LED indicator for 240V power

[David]

Just starting to work on the extractor hood fan that failed over Christmas.

It is a LAM2502/3 as detailed here

There is a 5 button control panel at the front of the hood.

Light push on/push off (works) Cancel Fan (mechanical) Fan 1 Fan 2 Fan 3 LED to indicate fan is on. (although you can hear it)

Each fan switch mechanically cancels whatever other fan switch was pushed in.

The symptoms are:

Light works

Fan does not work on any setting but LED comes on when any of the 3 buttons are pushed.

Suggestions so far have been the fan or the control panel. There were no obvious graunching noises from the fan to suggest impending failure.

My specific query is this; if the LED comes on does that mean there is

240V coming out of the control panel after the switch?

I ask this because if the control panel is working then I can save a degree of messing about taking the panel out and poking it with a meter.

As far as I can see so far there is a little circuit board with the LED on, and 3 wires, 2 black and 1 white, all of which go back up the wiring harness towards the motor and lights.

There are 3 wires black,red, purple which go into the three fan switches

There is an orange wire which goes into the light switch.

There is a light blue wire which disappears beneath these other wires.

My initial assumption (without stripping down further) is that the blue goes to some kind of common power rail and the other wires are the returns to the lights and the fan.

Given that the LED connections go back up the wiring harness I am guessing that they are connected to the motor.

If all this is correct then it looks as though the problem may be with the motor; just hoping to check my assumptions before getting further covered in old grease.

Cheers

Dave R

[David]

Noting that I've located the Wiki and am working my way through it,

I did want to check one thing.

It says that with a neon indicator on a light switch (find in the dark) it is wired between live and switched live and so is only lit when the switch is off.

I assume that this means that with current taking the path of least resistance, when the switch is on the current flows through the lighting circuit and not through the higher resistance of the neon.

All well and good until I consider an application where the neon comes on when the circuit is live. I assume there is a different resistance in the neon to allow it to light in parallel with the main circuit.

Is this correct?

Cheers

Dave R

[T i m]

Erm, with the switch open the neon uses the load to illuminate itself and when the switch is closed the neon is shorted out? I'm not sure if that confirms what you were suggesting or not. ;-)

Not sure? The only time the neon would illuminate with the switch on was if the switch was going high resistance?

Cheers, T i m

[Roger Hayter]

snip

A light that comes on when the switch is on requires a neutral wire, and is wired between neutral and switched live. Not a problem in your appliance, but can be a nuisance to supply at a light switch.

[David]

Yes, but do you see my point?

In a light switch the neon light only comes on when the circuit is broken (because it is bridging the switch but has high resistance so not much current flows). Allegedly it goes off once the circuit is made - that is, is is still in the circuit but not the preferred route for those pesky electrons.

The opposite occurs with switched power - that is it is bridged between switched live and neutral and only comes on when power is present; however the logic behind this seems the opposite of that above.

With the light, the neon goes out when live is connected to neutral (which is what happens with live and switched live - the connection through the light bulb ends up at neutral).

With the power switch, the light comes on when live is connected to neutral - through both the neon and the appliance. Suppose the power switch connected to a lamp; you would have the same load.

Is it because one is in series and the other is in parallel?

So long since I did any electrical theory.

Cheers

Dave R

[Harry Bloomfield]

It will be in parallel with the motor, what will be varied by the switches, is the selection of motor windings for the fan speeds.

Because of the very basic questions you are asking, I would suggest you leave it alone and just buy a new unit. The most likely fault is that the thermal fuse has gone o/c, buried deep inside the motor windings, which cannot be repaired. New units can be had for £55.

[T i m]

Ok.

By a factor of millions. The neon is in parallel with the switch that is in series with the load. With the switch open the load appears as a short circuit to the neon and with the load connected to neutral, gives you a full neon brightness.

With the switch on the neon is shorted out and so there is *no* voltage across it and so it doesn't light. It really couldn't be any easier and there is really no 'path of least resistance' discussion to be had (as it's not really one of those instances)?

The concept is exactly the same, it's just where the neon is located that is different.

If live was ever connected to neutral you would trip the fuse / MCB.

I doubt it (see above). ;-)

I think you meant 'when the live is connected to switched live there.

Yes, so as long as there is sufficient load to pass enough current to excite the neon then the neon will illuminate (and that's very little).

No, it's where the neon is located in the circuit.

It seems like it. ;-)

Imagine a fuse connected to a 12V supply (car battery) and then to say a lamp. The fuse has an LED and resistor in series across it. When the fuse is good the led is effectively shorted out by the fuse so it will

*never* illuminate. When the fuse blows and there is still a load (say a 21W lamp), then the 12V will then appear directly across the (blown) fuse and the LED will light (indicating which fuse has blown).

That's the exact same process as your neon and switch.

Cheers, T i m

[Roger Hayter]

Firstly, the neon always has a high resistance in series with it as part of the provided neon and leads assembly, so it only draws a very small current compared with what current the switched load (motor or light) would require to operate it.[1] If the neon is connecte across the open switch then current from the live terminal flows via the neon through the load (say, light bulb) and as the resistance of the light bulb is relatively low compared to the neon assembly most of the mains voltage is across the neon assembly. The very low neon current has little effect on the light bulb, producing a negligible voltage acress it (but see [1]). When the switch is closed the voltage is all across the load and the neon assembly is short circuited and has no voltage across it because of the negligible resistance of the closed switch.

When the neon is in parallel with the load connected from the switched live to neutral then it only glows when the switch is closed. Then the mains voltage is across the load and the neon assembly in parallel. The load draws its usual current, and the neon draws the very low current it needs to glow, because of its series resistance. When the switch is open there is no voltage on the switched live and its voltage falls to the same level as neutral. So neither the load nor the neon have voltage across them. lLive is never connected to neutral except via the load (or the neon, but that carries very little current).

[1] In real life the current through the neon may impart an eery glow to CFLs when they are supposed to be off.

[Brian Gaff]

Well, I'd have thought the obvious next step was to see if 240 v is present where you expect it. I would have thought if they are switching mains motors, they might well be using some kind of controller or set of relays, and that is where I'd expect issues. On the other hand it is entirely possible that the fans might be not running on mains at all, they might be being powered by some kind of power supply module. I think one needs to find out what the technology is if one can first then see if voltages that one would expect are present. After all the leds are probably fed from something low voltage in the first place. Brian

[Brian Gaff]

So are they neeons or leds? Normally neons can be run through a resistor to the mains to stop cooking the neeon. They work at lower voltages. Indee they are often used as voltage stabilisers as they strike at a certain voltage.

Brian