so there is always stuff that gets spilled migrates into crevices and it ended up gumming up the solenoid for the latches. We cleaned them out with alcohol and relubed them and they move much better. I will take my DE5000 to work tomorrow to see if the solenoid has been damaged (should have a different inductance if shorted). I have to say, most centrifuges I have taken apart have been complete crap inside. This one actually is pretty nicely made. Though they seemed to love those little SMD tantalums a bit too much (none are shorted I already checked). There is no room for a fuse, it basically goes (blue surface mount transformer --> NTC --> Connector --> wire to the Solenoid winding) WIth everything up to the connector directly on the PCB next to each other. The NTC is so close to the connector that it is actually browned the tab on the connector. Surprisingly, this setup worked for almost 10yrs. I guess they spec'd it decently well. I have yet to trace exactly how the switches work that are attached to the mechanism with the solenoid. I ordered a replacement NTC today to see if we can't get it back up and running.
Thanks again for your help!
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Ok, got the parts in and had time to solder it in. It now works perfectly! All of the random errors we were having are gone. So if you have a Beckman Coulter Microfuge 18 and are having issues with either the display going nuts (flashing or strobing) and/or the lid not opening properly the solution is really easy. Take it apart, clean and lube the solenoids, and replace the thermistor. Another device now back in full service instead of the trash pile.
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