Electronic Time Delay Switch with Fluorescent Light

[JohnW2]

Hi,

I've just bought what was meant to be a pneumatic time delay/lag switch, but what actually turned up was an electronic equivalent (of almost identical appearance). I'm therefore probably going to return it and request what I actually ordered.

However, it's got me thinking. It's a 'two-wire' (no neutral) switch, which claims to work with incandescent loads up to 5A and fluorescent loads up to 3A, with a note that a capacitor may be required for 'very small loads' such as CFLs and LEDs.

What I want to control is a single fluorescent tube with a conventional 'ballast'. If it would work, the electronic switch has some attractions, since it allows for considerable longer time delays than a pneumatic one. However, I'm wondering how the switch could be engineered to work with such a load. The only thing I can think of is that when one presses the button, the 'switch on' is mechanical (not electronic) and then, once the light is on, the electronics can get some power to run the timer. Does anyone have any experience using such a switch with a (conventional ballast) fluorescent load and/or know if it works as I have suggested.

I might just 'try it' and see if it actually does work!

Kind Regards, John

 

[Echo the husky]

The electronics are charged up by the connected load, when you push the button a latching relay closes and the time starts. After the time us up the latching relay opens again and the load can then charge the switch again. The very low current drawn by the switch will allow the live and switch live connections to appear to the switch as live and neutral to operate the circuit.

The fluorescent fitting should work fine as the ballast, 2 filaments and starter will be a complete circuit when the lamp isn't running. The electronic timers are often more reliable than the pneumatic type, and require less maintenance.

 

[JohnW2]

The fluorescent fitting should work fine as the ballast, 2 filaments and starter will be a complete circuit when the lamp isn't running.

... but I thought that, when the lamp was not running, the starter would be open circuit - and having just got my meter out and checked, that does, indeed, seem to be the case. That's why I ordered a pneumatic one, rather than an electronic one (probably more reliable, and with a wider timer range).

Kind Regards, John

 

[Echo the husky]

Yes you are right! When power is applied an arc appears between the starters open contacts, they heat up the starters bimetallic contacts which then close to complete the circuit to the tubes heaters to warm them up, as the arc is no more the contacts cool and open again. If the tube then lights the current then flows through the tube and not the starter, otherwise the cycle continues.

So it may not work! Try it and see...

 

[JohnW2]

Yes you are right!

I thought I was As I said, that's why I thought I probably needed (and hence ordered) a pneumatic one with such a load!

So it may not work! Try it and see...

Indeed. I need to try to find a way of getting it out of it's packaging without destroying the later - in case it doesn't work and I have to return it! As I said, I can see how it could work if the 'switch on' is mechanical, since once the light is running, there's a source of power for the timer. The button (which looks just like a 'pneumatic' one does appear (through the packaging at the moment!) to have a 'click' action, so it's possible that it is a mechanical 'momentarily on' switch.

Kind Regards, John

 

[Robin0577]

If the light fitting contains a PFC capacitor across the mains, that would probably be enough of a current path to supply the electronics of the switch.

 

[JohnW2]

If the light fitting contains a PFC capacitor across the mains, that would probably be enough of a current path to supply the electronics of the switch.

It probably would, but it's a pretty ancient fitting, so (without looking), I'd put no money on one being present!

My speculation about the possibility of a 'mechanical switch-on' was not correct. On the bench, pressing the button (which certainly does 'click') does not result in any continuity appearing between the L and S/L.

I'll try it on the fluorescent in a bit, and will report back! The supplier has agreed that he will take it back if it doesn't work for me, even if I have 'broken into' the packaging.

Kind Regards, John

 

[JohnW2]

Yes you are right! ... So it may not work! Try it and see...

It works!

It's still not clear why/how it works. When I get a chance I'll look inside the fitting (not too readily accessible!) to see if Robin's idea about a PFC capacitor is right and/or if I can see anything else which could provide a path through the fitting when it's off!

Kind Regards, John

 

[333rocky333]

Id say an ancient fitting is more likely to have one, maybe the chokes leaking a bit to earth or dodgy old wiring

 

[JohnW2]

Id say an ancient fitting is more likely to have one, maybe the chokes leaking a bit to earth or dodgy old wiring

Anything's possible! Getting inside the fitting will take a bit more effort, but I've just IR'd the fitting (with starter removed) and wiring (all the way back to the switch in question {but with the electronic switch disconnected!}), and there clearly is not any DC leakage path. I therefore guess that we really are looking for 'capacitance'!

Kind Regards, John

 

[JohnW2]

Id say an ancient fitting is more likely to have one, maybe the chokes leaking a bit to earth or dodgy old wiring

Yep, a big fat 4μF capacitor - so all explained!

A capacitor of that size would probably be expected to give a fairly low 'first IR reading'. However, because of that possibility, I'm in the habit of 'ignoring' (often not even looking at) the IR reading I get the first time I push the test button. I can but presume that's what I did the other day, since all the readings I 'saw' (presumably after the capacitor had been charged up by the first test) were >200MΩ.

Anyway, what I regarded as a 'mystery' is now solved.

Kind Regards, John

 

[333rocky333]

Getting inside the fitting will take a bit more effort

not too readily accessible!

You just couldnt resist knowing

 

[JohnW2]

Getting inside the fitting will take a bit more effort ... not too readily accessible!

You just couldnt resist knowing

Indeed not! Do you not share in my curiosity?

For those who get excited about 'running costs', I've also just done some measurements. With the light off, this switch is drawing about 9.3mA (through the capacitor!). That's about 2.14W, which equates to around £3 per year. Whether or not it is 'cost effective' is debatable. In order to 'break even' (and ignoring the capital outlay for the switch), the switch would have to result in the light (58W) being on for about 323 hours less per year - and appreciably more 'saved hours' than that to recover the cost of the switch. In my case, given that it has not infrequently been accidentally left on for 'days' (it's in a little-used room), it probably will result in some net saving, but clearly not enough to make me a millionaire

Given the proliferation of this sort of two-wire electronic device (dimmers, switches, timers etc.), if anyone needed reminding, this underlines the importance of properly isolating a circuit before working on it - since merely 'turning off' a functional switch of this sort still leaves a significant shock hazard at the fitting (and switch). A 4μF capacitor has a reactance of around 800Ω at 50Hz, not a million miles away from possible impedance through a human body. There is hence the possibility of someone getting a shock of anything up to around 9mA (certainly 'a few mA') if they found themselves connected between the feed to this 'switched off' fitting. and N or E - unlikely to be fatal (but not impossible), but nevertheless with the potential to result in 'consequential injuries'.

Kind Regards, John