Fuse failure question

[Connecteur]

I recently replaced a control board in a pellet stove, because the stove had gone completely dead. The new replacement board made the stove work again, properly, so my hunch turned out to be right.

The part # is SRV7077-050
https://www.amazon.com/dp/B08H448D3Z?psc=1&ref=ppx_yo2ov_dt_b_product_details

On closer inspection of the old board, the fuse had lost continuity, but it appears not to have blown.  Therefore I might presume that the board itself had not failed, but the fuse itself failed.  There appears to be a ball of solder inside the glass tube, which suggest that it failed over time from loss of conductivity and overheating.  The fuse is not the snap-in type, but the board-mount type.  I have ordered a replacement fuse in hopes that the old board can be kept on hand as a standby part in case the newer one fails.

Am I correct in my thinking?

[cruff]

It depends on if the ball of solder is actually the melted fuse wire? Did the solder melt away from the end cap due to heat from the stove itself rather than excessive current caused by another component's failure? If this was on the AC input and it feeds a rectifier, first check to see if any of the diodes in the rectifier have failed short.

[bdunham7]

Am I correct in my thinking?

How do you distinguish between a fuse that has gone open improperly and one that has blown?  I don't think you can conclude that the fuse "failed" without replacing it and seeing how much current the board draws. It is possible that the fuse is marginally rated and just blew after a long period of operation at or near its rating,  or that there is or was an overload or short. 

[Connecteur]

Am I correct in my thinking?

How do you distinguish between a fuse that has gone open improperly and one that has blown?  I don't think you can conclude that the fuse "failed" without replacing it and seeing how much current the board draws. It is possible that the fuse is marginally rated and just blew after a long period of operation at or near its rating,  or that there is or was an overload or short.

In any case, I believe a ball of solder in the middle of a glass fuse is not a blown fuse in the normal sense.  I think perhaps the fuse gradually failed as it heated up, while it was failing.  This leads me to suspect that nothing on the board caused the fuse to fail.
If nothing on the board caused the fuse to blow, it's reasonable to assume that the board might be OK.

Nevertheless, when my replacement fuse arrives, I'll be able to solder the replacement in place and test the board.  Then I'll know if I was right.

[Gyro]

A ball in the middle of the fuse element is an indication of a slow blow / time delay (T) fuse. Fuses can fail due to thermal fatigue over time.

[TimFox]

When  using glass cartridge low-current slow-blow fuses, I usually found that a coil spring exerting tension on a fine wire pulling against a blob of solder-like material.
The fine wire (or sometimes two wires) would blow quickly on a large overload, while the melting solder would blow only after a long overload.
Both cases are "blown":  depends on timing of the external problem.

[floobydust]

Your control board does have a MOV so there could have been a mains transient that caused the fuse to blow. There are 4 outputs, not sure what they are but any one of them could be drawing high current, overloading or pushing the fuse near it's limit. What happens with a pellet feeder jam?
A fuse can blow quite slowly, over many seconds which looks different inside compared to a fast clearing.

[Connecteur]

Your control board does have a MOV so there could have been a mains transient that caused the fuse to blow. There are 4 outputs, not sure what they are but any one of them could be drawing high current, overloading or pushing the fuse near it's limit. What happens with a pellet feeder jam?
A fuse can blow quite slowly, over many seconds which looks different inside compared to a fast clearing.

Auger jams have been frequent.  They originally supplied us with spring steel hairpin clips to use as shear pins, but we have since started using a weaker metal cut from welding rod.  It's reasonable to assume that auger jams have caused temporarily high current draw.  The board is specified to draw 3 amps, and the fuse is rated at 5 amps.

[TimFox]

When your auger jams and the motor stalls, there is no motor back-emf and the stall current is essentially the supply voltage divided by the copper resistance of the motor windings.
What DC resistance do you measure across the motor itself (no applied voltage, source disconnected)?

[Tomorokoshi]

How is the stove started? Electric igniter?

[Connecteur]

How is the stove started? Electric igniter?

Yes, I replaced it for the first time last year.  No idea the draw.   The transformer on the board says: 9VAC 3VA, so it must be switching 115VAC directly for the fans and igniter.

[floobydust]

In the Amazon pics of the replacement board, it is a 5x20mm in a fuseholder with cover.
If you keep having problems with the auger jamming, I would consider adding a circuit breaker for it. Limiting torque mechanically with shear pins is a last resort.

[Connecteur]

In the Amazon pics of the replacement board, it is a 5x20mm in a fuseholder with cover.
If you keep having problems with the auger jamming, I would consider adding a circuit breaker for it. Limiting torque mechanically with shear pins is a last resort.

The original design is using a shear pin.  I don't think I have the engineering skills to redesign it.