Thermal Fuse Testing - need best way to TEST.

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MattBuzzBurbank

Joined Nov 14, 2019
1
I repair home automatic coffee machines mechanical as well as electrical issues - .Just simpler basic stuff that can be diagnosed with a multimeter. I do not do any kind of board repairs.
Ok so I work on some coffee machines that have dual boilers and each takes 1-2 thermal fuses. Up to four.
I need to occasionally TEST these when the machine throws a specific error code indicating failure of a thermal fuse, however I am in a quandary as to how to BEST test them.
Removing them is easy, but this requires unbolting it, sliding off the teflon tube cover and then cutting it out of the circuit (and then redoing it if its good) and I really don’t want to do this
(Here is a photo of a 1 per boiler thermal fuse setup.)
As you know the thermal fuse when it fails will have one side hot and the other dead – but how to check in place without removal and jumpering / bypassing to test? Can this be reliably done?
What is the BEST and fastest way to check this?
 

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Doode

Joined Nov 15, 2019
6
I am very amateur at this, but when my cake pop maker broke down, I used multimeter set to ohms to find the issue. I bought a new fuse and it works. I have a few spares and will test continuity on the leads... Like any fuse, no continuity=no good?
 

Reloadron

Joined Jan 15, 2015
4,589
Just measure the resistance across them. The reading should be 0R, or very close to that.
That is what I would do when no power is applied or with power applied I would just measure the voltage drop across the fuse. Either method all you need is a cheap multi-meter.

Ron
 

SLK001

Joined Nov 29, 2011
1,529
The thermal fuse isn't a current type fuse, so the current value written on the fuse isn't a trip current, but is a minimum carry current. The fuse has a compound inside that turns liquid at the temperature set point written on the fuse. When it turns liquidous, it causes the fuse to open. This means that any good thermal fuse should read "0" ohms across it, whether in or out of the circuit (like the others have said). If you get a large resistance across one of these, it has opened and needs to be replaced.
 

narkeleptk

Joined Mar 11, 2019
375
I read once that you could do a very rough testing on a thermal fuse by measuring the resistance at normal temperature and then use a hot air gun to heat it up. The resistance should go up, then after it cools should go back down to 0'ish.
 
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A circuit with "say a switch" or thermal fuse (switch) will generally read the voltage it''s switching when it's open and all other parts of the circuit is included.

e.g. 120, switch, resistance heater. When on, the voltage across the switch would be ~0. When off - line voltage.
That method can be used to check fuses, crimped connections etc. but you have to know what the circuit looks like.
 

dendad

Joined Feb 20, 2016
3,075
I read once that you could do a very rough testing on a thermal fuse by measuring the resistance at normal temperature and then use a hot air gun to heat it up. The resistance should go up, then after it cools should go back down to 0'ish.
I think that will only apply to Polyswitch type re-settable fuse, not the one in question here. These are a one time device.

If you are going to replace the fuse, do not solder leads to it as that may well "blow" it.
 

narkeleptk

Joined Mar 11, 2019
375
I think that will only apply to Polyswitch type re-settable fuse, not the one in question here. These are a one time device.

If you are going to replace the fuse, do not solder leads to it as that may well "blow" it.
You are right, thank you for correction. I was on my phone and posting with out thinking to much while working.
 
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