I have a couple of light fittings with 4 12v spotlight bulbs (12v 20 watt)
- the sort that slide and swivell along bare metal bars..
I realise that if I reduce the load by fitting LED 7 watt lamps then I have changed the load and that the voltage may rise. The transformers are thise circular ones (toroidal??)
Are these more - or less prone to giving an unregulated voltage? Can I safely replace the 20watt halogens with LEDs - or will they fail? I have tried replaceing one lamp with an Aldi lamp and I like the result. Can I go further?
This brings me up to a similar query. do 12v LEDs have some form of brdge rectifier inside so they can work on AC, or are they effectively half wave rectified devices working on one half cycle?
Toroids are unregulated, and do rise a little in voltage if lightly loaded. LEDs suffer shorter life when overdriven. Whether it's all significant is another matter.
They are almost certainly full wave rectified smoothed and in chains three LEDs long with a small series resistor to limit current (bad design but quite possible) or a switched mode constant current source. The basic current controllers tend to fight with phase angle dimmers drawing an ever larger current to maintain constant light output.
I suspect they are moderately tolerant of mild over voltage +/-15% since such things do occur on the mains. It is possible that a big hefty transformer for many halogen lights might be above their rated maximum voltage when severely under loaded.
The constant current ones will behave with near constant light output irrespective of input voltage whereas the resistive ones their output will increase with increasing voltage until it goes pop.
So you are going from 4 x 20 = 80 W to 4 x 7 = 28 W. Dig out your voltmeter, remove three of the 20 W bulbs and see what the voltage is. You may sacrifice that 20 W bulb...
Maybe it would be safer to start with all four in, see what the voltage is and how it changes as you progessively remove bulbs down to one.
I should think so, that's 20W so (in theory) roughly the same as three 7 W LEDS.
Ah, snagette, are these 12 V LEDs DC or AC? I'd expect the transformer output to be AC, though ISTR you saying you'd tried an LED and it worked with three filament bulbs. Worth checking.
All the GU5.3 LEDs I have bought work off AC or DC. I gave up on them in one set of fittings as the integral PSU caused the integral transformers to buzz at some frequency I can't hear but my daughter can. It was a real sound as it was picked up on my frequency analyser app on my phone.
I have some LED drivers bought from Ebay - for some projects car wise. They appear to be the guts fitted to the base of a 12v bulb by the pins fitted to the PCB.
When used on the car, there is no difference in brightness with the engine running or stopped, so I'd guess are designed to cope with a wide variation of input voltage.
Automotive ones have to be. There are some insane voltage back emf spikes around when the starter motor is being turned over. I am not convinced the ones for domestic use are anything like as rugged.
When we had our shower room revamped some 4 or 5 years ago the single ceiling batten lamp socket was "upgraded" to 4 35W 12v tungsten halogen down-lighters which, at my insistence, were specified as 12v rather than the cheaper 240 volt type.
Each luminary was fed by its own individual 60W max rated smpsu "transformer" rather than having the whole lot fed from a single common transformer rated to supply the total 140W loading.
To date, I've not yet had to replace a single lamp and the reason for this longevity is quite obvious to anyone knowledgeable and observant enough to spot the obvious half second or so warm up delay upon switch on as a direct result of choosing the 12v option over the "cheap 'n' cheerful" 240v one.
The only surprise to me was the choice of smpsu "12v transformers" over the conventional 50Hz ac mains transformers I'd seen used in earlier installations. A choice, I've assumed, that was made on the grounds of 'economy'. Despite the utter simplicity of a 50Hz mains transformer, the cost of the materials alone in a decent quality unit would have far outstripped the total manufacturing costs of a modern 60W 12v SMPSU even
5 years ago.
Not only does a smpsu offer cost savings over the conventional transformer type it also offers better voltage regulation as well as surge current limiting on switch on (a best of *both* worlds feature not possible with a conventional transformer designed to cover a range of lamp wattage loadings). I suppose that smpsu "lighting transformers" are the only type available these days for new 12 and 24 volt lamped lighting installations.
But these ain't automotive. They appear to be from an MR16 lamp by the pins. Spec given as for 2 watt LEDs, which is what I'm using.
The interior lights on the old Rover were originally 6 watt tubular. So useless for actually reading anything. The fitting is plastic, and a larger tungsten melts it.
A couple of Star 2w LEDS per fitting works very well. I've also done the under bonnet lights the same, and the boot one.
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