Excuse a question from a LED novice.

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?

Brian

--
From the Sofa of Brian Gaff Reply address is active
Remember, if you don't like where I post
or what I say, you don't have to
read my posts! :-)
"DerbyBorn" wrote in message
2.236...
Excuse a question from a LED novice.

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?

On Monday, 16 November 2015 17:19:12 UTC, DerbyBorn wrote:
Excuse a question from a LED novice.

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?

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.


NT

On 16/11/2015 17:43, Brian-Gaff wrote:

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?

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.

--
Regards,
Martin Brown

"DerbyBorn" wrote in message
2.236...
Excuse a question from a LED novice.

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?


Any reason you cannot swap the transformer for an LED driver?

--
Adam

On Mon, 16 Nov 2015 17:13:46 GMT, DerbyBorn wrote:

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??)

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.

--
Cheers
Dave.

On 16/11/15 17:13, DerbyBorn wrote:
Excuse a question from a LED novice.

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?

good regulation is a function of good magnetics. Of course it depends
how much they skimped on iron.

less iron = cheaper price, more copper= more price.
Its a balance thing.





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?

Probably, but can you get 12V LED units?


--
the biggest threat to humanity comes from socialism, which has utterly
diverted our attention away from what really matters to our existential
survival, to indulging in navel gazing and faux moral investigations
into what the world ought to be, whilst we fail utterly to deal with
what it actually is.

"ARW" wrote in
:

"DerbyBorn" wrote in message
2.236...
Excuse a question from a LED novice.

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?


Any reason you cannot swap the transformer for an LED driver?


Depends on the responses. It would be a case of seeing what would fit
into the space.

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.


Sounds a godd idea - thanks.

10.9 volts with 4
12.1 with one

I am guessing that 4 LEDs woud be okay?

On Monday, 16 November 2015 23:21:38 UTC, DerbyBorn wrote:
10.9 volts with 4
12.1 with one

I am guessing that 4 LEDs woud be okay?

no problem at all

NT

On Mon, 16 Nov 2015 23:21:34 GMT, DerbyBorn wrote:

10.9 volts with 4
12.1 with one

I am guessing that 4 LEDs woud be okay?

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.

--
Cheers
Dave.

On 17/11/2015 08:55, Dave Liquorice wrote:
On Mon, 16 Nov 2015 23:21:34 GMT, DerbyBorn wrote:

10.9 volts with 4
12.1 with one

I am guessing that 4 LEDs woud be okay?

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.

In article ,
Martin Brown wrote:
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 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.

--
*Change is inevitable, except from a vending machine.

Dave Plowman London SW
To e-mail, change noise into sound.

On 17/11/2015 11:11, Dave Plowman (News) wrote:
In article ,
Martin Brown wrote:
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 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.

--
Regards,
Martin Brown

On Mon, 16 Nov 2015 22:44:45 +0000, The Natural Philosopher wrote:

On 16/11/15 17:13, DerbyBorn wrote:
Excuse a question from a LED novice.

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?

good regulation is a function of good magnetics. Of course it depends
how much they skimped on iron.

less iron = cheaper price, more copper= more price.
Its a balance thing.


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.

--
Johnny B Good

I have just had my meter on another fitting. The highest reading I get is
7.6 volts and it seems to reduce as I remove lamps. With no load I am
getting 0.2 volts

In article ,
Martin Brown wrote:
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.

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.

--
*And don't start a sentence with a conjunction *

Dave Plowman London SW
To e-mail, change noise into sound.

On 17/11/2015 10:28, dennis@home wrote:


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.

You should publish that. It might be a good way to get the kids to move
out...

Andy