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A thermistor just burnt out..........

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Kal_B

Member
I have a very expensive board where a Thermistor just burn to a crisp. As I start hunting for parts I was wondering what the likelihood is that the rest of the board is good.
 

kubeek

Well-Known Member
Most Helpful Member
Looks like an inrush NTC thermistor, so the board will most likely have many more issues.
 

alec_t

Well-Known Member
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Something on the board has shorted/ gone low resistance. Likely candidates are power semiconductors.
 

dr pepper

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Sounds like a power supply board, as alec says could well be a shorted switching tranny, or maybe a shorted rectifier diode or smoothing cap.
A bulb in place of the thermistor might tell you something, wattage depends on the current draw of the supply.
 

Nigel Goodwin

Super Moderator
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There seems little point in making blind guesses, the OP has told us nothing, he needs to at least post a picture of the board to give us some sort of clue as to what it might be, and why it might have failed.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Well, someone on AAC just replaced one (in-rush current limiter) on a 12 V Fridge and he just reported all is well.

Thermister means different things to different people. It could mean a temperature sensor or an in-rush current limiter or a meat thermometer.
If you can't cross the part, you need to know a bit about the steady state current and the size of the part to guess what a good replacement would be.
 

Flyback

Well-Known Member
thermistor often at offline SMPS input.....they often undersize them , and due to heating , they weaken, become too high resistance even when hot, and just fail eventually. Just replace it and see...use eye protectors. Ensure correct fuse. The MOV might have gone and that possibly blew the thermistor when it died...so replace the MOV too.....the thermistor might have actually acted as the fuse when the mov died, if the fuse say was over-rated.

Also, with any offline smps, may as well replace the big electrolytics if you want.
 

Kal_B

Member
Thanks guys. Here's a picture of the entire board. It is driving and a 2.5 KW motor and gear box on a rolling door. My understanding is that this is the second time the board goes bad but I don't know what happened the first time. I understand they suspect that the power supply on the board is too small but I am doubtful of that assessment.

The board must have a variable speed drive incorporated in it to drive the motor and control its speed. The door is not balanced and weighs a lot so on its way up it simply heavy and I hope the motor and drive are sized appropriately but on the way down the door pick up quite a bit of speed and my concern is that it doesn't have a braking resistor at least not that I can see.

When the board burnt I installed a controller and 2.2KW VFD because it was the biggest I could get a hold of quickly and thought if I set the drive at low Hz I would have no problem driving a 2.5 KW motor and to my surprise it ran the door at 50Hz going up with no problem but going down I got an overload fault at 15Hz. The door start well and until it picks up speed and bang it tripped.

Thank a lot for all your help and feed back.

Kal
 

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dr pepper

Well-Known Member
Most Helpful Member
I think I recognize that, a klimate door by any chance?
The drive will use an igbt module, big square thing on the back of the board, all rectifiers and igbt's are encased in a monolithic module.
The motor most probably has a worm drive, this will take out any pulling force on the motor so regen shouldnt be a problem.
15hz is a very low frequency for an induction motor, the stator probably saturated and caused an overcurrent.
 

Kal_B

Member
Thanks dr pepper, the gear doesn't take out the pulling force. It's a rubber door and weighs a ton and as it rolls down and it picks up a lot of speed. It's similar to a driving a crane.
The door is not Klimate door but they may very well be using the same board.

Kal
 
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