Fixing electric blanket controller. Need to replace a fuse, but trouble finding

[LeWidget]

Hi all,

My electric blanket has four settings. 0(off), 1(low), 2(med), 3(high). Low setting works, but 2 & 3 stopped working. odd. I took it a part, not a lot going on. I removed the components & tested them, they all seem to be working (diodes, resistors, thermal fuse etc..). Looking at the contact pads on the board, it looks like it's worn away, but using MM on continuity it seems fine. Looks like it's still copper. (I have since re-tinned (is that the term) the contacts.)

Switch seems to be functioning ok. I soldered a wire to the live & a wire to the neutral then using alligator clips, jumped the contacts to each setting (low, med, high). Seems to detect doing that (led indicator light comes one, whereas it would only come on using low setting), so I suspect contact between the switch and the pads being the issue. Unfortunately I must have touched something, blew the fuse. 

Looking online, I'm having trouble sourcing one. Closest is Mouser but they don't carry in stock so would be 12week wait (plus post). Most fuses I've come across seem to be the cylinder type that needs to go into a fuse holder, I need one with the axial legs to solder through-hole.

Written on the fuse itself is, "T800mA 250v". Not 100% sure if it's a 'timed' or 'fast blow'. Some things I've read is 'F' means fast, so maybe 'T' means timed? It blew pretty fast so I dunno 

The fuse measures at roughly 4mm x 11mm

Are there alternative fuses I can use in its place if I can't locate one?

Thanks in advance,

[fzabkar]

T = time delay

[Shonky]

T = slow blow, yes

Just find anything say 800mA (maybe 750mA will be easier) rated 250VAC or greater that will fit. Prefer ceramic.

What is the spacing? Perhaps you can fit PCB mount clips instead?

[eurgenca]

F = flink = fast
T = trage = slow

[FIXITNOW2003]

you can buy endcaps to fit standard 20mm fuses then use standard 20mm T800mA fuse if there is room
https://www.mouser.co.uk/ProductDetail/Schurter/13310044?qs=oY64LhD902k5UFq8Ydr2cQ%3D%3D&mgh=1&vip=1&gclid=Cj0KCQjwmICoBhDxARIsABXkXlJCT-iCruzsH6HxC_sFcORpurXb7TGOMGGhFf-mPnvHHZe4ttWK-38aAnyAEALw_wcB

[wasedadoc]

It is not difficult to solder leads to the metal end caps of glass fuses.

[u666sa]

Just purchase a regular, glass, 800mA 250v fuse, and solder some wire to it. It will work just fine.

You can even solder any resistor in place of fuse, cut it off, and insert fuse, and then solder fuse to wires. 

[Swake]

These are in-line ceramic tube fuses. The size is 3.6mm x 10mm.

Like these, unfortunately they don't have 800mA versions and I don't know how much we can trust the ratings of these fuses: https://www.aliexpress.com/item/1005005464493861.html

I can not recommend soldering the wires to a generic glass fuse. You risk changing the properties of the fuse and would only know after your house has burned down.

The fuse in the pictures is a ceramic version. Make sure to replace by the same type.

[TimFox]

Mouser listings:
https://www.mouser.com/c/circuit-protection/fuses/cartridge-fuses/?current%20rating=800%20mA&fuse%20size%20%2F%20group=3.6%20mm%20x%2010%20mm&fuse%20type=Time%20Delay%20%2F%20Slow%20Blow

The first line item (from Schurter) is listed in stock.
The Littelfuse are not stocked.

[LeWidget]

Thanks guys . I'll hed up to the local Jaycar, see what they carry in stock.

T = slow blow, yes

Just find anything say 800mA (maybe 750mA will be easier) rated 250VAC or greater that will fit. Prefer ceramic.

What is the spacing? Perhaps you can fit PCB mount clips instead?

Thanks Shonky. Would there be any potential issue choosing a lower 750mA or higher 850mA fuse instead of sticking to an 800mA ?

In regards to the spacing you asked, you mean distance between each hole for the fuse legs? 17mm I measured.

The first line item (from Schurter) is listed in stock.
The Littelfuse are not stocked.

Thanks for the link . I might have to try and find that part in Mainland Australia. The shipping alone costs as much as the blanket lol.

[edit] What's the difference between fast blow and slow blow ?

[Shonky]

Thanks guys . I'll hed up to the local Jaycar, see what they carry in stock.

T = slow blow, yes

Just find anything say 800mA (maybe 750mA will be easier) rated 250VAC or greater that will fit. Prefer ceramic.

What is the spacing? Perhaps you can fit PCB mount clips instead?

Thanks Shonky. Would there be any potential issue choosing a lower 750mA or higher 850mA fuse instead of sticking to an 800mA ?

In regards to the spacing you asked, you mean distance between each hole for the fuse legs? 17mm I measured.

The first line item (from Schurter) is listed in stock.
The Littelfuse are not stocked.

Thanks for the link . I might have to try and find that part in Mainland Australia. The shipping alone costs as much as the blanket lol.

[edit] What's the difference between fast blow and slow blow ?

Generally if you can't replace a fuse exactly it's better (well safer) to go lower at the risk maybe blowing a bit easier. 50mA up or down from 800mA is really stuff all so I would have no problem using 850mA if that was all you could get. 0.75A/750mA is just more likely to be available.

Slow blow is designed to allow for a small inrush or surge of current beyond its rating at the expense of slightly extending the time before it blows.

Yeah 17mm spacing might allow fitting the inner pins of clips like these https://www.altronics.com.au/p/s5983-m205-pcb-mount-fuse-clip-pk-8/ and then using easy to get M205 fuses. Maybe drill for the "outer" pins for a solid mount.

[fzabkar]

I confess that I don't understand the need for a slow blow fuse in this application. Isn't the blanket just a fixed resistor? It's not like an incandescent bulb with a low cold resistance, or an electric motor, or a large capacitor that needs to be charged.

:-?

[TimFox]

Wire resistors that are designed to be heaters usually have a positive temperature co-efficient and their cold resistance is lower than operating (hot) resistance.

[fzabkar]

Wire resistors that are designed to be heaters usually have a positive temperature co-efficient and their cold resistance is lower than operating (hot) resistance.

Nichrome wire doesn't have much variation in resistance over a wide temperature range.

https://auremo.biz/reference/zavisimost-soprotivleniya-nihroma-ot-temperatury.html

[soldar]

I confess that I don't understand the need for a slow blow fuse in this application. Isn't the blanket just a fixed resistor? It's not like an incandescent bulb with a low cold resistance, or an electric motor, or a large capacitor that needs to be charged.

There can be line transients. I believe the thing to do is to put a slow fuse unless there is a reason that requires a fast blowing fuse.

Nichrome wire doesn't have much variation in resistance over a wide temperature range.

About three years ago I repaired several of these electric blankets and also an electric back and neck warmer which I found out was electrically exactly the same. They are sold by LIDL here. One of mine stopped working and when I mentioned it other people gave me theirs to repair. But they are so cheap they are not really worth repairing. Only for the learning experience.

Everything I am saying now is from memory and there could be mistakes.

I never could understand or figure out how the controller thing worked and they are what failed so I just replaced it with dimmer controls. I tried to contact Big Clive of Youtube to see if he would be interested in dissecting the controllers because this is the kind of thing he does but I was never able to contact him. If someone knows how to contact him please let me know.

I do seem to remember the resistance did go up with temperature so this would provide a level of auto regulation. When the blanket or pad is cold it would use more power and as it heated up it would use less power. I figured it was not nichrome but some other alloy.

What I found interesting is that the cold resistance was always about 200 ohm, in the blankets and in the heating pad. This is too low to connect directly to 230V but can be connected to 127V for about 80W. I found this interesting. I do not know if the reason is that they have the same blanket for all markets but then they need to change the controller. I would have thought they would use a blanket which could be connected to 230V and then trim down power from there with the controller.

I built a controller using old SCR so that it has three positions that gave, say, 90, 60, 30W. Something I put together on perfboard, quick and dirty, and it is still working fine years later.

But I am very curious as to deciphering how the original control with the diodes works. It might explain why the resistance is set the way it is.

[fzabkar]

For me, the most intriguing aspect of the design is the 6-leaded white component at the bottom right corner. After examining PCBs from other electric blankets, this device appears to consist of two parallel power resistors sandwiching a thermal fuse.

https://www.diychatroom.com/attachments/1665173509791-png.714510/
http://i1241.photobucket.com/albums/gg504/reynierl/cb.jpg
https://www.glodark.com/images3/Blanket_control.jpg
https://allanfaulds.weebly.com/uploads/2/1/2/7/21271896/6098376.jpg?497

https://img.directindustry.com/images_di/photo-mg/160622-17846652.jpg
https://www.kwxcom.com/frw-fusibleresistor.html

[mikerj]

For me, the most intriguing aspect of the design is the 6-leaded white component at the bottom right corner. After examining PCBs from other electric blankets, this device appears to consist of two parallel power resistors sandwiching a thermal fuse.

This scheme provides similar protection to a domestic circuit breaker, though non-resettable.  The fuse provides protection for gross overloads and the thermal fuse + resistors provides protection against smaller sustained overloads (e.g. partially shorted heating element) that may not draw enough current to blow the fuse in a reasonable time.  An MCB includes both a magnetic and a thermal trip to cover the same situations.

[TimFox]

Wire resistors that are designed to be heaters usually have a positive temperature co-efficient and their cold resistance is lower than operating (hot) resistance.

Nichrome wire doesn't have much variation in resistance over a wide temperature range.

https://auremo.biz/reference/zavisimost-soprotivleniya-nihroma-ot-temperatury.html

Some electric blankets and heating pads use PTC materials in their design.
See this patent:  https://patents.google.com/patent/US5770836A/en