Resistor hot in bathroom extract

[JohnW2]

It gets more complicated.

I'm not sure I would necessarily say that, since most of what you go on to say is very logical.

Given that two capacitors in series ( with no other components ) will have the same amount of charge ( current x time ) then the smaller capacitor will have a higher voltage ( V = charge / capacity ) than the larger capacitor

Exactly. That's why/how the smaller capacitor is being used as a 'dropper'.

But with 1 Watt LED lamps with a full wave rectifier between the capacitors the circuit becomes a charge pump. The charge that passes through the 0.47 uF capacitor passes into the 10 uF capacitor. The rectifier bridge prevents that charge leaving the 10 uF when the AC reverses. Instead when the AC reverses the 10 uF capacitor gets another 0.47 uF worth of charge thus increasing the voltage on the 10 uF capacitor. Eventually the voltage on the 10 uF capacitor becomes high enough for the LED elements to light and also act as a pseudo Zener diode.

Again, exactly. You're really just describing the function of the 10 uF 'smoothing capacitor.

Do you therefore agree that, with the rectifier diode changed to a bridge, and with the resistor replaced by a ('fairly low value') capacitor, that the fan timer module would work satisfactorily?

These LED lamps are rated at 230 V but two in series ( 115 V each ) appear be as bright as they are on 230 V.

I suppose that's not all that surprising given that, as you say, the LED elements are essentially acting like a zener, despite the fact then you then have two 0.47 uF capacitors in series in the current path. However, if you started reducing the value of those 0.47 uF capacitors (or, as I do for dimming, add a further capacitor in series with it all) I would expect that you would fairly soon reach the stage at which the LEDs did start getting dimmer.

Kind Regards, John

 

[DetlefSchmitz]

This seems to work. About 5mA from the mains. You must have the cap before the bridge else there is no changing of current direction to resist the flow.

 

[RandomGrinch]

Could I have a discharge resistor over the cap as well please?
I wouldn't want any nasty zaps!

 

[DetlefSchmitz]

Well, yes. I would want to spend a lot more time checking things before production!

 

[JohnW2]

This seems to work.

Thanks - so, as I expected.

About 5mA from the mains.

Not far off what I would predict/guess. With around 215V across a 0.1 μF capacitor (reactance/impedance = 31.8 kΩ at 50 Hz) it would be about 7.2 mA.

You must have the cap before the bridge else there is no changing of current direction to resist the flow.

Well, yes, given the big 'smoothing capacitor, everything 'after the bridge' is essentially DC, and one wouldn't expect that to get through a capacitor.

So, if I understand correctly, we now all seem to be agreed that my original suggestion could be made to work in the timer modules (just by changing diode to bridge and changing resistor to capacitor) - so, as I asked in the very first place, why don't they use a 'capacitive dropper' instead of the heat-producing (and 'energy wasting') resistor - it surely can't be just the cost difference between diode and a little bridge, can it?

Kind Regards, John

 

[JohnW2]

Could I have a discharge resistor over the cap as well please?
I wouldn't want any nasty zaps!

You may

Kind Regards, John

 

[DetlefSchmitz]

why don't they use a 'capacitive dropper' instead of the heat-producing (and 'energy wasting') resistor

I imagine that after the vacation student left, the simplest thing was to use and reuse his/her design. Other manufacturers would simply have copied it to save hiring someone.

 

[JohnW2]

I imagine that after the vacation student left, the simplest thing was to use and reuse his/her design. Other manufacturers would simply have copied it to save hiring someone.

I wish it were not the case, but I strongly suspect that the answer may, indeed, be some variation on that theme.

"This is the first approach that anyone thought of, so it's how we've always done it in the ensuing decades" is a unbiquitous phenomenon in very many walks of life (from medicine downwards) - and, as you say, exacerbated by others (who might have come up with 'better ideas') simply copying 'the original'.

I think that vacuum cleaners are a good case in point. Dyson didn't know anything which wasn't known about a century earlier, but he did have the ability to close his eyes to what was 'established'' and actually do some thinking!

In some disciplines (again, medicine being a good example), there has been a progressive move to 'reviewing' many of the 'long established 'beliefs and practices', as the discipline moves closer to being 'evidence-based' - but there are an awful lot of instances of that not happening!

Kind Regards, John

 

[JohnW2]

View attachment 312536
The capacitor can be charged once and only once during the first positive cycle of the supply, it cannot lose that charge as the diode prevents any current from leaving the capacitor.

.... But with 1 Watt LED lamps with a full wave rectifier between the capacitors the circuit becomes a charge pump.

@bernardgreen in passing, I was interested to see the different ways in which you and I think/talk about these situations.

Perhaps more like a physicist than an engineer, you seem to think in terms of charging and discharging of capacitors, whilst I tend to think of 'circuits' containing components which have various impedances.

As well as obviously working for DC circuits with all the components being resistive, my 'way of thinking' also works fine when there are capacitive/inductive components, provided that the current flowing is AC (as per my initial 'ignoring rectification' descriptions and calculations).

What I initially failed to take into account was the fact that a "simple unbranching circuit" (i.e. one with all components in series) is a simple unbranching circuit, such that the current has to be exactly the same at every point in that circuit. Given that (in my initial discussion), one of the components was a diode, that meant that the current in every part of the circuit (including on the supply-side of the diode) had to be DC - and I clearly would not have expected DC to go through a capacitor. As Detlef's 'extended simulation' showed, all that then happens is that one gets an initial charging of the capacitor, after which no further current flows.

Changing from a diode to a bridge rectifier obviously changes that, since it is no longer a 'simple unbranching circuit', so the current upstream of the rectifier remains as AC, and hence can pass through (and be controlled by the impedance of) the capacitor, even though the current downstream of the dropping capacitor is essentially DC.

I'm not sure it would be right to say that this is a case of 'learning something new every day', since I suppose that already 'knew' it, but I was certainly 'reminded' of its relevance by what you wrote - so thanks!

Kind Regards, John

 

[bernardgreen]

The physics master at school taught us that electricity was electrons flowing from Negative to Positive and not "holes" flowing from Positive to Negative. ( "holes" being atoms missing an electron ). Electrons were also very unfriendly things and would do all they could to get away from other electrons.

That made more sense than the commonly accepted analogy of electricity flowing like water.

 

[ebee]

Analogies can be a very good way for us to imagine things but we must always bear in mind they may only be helpful in a certain way and by no means totally accurate and often have the ability to mislead us.

In my own case I, sort of, invented an analogy I suppose.
Regarding power factor. I have found that even some electricians etc tend not to grasp it, some even make some big mistakes.
Years ago I too struggled with the concept of power factor.
There is a big flaw in my analogy, it is totally wrong. However it does help me to think about it even though it is wrong.
I consider the total power or current or voltage needed to be a certain amount that say a battery might need to deliver to the load.
Then I imagine that the load sends power/voltage current/back to the battery.
So the entire system of battery and cables needs to be capable of sending this high amount to the load.
But the load only really gets the lower amount (what it receives less what it sends back)
That way I start to understand the concept of Apparent Power, Real Power and therefore start to appreciate Reactive Power.
Obviously it is totally wrong, not least because I am thinking in DC and this subject only concerns AC.
However if you think in DC then think of quickly disconnecting the battery then connecting again in the opposite polarity I suppose it makes it a little bit more acceptable.
Anyway, all of those years ago, it helped me and also some I mentioned it to.
So I think that analogies do have a place if used wisely.
Thinking of Voltage as a water tank raised higher than the pond it is filling is another good one. etc etc etc

When I went to school we were taught about Centrifugal Force.
Nowadays we are told that Centrifugal Force does not exist.
But the calculations we used to make about Centrifugal Force still hold us in good stead.
I wonder if we can think of those as an analogy?

 

[bernardgreen]

Centrifugal Force

My head spins when thinking about centrifugal force and centripetal force.

 

[ebee]

My head spins when thinking about centrifugal force and centripetal force.

Oh Bernard lad - I did like that one, it must rank upon the likes of "I`d give my right arm to be ambidextrous and thank god I`m an atheist

 

[JohnW2]

The physics master at school taught us that electricity was electrons flowing from Negative to Positive and not "holes" flowing from Positive to Negative. ( "holes" being atoms missing an electron ).

I suppose that 'holes' and electrons are just two ways of looking at the same thing - in some senses similar to the way in which one can talki of the amount of water in a glass by either describing 'how full' or 'how empty' it is.

One of the important things to grasp is that the speed with which electrons (or 'holes') 'come out of the other end of a conductor' is very much faster than the speed with which individual electrons/'holes' actually move.

I recall that, back in the 60s, when I started reading and learning about semiconductors, virtually all the talk was of 'movement of holes', and I had some difficulty in getting my head around that!

That made more sense than the commonly accepted analogy of electricity flowing like water.

I must say that my experience is that most people seem to be very much helped by the 'hydraulic' analogies - at least, for DC circuits. However, they will often find it much more difficult to get their heads around AC, and I can't think of any particularly good analogies for that - other than to just consider a series of instantaneous points in time, in which case the AC effectively 'becomes DC'.

Kind Regards, John

 

[JohnW2]

[ deleted - pressed the button prematurely ]