Bathroom Fan not switching off with LED Lamp - Page 1
Forum section: electronics / projects

author: paul_g_787
author_rank: Frequent Contributor
timestamp: December 24, 2020, 07:19:47 pm
content: Hi Everyone.In our house we have the typical bathroom light with an extractor fan that stays on for say, 5 minutes or so after the light goes off. (UK 240V 50Hz).However these do not work with LED lamps and the fan stays on indefinitely, unless I turn off the fan isolator switch above the bathroom door!Whereas with a traditional tungsten lamp it works just fine!!The fan is a Newlec NL850T - Made in UK approx. 1995.After messaging Big Clive from YouTube, I tried his suggestion with no lamp in the light fitting and the problem is still present!He suggested "the switch cable may be coupling enough current capacitively to make the module think the light is on when there's not a tungsten lamp to decisively shunt it to neutral".So today I took the fan down and wired an old light switch and a pendant up on the kitchen table the same as in the bathroom, but of course the wires in this situation are nice and short so have a lot less capacitance between them. And it works perfectly fine like this with the tungsten lamp, LED lamp and no lamp!!So I believe Clive's theory is correct!I have drawn the schematic of the fan circuit and attached it below.What does everyone think? And is there a way to adapt the fan to work with LED lamps?

author: Gyro
author_rank: Super Contributor
timestamp: December 24, 2020, 07:51:18 pm
content: First observation, that 27k resistor needs swapping (preferably for a flameproof type) before it becomes less than a 27k resistor or the board smokes. [Edit: it must be pretty much on its 2W dissipation limit too!].Capacitive couplingcouldbe the problem, but then again, so could resistive leakage. Bathrooms tend to be humid places and light pull switches can degrade. Humidity also causes leakage in dusty fittings. Ideally you would disconnect the light switch cable at the ceiling rose and perform a high voltage insulation test on it.In terms of a practical solution, after checking, and if necessary, cleaning the switch and the ceiling rose / junction box. I would try turning down the 4M7 on-time pot. By the time you get that down to zero, it must surely turn off - if the switch wiring is capable of putting nearly 1mA into the S/W live input (260k at that the minimum time setting) then youdohave a problem.I think it's just a case that you need to set the on-time pot for a shorter time setting than you would with a filament lamp.P.S. Cleaning that fan board could help too, there could be leakage across the dusty surface.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: December 24, 2020, 08:15:12 pm
content: Yes I did notice the resistor and it is cracked the same on all 3 fans. Just age I guess! I intend to replace it on all 3 if I can get the fan working properly. How can I tell it is a 2W one?I do not believe the problem is resistive leakage. We have the same issue in all 3 rooms, bathroom, master en-suite and downstairs toilet which we do not use very often and as such does not have a high humidity like, say the main bathroom where everyone showers. Everything is looking good to me. All connections are great, no corrosion and everything is clean inside (even before I cleaned it anyway). It would not hurt to do an insulation resistance test but I do not have the equipment for this.I have tried fiddling with the timer resistor position on all 3 fans already and regardless of where this is set including fully up or down the fan still stays on indefinitely with LED or no lamp. Only working as expected with tungsten lamps.When I was testing with the pots in the fully down position, the fans ran for over 72 hours before I gave up and turned them off at the isolator. Again, same behaviour on all three bathrooms. So I am strongly suspecting capacitive coupling as the most likely cause of the issue.This issue has been there since we bought our first LED lamp in 2006 and it would not work properly as above, hence we have always used the old style, which I would just continue to use, but I cannot get these any-more.Also, I have cleaned the board thoroughly with isopropyl alcohol since I took the photo. That was taken immediately after I removed it from the fan so it was a bit dirty!!

author: Gyro
author_rank: Super Contributor
timestamp: December 24, 2020, 08:57:32 pm
content: I'm working on the basis that they have been dissipating nearly 2W in small plastic unventilated enclosures for the past 25 years. It they are lower rated, then it's a miracle!I would rate any replacement at 3W [Edit: Preferable fusible], on long leads.I am surprised that they do not turn off, regardless of the pot position. Analysing a but further, with the pot at the minimum position, to keep the inputs of the IC energised with a 16V supply, it needs to be developing about 10V across R3, which at 130k is about 77uA (ok, not 1mA, but that's what the S/W live input is going to be pulling, with the rest of the current going through D4).The characteristic (not problem) of LED lamps is that they will not start to pull current until the voltage gets close to the accumulated LED forward voltage drop (simplified). You would probably see something similar with CFLs, only a tungsten filament lamp acts as a traditional pull-down resistor all the way to zero volts.The problemcouldbe fixed by fitting a suitably rated resistor between the S/W live input and Neutral (before D2 so that it wouldn't affect the timing). Unfortunately there are twomajorproblems with doing this...1. You would further increase the heat level inside what is already an oven of a PCB enclosure.2. Modifying such a product would invalidate your house insurance.I think you need to accept that the fans are 25 years old. They have given decent service but deserve a dignified burial. The PCBs are delaminating and brown, which will lead to increased leakage, they used flammable parts and didn't include any fault protection. In short, a bit of a fire risk. At 2W standby dissipation, they don't meet modern standards and their design isn't compatible with modern lighting (without unauthorised modification, as above). If you are really set on keeping them, then maybe look for an external fan controller (designed for non-timer fans)I'm still leaning towards resistive leakage, only because it would take a few nF of capacitance to cause the same effect (and presumably on three different wiring runs), but I could easily be wrong.

author: themadhippy
author_rank: Super Contributor
timestamp: December 24, 2020, 09:59:57 pm
content: Quoteinstead of a resistor try a neon indicator or an X2 0.1uf capacitor

author: Gyro
author_rank: Super Contributor
timestamp: December 24, 2020, 10:06:18 pm
content: Yes, that might work. The cap more likely than the neon, which wouldn't conduct below about 60V (once lit). It could maybe be fitted in the back of the fan isolator.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 24, 2020, 10:06:59 pm
content: Perhaps increase the value of R2 until you find a value that barely triggers it at 80% of the nominal mains voltage? (Make a voltage divider for testing if you don't have a variac and preferably power the whole setup from an isolated AC supply for testing, such as an inverter connected to a computer power supply.)Quote from: themadhippy on December 24, 2020, 09:59:57 pmNeon indicator won't work since it takes 60-70V for one to start conducting, the capacitor might be a good idea.

author: themadhippy
author_rank: Super Contributor
timestamp: December 24, 2020, 10:14:40 pm
content: Quotemaybe not in 110V land but they work a treat here in 240v world curing capacitive bleed on lighting circuits

author: amyk
author_rank: Super Contributor
timestamp: December 24, 2020, 10:42:10 pm
content: D3 and D4 are 12V zener diodes, not regular ones. They provide a 12V supply to power the rest of the circuitry.You could replace the dropper resistor with a suitable value of capacitor instead, which should eliminate the heat dissipation; or alternatively, increase its value - the triac needs 3mA to switch on, and the 40106 draws practically no current, but the design as-is will result in a constant current supply of over 8mA.1M between SW_LIVE and NEUT should stop any stray currents; additional heat dissipation will be 50mW which is negligible.

author: IanB
author_rank: Super Contributor
timestamp: December 24, 2020, 10:44:16 pm
content: Quote from: paul_g_787 on December 24, 2020, 08:15:12 pmNot only cracked, but look at the heat damage on the circuit board around it!If you put the mains across a 27k resistor it will dissipate over 2 W (240^2 / 27000). It should be rated over 2 W to have a margin of safety, and should be stood off from the board to improve heat dissipation, not flush with the board as in the picture.A dropper capacitor as just mentioned by amyk would have a much lower heat dissipation.

author: Ian.M
author_rank: Super Contributor
timestamp: December 24, 2020, 11:13:31 pm
content: However if you convert it to a capacitive dropper, you'll need to add another diode between neutral and the anode of D1, (cathode up), and you'll still need a series resistor albeit a much smaller lower value one to prevent fast mains spikes blowing the diodes.You can get RC snubbers consisting or a resistor and capacitor in the same package, and if you can find a 150nF one rated for 250V AC use, with anything between 100R and 470R internally in series it would make a great capacitive dropper for this application. Another one in parallel to the lamp would probably cure the phantom voltage / not turning off problem.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: December 27, 2020, 03:22:21 pm
content: Quote from: Ian.M on December 24, 2020, 11:13:31 pmThis sounds like an elegant solution.So do you mean like this (see attached schamatic)?I cannot find a 150nF snubber but I can easily get a cheap pack of 100nF / 100R onesAnd would the 1N4007 be suitable for the diode? I have tons of these.Also do you think it would be a good idea to put a fuse in series with both the live and switched live? What value should be appropriate? I imagine this circuit would not draw much power at all.As correctly pointed out, there is no fault protection and the lighting circuits are not RCD protected due to the age of the house.

author: Ian.M
author_rank: Super Contributor
timestamp: December 27, 2020, 04:33:00 pm
content: The revised schematic looks reasonable. If a single 100nF snubber doesn't provide enough 'juice', try two in parallel. Worst case, you'll need to get a 150nF or 220nF class X capacitor and use a separate resistor.For the extra diode, even a 1N4001 will do. It never 'sees' more than 16.7V in reverse. You could *probably* even get away with a 1N4148, but there is a small risk of failure if your mains supply is particularly noisy.The unswitched Line feed (not 'Live' as that's *both* Line and Neutral) needs to be fused, but as an isolator is required anyway to meet code you might as well use a switched FCU fitted with a 3A fuse to feed the fan + run-on circuit *and* the light, so both the switched and unswitched Line are fused and isolatable. If you can't re-do the light wiring the fan will need an isolator with enough poles to break both the switched and unswiutched Line, (three pole ones, also breaking Neutral) are suitable) with a separate unswitched FCU.

author: Gyro
author_rank: Super Contributor
timestamp: December 27, 2020, 09:58:16 pm
content: Quote from: paul_g_787 on December 27, 2020, 03:22:21 pmIt's possible to fuse at least the fan live using the existing mains isolator single back box. Toolstation stock the Click modular switch system. You need (one of each)...https://www.toolstation.com/click-mode-grid-front-plate/p75262https://www.toolstation.com/click-mode-grid-module/p93193https://www.toolstation.com/click-mode-grid-module/p49473Fitted with a 3A fuse or lower (if you can find a genuine BS1362 1A fuse).Fans are supposed to be fused down to their manufacturers' nstructions anyway under the wiring regs but this often overlooked. It is at least an improvement on the 6A lighting breaker - or 5A rewireable fuse if is an old house.For the S/W live, there's not much you can do with a fuse - probably best to put the 130k resistor first (probably split into a couple of 70k resistors in series to improve the breakdown voltage) and carefully insulated. Worst case, if one of the resistors went short, and the other end touched neutral, the remaining resistor would dissipate 430mW, so a pair of 1W flameproof resistors would be appropriate.The 100R resistors should be fusible too - X caps will only reliably self-heal if there is sufficient current available (as in the intended across the mains application).As I previously mentioned, you are dicing with your house insurance somewhat as this is a fixed item, sitting in the ceiling or loft space.

author: themadhippy
author_rank: Super Contributor
timestamp: December 27, 2020, 10:23:06 pm
content: Quotewith the move to more energy efficient lighting i wouldn't be surprised to see 2A or 3A mcd's starting to appear on domestic lighting circuits,even with the maximum 10 outlets on a circuit theirs still enough current to get the equivalent brightness of the old 100w incandescent lamps

author: Gyro
author_rank: Super Contributor
timestamp: December 27, 2020, 10:52:35 pm
content: Agreed. Large incandescent loads and 700W radiant bathroom heaters hanging off the lighting circuit are pretty much a thing of the past. At the same time though, the breaker is there to protect the downstream cabling and it's probably not practical for sparks to wire a house with anything more fragile than 1mm2, so I don't know. 6A is a suitable rating for that cable size.Two lighting circuits in a two story house (12A total) is massive overkill, but at the same time, do you want all your lights going out at the same time if a breaker trips?

author: CaptDon
author_rank: Super Contributor
timestamp: December 28, 2020, 08:02:36 pm
content: The filament resistance was a pull-down for the leakage pathto de-activate the circuit. Lamp not in socket, no pull down, no turnoff, no shit!!! LED unit in socket, still no low resistance pull downand no time out. Circuit was never designed to be compatiblewith LED/CFL lamps. I had one similar to this and about every20 seconds the CFL lamp would flash as the leakage current chargedthe capacitor inside the CFL until it reached a proper 'turn-on' voltage,but as soon as he lamp tried to turn on it would go back off becausethe leakage current couldn't sustain the CFL 'on' current. I'm afraidyou are stuck with using the fixture as it was designed with a tungstenlamp, or just upgrade to something modern.

author: paul_g_787
author_rank: Frequent Contributor
timestamp: December 31, 2020, 03:44:08 pm
content: Quote from: Ian.M on December 27, 2020, 04:33:00 pmThere is already a 3-pole isolator switch in place for all the fans. I will check to make sure that both the Live and Switched Live are fused in the isolator. If not I will fit a couple of fuses to the circuit to be safe as the only protection on the lighting circuit is the 6A circuit breaker (old type without RCD).As for the two new resistors. What would the power rating need to be? Would these be lower than the original 2W? I have plenty of 1/2W and 1/4W ones.