Can anyone explain why this would occur

[Questforknowledge]

Good evening all

I'm training to become and electrician and last week I came across an issue when we were attending to some outside lights.

Basically the scenario was there was two outside lightsne was a small upright bulkhead fitting with an inbuilt photocell and pir this brought itself and the other fitting (which was a basic drum fitting) on. There was a flickering issue with the lights and when the lamp was removed from the drum fitting (non sensor light), the bulkhead flickered but when the bulb was placed back in the flickering would stop.

I have no clue why this would happen and due to the nature of getting jobs done I have never really received a definitive answer.

Would be very grateful if anyone could shed some light on this.

Regards harry

 

[Deleted member 105166]

Have you checked all terminations for tightness?

 

[Questforknowledge]

Have you checked all terminations for tightness?

Thanks. For the response it was actually a job a colleague went on with and so I didn't have much input. I understand how a loose connection could cause flickering but just can't fathom why when the bulb was inserted into one light fitting it stopped the flickering on another

 

[Cid]

I think it's something to do with disturbing the ballast/transformer thing

 

[Phil Thompson]

Where they led bulbs?

 

[Questforknowledge]

Where they led bulbs?

Yes they were led

 

[Lucien Nunes]

The PIR unit probably uses a triac to switch the lamps rather than a relay. Triacs have the advantage of not wearing out, but have a minimum load requirement below which their output is unstable. The lamp in the PIR fitting was probably too low a wattage to ensure reliable operation of the triac on its own, but the total load of the two lamps was sufficient for it to fire reliably.

It's the same constraint as the minimum load requirement on most small dimmers. Historically, pretty much any filament lamp was enough to keep the triac happy (minimum loads were often 25 or 40W depending on the dimmer) but most LEDs fall below this.

 

[Questforknowledge]

The PIR unit probably uses a triac to switch the lamps rather than a relay. Triacs have the advantage of not wearing out, but have a minimum load requirement below which their output is unstable. The lamp in the PIR fitting was probably too low a wattage to ensure reliable operation of the triac on its own, but the total load of the two lamps was sufficient for it to fire reliably.

Thanks for the response. It's a bit over my head but I understand the basics of what your saying and has definitely given me some clarity.

 

[Lucien Nunes]

Relays and triacs are the two most commonly used devices to switch a 230V AC circuit and both are used in PIR sensors. The relay is an electromechanically operated switch, that has two completely separate circuits through it; the coil (control circuit) and the contacts (load circuit.) When the coil is energised by the control circuit, in this case the PIR, the contacts close just like an ordinary switch and make a solid connection from permanent live to lamp. So long as it is not overloaded, the relay doesn't care how much current the load takes, right down to zero, it will still switch on because the coil and contacts are independent.

The triac is an electronic component made of silicon, a kind of tough, AC-compatible transistor. Unlike the relay, the control and load circuits are not electrically separate, and its switching behaviour depends on both of them. The control input can turn it on, but it won't stay on unless a certain minimum current flows through the load. This means that it will unavoidably turn off each time the AC current waveform passes through zero, as it does 100 times per second on 50Hz. The control circuit is designed to trigger it 100 times per second, on every half-cycle of the mains waveform, so that it is effectively on all the time. However, if the load is below the minimum requirement, it will tend to turn-off part-way through the half cycle or may not trigger at all. This is what causes the flickering - sometimes it triggers, sometimes it doesn't.

 

[Shoei]

Good evening all

I'm training to become and electrician and last week I came across an issue when we were attending to some outside lights.

Basically the scenario was there was two outside lightsne was a small upright bulkhead fitting with an inbuilt photocell and pir this brought itself and the other fitting (which was a basic drum fitting) on. There was a flickering issue with the lights and when the lamp was removed from the drum fitting (non sensor light), the bulkhead flickered but when the bulb was placed back in the flickering would stop.

I have no clue why this would happen and due to the nature of getting jobs done I have never really received a definitive answer.

Would be very grateful if anyone could shed some light on this.

Regards harry

Just to be clear, did the flickering happen when the lights were meant to be off or was it a problem only when the pir was turning them on? Flickering in LEDs that are nominally off is often due to capacitive leakage in cables, adding another load in parallel can make it stop.

 

[Lucien Nunes]

Ah, yes, good point, the OP doesn't make it clear. If the flickering occurred when the lamps were off, then my explanation doesn't apply in this case.