Hi Everyone,

My first time posting with AAC! I was wondering if you can help me figure out what component this is with reference designator "THR1"? Pictures are attached.

The component has two terminals and is connected to the input of a 120V AC adapter. It is connected directly to the rectifier circuit (yellow and blue caps, and inductor). All the current passes through this component. It was a disc shape throughole component but I can't remember the color. The output voltage is 42V DC for battery charging applications.

Could be a thyristor? If so it would have to be a diac or dynistor. Or perhaps this is a thermistor? My guess is this is some sort of surge, in-rush current limit component because when I solder a wire across it and change the fuse it blows right away when plugged in. I also suspected a varistor but why ref des THR1?

Thanks!
Nick

 

Welcome to AAC!

If it is in series with incoming AC power then it is likely a resettable fuse.
Companies such as Polyswitch and Littlefuse make them.
If it is blowing then that is an indication that there is a fault on your gadget. You will have to correct the fault first otherwise you will keep on blowing fuses.

 

Looks like the board may have done some burning. You said when you bridge the component the fuse blows. So I don't think it's a fuse. Would have to see a schematic of how it's wired. Just looking at the pictures makes deciphering difficult, but doable. But I don't have the time.

My first guess was a thyristor too. But it could possibly be a capacitor. Doubtful, but may be possible.

 

Welcome to AAC.

If it is an NTC thermistor, its job would be to limit inrush current as the capacitors get an initial charge. Shorting it would blow the fuse since that is what it is mean to prevent.

 

agreed. based on available clues it is most likely thermistor placed there to prevent high inrush current.
on powerup large filter capacitor capacitor is empty and practically a short circuit. so it need to charged gradually until current is reasonable and thermistor will do just that (must be NTC).

 

Welcome to AAC.

If it is an NTC thermistor, its job would be to limit inrush current as the capacitors get an initial charge. Shorting it would blow the fuse since that is what it is mean to prevent.

agreed. based on available clues it is most likely thermistor placed there to prevent high inrush current.
on powerup large filter capacitor capacitor is empty and practically a short circuit. so it need to charged gradually until current is reasonable and thermistor will do just that (must be NTC).

That makes a lot of sense! Thanks for your input, I'll go ahead and order some NTC thermistors online and give it a try.
Great experience with my first post!


I don't think it's a resettable fuse because it looked disc-shaped when I took it out.

Nick

 

resettable fuse works the other way (PTC = Positive Temperature Coefficient). normally it is low resistance. but when there is a high current, it gets warm and resistance increases and limits that current. it was common in CRT television sets for degaussing of the tube on startup.