Issue with hot air gun soldering iron's thermistor

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Issue with hot air gun soldering iron's thermistor

Postby SemiconductorCat » Thu Sep 11, 2014 11:18 pm

Hi all, I think my soldering iron thermistor is gone. I need a way to verify it.
I have quickly reviewed this reference design here.
Image

And the pinouts here.
Image


When I put the ohm meter between TH1 and TH2 pins it shows open circuit. What I don't know is the default
value of the thermistor. Any idea ?

I need to know it , because I'm going to invest to buy new filament unit and it's higher in cost.
I think I need to replace both filament and and solder it back. The thermistor assembly is so tide and complicated
to home hack in.

Is anyway to know precisely what's the default ohmic value of that thermistor ?


As I guess the issue is when the thermistor  have infinite resistance [open loop] it will not let the feedback to let it
switch on the traic there.

Asking for suggestions , and also need to know places to buy that item.
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Re: Issue with hot air gun soldering iron's thermistor

Postby SemiconductorCat » Thu Sep 11, 2014 11:27 pm

SemiconductorCat wrote:Hi all, I think my soldering iron thermistor is gone. I need a way to verify it.
I have quickly reviewed this reference design here.
Image

And the pinouts here.
Image


When I put the ohm meter between TH1 and TH2 pins it shows open circuit. What I don't know is the default
value of the thermistor. Any idea ?

I need to know it , because I'm going to invest to buy new filament unit and it's higher in cost.
I think I need to replace both filament and and solder it back. The thermistor assembly is so tide and complicated
to home hack in.

Is anyway to know precisely what's the default ohmic value of that thermistor ?
Please review and add suggestions to my calculation on that loop.

where  if the thermistor resistance is 'x', then

Image

Is that simple to calculate like that using the reference schematic ? Will that value should be there and work ?
How to find graphs or particle values on that thermistor ?


As I guess the issue is when the thermistor  have infinite resistance [open loop] it will not let the feedback to let it
switch on the traic there.

Asking for suggestions , and also need to know places to buy that item.
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Re: Issue with hot air gun soldering iron's thermistor

Postby Neo » Sun Sep 14, 2014 9:38 pm

If the thermistor is open circuit, that means it's gone. There should be a value that varies over heat. And when it is open circuit either your iron continuously heat up or most probably doesn't work at all. You will need to disassemble it carefully and find the part number. Then you can give a try on eBay to order the replacement part. May be you can try Pettah too.
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