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UK toilet extractor fan
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james_s:

--- Quote from: floobydust on August 17, 2021, 07:36:33 pm ---In North America there have been bathroom fan fires, they are fed by the branch circuit 15A breaker, and locked-rotor causes them to overheat, so I would welcome any small fuse addition. I think newer fan motors have internal TCO but not older ones made for many decades.
It was not the hot chili and burritos that fueled the fire.

--- End quote ---

I've seen the result of it. Those little shaded pole motors run hot under the best of conditions, I don't think a locked rotor greatly increases the current draw of one but it does remove the cooling airflow. The fact that it is normally full of dust bunnies adds additional thermal insulation and fuel and then it sits there and cooks until it catches fire. I'm not sure a fuse would do much good actually, but a thermal fuse in the motor would certainly be a good idea. I would assume the modern ones have one but I have not looked.
Zero999:

--- Quote from: Gyro on August 19, 2021, 06:48:17 pm ---
--- Quote from: Zero999 on August 19, 2021, 06:18:15 pm ---That's true. It does seem odd that the fan requires a 3A fuse, yet the timer is designed to connected to a standard 6A lighting circuit. If you're really paranoid, you could put the light on a 3A spur.

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They're normally combined units these days (ie. timed [Edit: run-on] fans), with the fuse protecting both the fan motor and the timer circuit. 3A does seem high considering the contents - normally a resistor dropper, shunt zener regulation, CD4xxx IC and a TO92 Triac a few passives and a preset pot, but 3A is the lowest commonly available value so I guess that's the reason.

Most (I can't say all) fans just call for a fuse on the permanent line - otherwise you would be fusing the light fitting too (not that it would be a bad thing though).
--- End quote ---
When I said the timer being connected to a 6A lighting circuit, I was talking about the timer input to the light switch.


--- Quote ---I recently had to repair one of our fans, the under-rated electrolytic across 15V CD40xx logic supply had gone short (not leaked). The mains dropper resistor was still merrily dropping mains with a few % additional dissipation. After a decade or so of use, the phenolic PCB and plastic casing around and above it was a little browned, but no fuse on earth would really stop it melting down if it wanted to. The switched line isn't much of a risk because it goes straight to a decent body size high value series resistor to be clamped and rectified as a logic input.
--- End quote ---

I haven't wired up one of those fans before though. I did install one in my parents' house, but it was on a pull cord, rather than a timer. I'd be surprised if they still use CMOS logic ICs, rather than a microcontroller these days.
Gyro:
I don't think these things change much. A micro would require reasonable regulation rather than a simple 12-15V zener. It must also directly drive a mains Triac - a few more volts helps you get to a safer value gate resistor. The 40xx IC is nothing more than a cheap impedance converter / Schmitt trigger. The run-on time is set by simple electrolytic R/C network with preset pot. Easier than a dip switch, strange light switch on-off sequence etc. They normally come preset to the standard 20 mins anyway. It's a tried and tested reliable solution (even using a resistive rather than capacitive dropper helps there too, no self-healing failures). It would be hard to justify a micro on the basis of a BOM cost reduction here. The PCB must be big enough to hold the mains terminals too, so no board area reduction.

Example...

Zero999:

--- Quote from: Gyro on August 20, 2021, 09:57:38 am ---I don't think these things change much. A micro would require reasonable regulation rather than a simple 12-15V zener. It must also directly drive a mains Triac - a few more volts helps you get to a safer value gate resistor. The 40xx IC is nothing more than a cheap impedance converter / Schmitt trigger. The run-on time is set by simple electrolytic R/C network with preset pot. Easier than a dip switch, strange light switch on-off sequence etc. They normally come preset to the standard 20 mins anyway. It's a tried and tested reliable solution (even using a resistive rather than capacitive dropper helps there too, no self-healing failures). It would be hard to justify a micro on the basis of a BOM cost reduction here. The PCB must be big enough to hold the mains terminals too, so no board area reduction.

Example...



--- End quote ---
That's a 14 pin DIP. I wonder if it's the CD4541B.

I suppose it's not surprising it's still done the old fashioned way: no software or programming to worry about.
Gyro:
No I think it's a CD4093 if memory serves, no counting happening, just the RC time constant. Switched line charges it up via a resistor, diode, and clamp and then it slowly discharges when the light is turned off. There are 2 electrolytics, one for the supply and one for the RC. A couple of diodes, zeners and resistors, a dropper resistor and TO92 triac and you're done. You're always going to be lumbered by through hole stuff anyway.

Yes, there's a lot to be said for not being dependent on a single silicon manufacturer (particularly right now) and mask / flash or whatever. It's all generic commodity stuff.

P.S. The dropper only needs to supply enough current for the 4093 quiescent and the triac trigger current, plus a bit of safety margin.
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