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Posted to uk.d-i-y
Ben Mack
 
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Default Low Voltage Lighting transformer problem

Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)

However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable

So my question is - do all compact dimmable 12V transformers have high
frequency outputs?

I don't have space for an old-fashioned 50Hz transformer

Cheers
--
Ben Mack
Watchfront Electronics - Bespoke R&D - http://www.watchfront.co.uk/
Watchfront Internet - ADSL, Colo - http://www.watchfront.net/
Are you bricking it? - Firewalls - http://www.firebrick.co.uk/
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fred
 
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Default Low Voltage Lighting transformer problem

In article , Ben Mack
writes
Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)

However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable

Look for another cause to the problem, 100pF/m for the T/E over 3m still
gives 4k impedance at 125kHz so unlikely to be causing your 6V problem.
Look at minimum load, maximum load, wiring faults & faulty transformer.
--
fred
Plusnet - I hope you like vanilla
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Dave Plowman (News)
 
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Default Low Voltage Lighting transformer problem

In article ,
Ben Mack wrote:
I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)


However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable


So my question is - do all compact dimmable 12V transformers have high
frequency outputs?


I don't have space for an old-fashioned 50Hz transformer


TLC have a cable calculator for LV lamps. Dunno if it will help with your
problem.

http://www.tlc-direct.co.uk/Technica...ltageDrop.html

--
*You sound reasonable......time to up my medication

Dave Plowman London SW
To e-mail, change noise into sound.
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Ian Stirling
 
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Default Low Voltage Lighting transformer problem

fred wrote:
In article , Ben Mack
writes
Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)

However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable

Look for another cause to the problem, 100pF/m for the T/E over 3m still
gives 4k impedance at 125kHz so unlikely to be causing your 6V problem.
Look at minimum load, maximum load, wiring faults & faulty transformer.


http://en.wikipedia.com/wiki/skin%20effect
  #5   Report Post  
Posted to uk.d-i-y
 
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Default Low Voltage Lighting transformer problem

Ben Mack wrote:

Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)

However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable


it doesnt at all. Look for a bad connection or too many bulb watts for
the transformer. Bear in mind multimeters may read the 12v wrong if its
not sine wave.


So my question is - do all compact dimmable 12V transformers have high
frequency outputs?

I don't have space for an old-fashioned 50Hz transformer


tronic TFs are high frequency, toroidals are 50Hz.


NT



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fred
 
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Default Low Voltage Lighting transformer problem

In article , Ian
Stirling writes
fred wrote:
In article , Ben Mack
writes
Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)

However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable

Look for another cause to the problem, 100pF/m for the T/E over 3m still
gives 4k impedance at 125kHz so unlikely to be causing your 6V problem.
Look at minimum load, maximum load, wiring faults & faulty transformer.


http://en.wikipedia.com/wiki/skin%20effect


Only 5% increase in impedance on 1.5mm2 @ 125k
--
fred
Plusnet - I hope you like vanilla
  #10   Report Post  
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Andy Wade
 
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Default Low Voltage Lighting transformer problem

fred wrote:

Only 5% increase in impedance on 1.5mm2 @ 125k


Only 5% increase in *resistance* due to skin effect, perhaps, but a huge
increase in *impedance* due to the inductive reactance...

--
Andy


  #11   Report Post  
Posted to uk.d-i-y
fred
 
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Default Low Voltage Lighting transformer problem

In article , Andy Wade
writes
fred wrote:

Only 5% increase in impedance on 1.5mm2 @ 125k


Only 5% increase in *resistance* due to skin effect, perhaps, but a huge
increase in *impedance* due to the inductive reactance...

You're quite right about the inductance, I was just dissing the skin effect
theory.

I've just had a look at a the spec for one of the TLC transformers and it is a
35k switcher which would improve things quite a bit, splitting the load over
a couple of cables would help too.

Switching to adjacent cores of a triple + earth makes only a little difference
due to the Ln (or Log) term in the ratio of radius to spacing, I got a
reduction of only 15% there.

So, replacing the transformers with 35kHz models and splitting the load
into 2 T/E (or similar) feeds should reduce the inductive drop to 0.75V. That
drop is at 90deg phase and lossless so the lamps should actually close to
12V.

The data I looked at was for the Intram Barwell models on this page:
http://www.tlc-direct.co.uk/Main_Ind.../Lighting_Tran
sformers/index.html

As and aside, I'm a bit taken aback that this is an issue at all as this can't
be an uncommon situation and this is the first word of it I've ever seen on
the group, there's always a first time I suppose.
--
fred
Plusnet - I hope you like vanilla
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Posted to uk.d-i-y
 
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Default Low Voltage Lighting transformer problem


fred wrote:

So, replacing the transformers with 35kHz models and splitting the load
into 2 T/E (or similar) feeds should reduce the inductive drop to 0.75V. That
drop is at 90deg phase and lossless so the lamps should actually close to
12V.


As and aside, I'm a bit taken aback that this is an issue at all as this can't
be an uncommon situation and this is the first word of it I've ever seen on
the group, there's always a first time I suppose.


This may be horribly simplistic, but as these are 12V Halogens, would a
possible approach simply be to rectify the current, so instead of
125kHz AC, it becomes DC? Or have I missed the point somewhere?

Sid

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The Natural Philosopher
 
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Default Low Voltage Lighting transformer problem

Ben Mack wrote:
Hi

I've installed some 12V halogen lights in some shelves, such that I have
to mount the transformer remotely. I used 3m of normal 1.5mm^2 lighting
cable to connect the transformer to the lights, having calculated a
300mV drop across the cable (30mV/A/m x 3.3A x 3m)


What makes you think that the lights are only taking 3.3A?


However in practise I'm losing around 6V in the cable, and have
discovered that the transformer output is 125kHz, not 50Hz, explaining
the bad attenuation in the cable


Irrelevant. That's not the problem. I don't know what is, but its not that.



So my question is - do all compact dimmable 12V transformers have high
frequency outputs?

I don't have space for an old-fashioned 50Hz transformer

Straneg..they are not much bigger than teh HF units...however for
dimmable HF is usually better.

Anyway, I did this for 3x50W units, and used similar run of 2.5mm per
lamp starred off a junction box with cooker type cable feeding it.

It's fine..
Cheers

  #18   Report Post  
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The Natural Philosopher
 
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Default Low Voltage Lighting transformer problem

wrote:
Andy Wade wrote:
wrote:

if its 2 core cable the L shuold be vanishingly small, as i is flowing
in opposite directions in each core.

There's a current loop of finite size, so the inductance is finite, not
zero. Whether or not it's vanishingly small depends on the application.
In 50 Hz work we're used to being able to neglect wiring inductance,
but it ceases to be negligible at higher frequencies, or even at 50 Hz
for large cables (50mm^2 upwards, say). In this case the frequency is
over three orders of magnitude above mains frequency.

How do you get your 0,9uH figure?

Any text book on E-M theory /transmission line theory will give you the
following expression for the inductance per unit length of a
parallel-wire line

L = (mu / pi) * ln (s/r) [for s r]

where

mu is the permeability, in this case = mu_0 = 4*pi*10^-7 H/m
(so mu / pi = 0.4 uH/m),
s is the spacing (between centres) of the conductors, and
r is the radius of each conductor.

For 1.5 mm^2 T&E cable the wire diameter is 1.38 mm, so r =~ 0.7 mm.
I guessed s = 6 mm.

Substituting these values, the ln() term evaluates to 2.15, so
L = 0.4 * 2.15 = 0.86 uH/m. QEF.



I see, he said looking surprised, thanks for the details. But one
critical detail isnt mentioned the is this figure the L of both
wires carrying i in same direction, or for counter flow? The two Ls
will be very different.


NT

Of course shoving a power factor correction capacitor on the end might
assist boosting the output..
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Andy Wade
 
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Default Low Voltage Lighting transformer problem

The Natural Philosopher wrote:

I still think Andy should amaze us by calculating the correct capacitor
to put on the end to make it a zero reactive load..

Lets see. 2 ohms at 125 KHZ..

I make that 0.6uF.

So sticking a 0,6uF capacitor across the far end of the cable should
make it all soring to brightness again.

?


Less than that, I reckon. What you need to do as to add C to make the
Zo of the transmission line equal to the load resistance. As any fule
no, Zo = sqrt(L/C), so C = L / Zo^2. In this case Zo needs to be 3.6
ohms (12 V /3.3 A), so C = 0.9E-6/3.6^2, which is about 70 nF/m - so a
220 nF cap should be about right for the 3m run. Might be worth a try,
although, as others have pointed out, the 'transformer' could object to
the reactive load presented at higher harmonic frequencies by such an
arrangement.

--
Andy


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Default Low Voltage Lighting transformer problem

Andy Wade wrote:
The Natural Philosopher wrote:


The cap on the wire sounds like a clever idea.

The rectifier option suffers one basic flaw: the rectifier voltage
drop. At just 1v per diode thats 2v per bridge rec, which is
unacceptable on 12v halogen lights.

I would hesitate to suggest a very small cap after the rectifier to up
the rms, as the psu might well very much object. Maybe - or it might be
ok. The bigger the cap the more the rms v will increase, as it fills in
the lows, so only a very small one would be wanted.


NT

  #22   Report Post  
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Ben Mack
 
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Default Low Voltage Lighting transformer problem

Well thanks all for your most informative replies, most impressed by the
knowledge on this group, particularly Andy's maths!

The problem was due to frequency (see below) and is now sorted :-)

In article , Andy Wade
writes
The problem here is caused by the inductance of the cable, which I
figure will be roughly 0.9 uH per metre. For a 3 m run that's a
reactance of about 2 ohms at 125 kHz, so with the 3 A load current
mentioned the voltage drop will be about 6 V - more or less as observed :-)


Excellent :-)

The only solutions are to get the electronic 'transformer' nearer to the
luminaires, or use a real 50 Hz (iron-cored) transformer.


We found some 50Hz toroids from Maplins that work a treat over the same
wiring, proving the wiring was OK and frequency was the issue. So well
done Andy - your maths was spot on

We had some fun and games finding space for the bigger transformers, but
now all sorted

Regarding the idea of a terminating capacitor, I will have a play with
that when I get a mo, sounds an interesting possibility

Oh, and the reason for 12V lamps - we needed some very shallow
downlighters to fit into milled recesses in the underside of shelves,
and couldn't find any 230V lamps slim enough. And halogens for
aesthetics

In article , Dave Plowman (News)
writes
TLC have a cable calculator for LV lamps. Dunno if it will help with your
problem.

http://www.tlc-direct.co.uk/Technica...ltageDrop.html


Mmmm, seems they made the same mistake I did - they reckon 3m of 1mm^2
T&E will drop 0.4V driving 12V 40W

In article , fred writes
As and aside, I'm a bit taken aback that this is an issue at all as this can't
be an uncommon situation and this is the first word of it I've ever seen on
the group, there's always a first time I suppose.


So am I, I was most surprised!

I guess 125kHz is pretty high, I checked some other transformers that
ran around 30 - 50kHz so less of a problem, though still noticeable. And
sparkies normally seem to fit a transformer by each light fitting,
certainly for ceiling downlighters which I guess is the most common app

Well you live and learn, many thanks all!

Cheers
--
Ben Mack
Watchfront Electronics - Bespoke R&D - http://www.watchfront.co.uk/
Watchfront Internet - ADSL, Colo - http://www.watchfront.net/
Are you bricking it? - Firewalls - http://www.firebrick.co.uk/
  #23   Report Post  
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Andy Wade
 
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Default Low Voltage Lighting transformer problem

Ben Mack wrote:

We found some 50Hz toroids from Maplins that work a treat over the same
wiring, proving the wiring was OK and frequency was the issue.


A very sensible solution I must say, using a transformer rather than a
long-wave transmitter to power your lighting :~)

A further thought is that at such a high frequency (125 kHz) the
inductance of the bulb filaments could also be significant, meaning that
(a) the inductive voltage drop in the wiring is not necessarily in phase
quadrature with the load voltage, thus making its effect worse, and (b)
the voltage across the lamp terminals might need to be significantly
higher than 12V to get the lamp running at the correct power.

--
Andy
  #24   Report Post  
Posted to uk.d-i-y
Dave Plowman (News)
 
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Default Low Voltage Lighting transformer problem

In article ,
Andy Wade wrote:
A very sensible solution I must say, using a transformer rather than a
long-wave transmitter to power your lighting :~)


A further thought is that at such a high frequency (125 kHz) the
inductance of the bulb filaments could also be significant, meaning that
(a) the inductive voltage drop in the wiring is not necessarily in phase
quadrature with the load voltage, thus making its effect worse, and (b)
the voltage across the lamp terminals might need to be significantly
higher than 12V to get the lamp running at the correct power.


It certainly seems strange. Most SMPS are just outside the audio range but
well below RF. About 30 kHz being typical.

--
*I don't have a license to kill, but I do have a learner's permit.

Dave Plowman London SW
To e-mail, change noise into sound.
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Posted to uk.d-i-y
 
Posts: n/a
Default Low Voltage Lighting transformer problem

Ben Mack wrote:
Well thanks all for your most informative replies, most impressed by the
knowledge on this group, particularly Andy's maths!

The problem was due to frequency (see below) and is now sorted :-)

In article , Andy Wade
writes
The problem here is caused by the inductance of the cable, which I
figure will be roughly 0.9 uH per metre. For a 3 m run that's a
reactance of about 2 ohms at 125 kHz, so with the 3 A load current
mentioned the voltage drop will be about 6 V - more or less as observed :-)


Excellent :-)

The only solutions are to get the electronic 'transformer' nearer to the
luminaires, or use a real 50 Hz (iron-cored) transformer.


We found some 50Hz toroids from Maplins that work a treat over the same
wiring, proving the wiring was OK and frequency was the issue. So well
done Andy - your maths was spot on

We had some fun and games finding space for the bigger transformers, but
now all sorted

Regarding the idea of a terminating capacitor, I will have a play with
that when I get a mo, sounds an interesting possibility

Oh, and the reason for 12V lamps - we needed some very shallow
downlighters to fit into milled recesses in the underside of shelves,
and couldn't find any 230V lamps slim enough. And halogens for
aesthetics

In article , Dave Plowman (News)
writes
TLC have a cable calculator for LV lamps. Dunno if it will help with your
problem.

http://www.tlc-direct.co.uk/Technica...ltageDrop.html


Mmmm, seems they made the same mistake I did - they reckon 3m of 1mm^2
T&E will drop 0.4V driving 12V 40W

In article , fred writes
As and aside, I'm a bit taken aback that this is an issue at all as this can't
be an uncommon situation and this is the first word of it I've ever seen on
the group, there's always a first time I suppose.


So am I, I was most surprised!

I guess 125kHz is pretty high, I checked some other transformers that
ran around 30 - 50kHz so less of a problem, though still noticeable. And
sparkies normally seem to fit a transformer by each light fitting,
certainly for ceiling downlighters which I guess is the most common app

Well you live and learn, many thanks all!

Cheers



other shallow lights:
screwfix.com numbers

round:
38701
87529
6000hr bulbs: 21975

strip:
22239
98307
68183
95961
23953
17886
15608

and if youre vision is really going, 10383


NT



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Ben Mack
 
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Default Low Voltage Lighting transformer problem

In article , Dave Plowman (News)
writes
It certainly seems strange. Most SMPS are just outside the audio range but
well below RF. About 30 kHz being typical.


As with everything else in this world, they just keep getting faster.
The faster they run, the smaller the transformer, and speed is mainly
limited by the semiconductors which are getting quicker all the time.

--
Ben Mack
Watchfront Electronics - Bespoke R&D - http://www.watchfront.co.uk/
Watchfront Internet - ADSL, Colo - http://www.watchfront.net/
Are you bricking it? - Firewalls - http://www.firebrick.co.uk/
  #27   Report Post  
Posted to uk.d-i-y
 
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Default Low Voltage Lighting transformer problem

Dave Plowman (News) wrote:
In article ,
Andy Wade wrote:


A very sensible solution I must say, using a transformer rather than a
long-wave transmitter to power your lighting :~)


A further thought is that at such a high frequency (125 kHz) the
inductance of the bulb filaments could also be significant, meaning that
(a) the inductive voltage drop in the wiring is not necessarily in phase
quadrature with the load voltage, thus making its effect worse, and (b)
the voltage across the lamp terminals might need to be significantly
higher than 12V to get the lamp running at the correct power.


It certainly seems strange. Most SMPS are just outside the audio range but
well below RF. About 30 kHz being typical.


cost cutting exercise, higher frequency equals less inductor. Would
work if short leads used. If they marked the transformers clearly as
only for use with say 2' leads they could avoid a lot of returns and
complaints.


NT

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Dave Plowman (News)
 
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Default Low Voltage Lighting transformer problem

In article .com,
wrote:
It certainly seems strange. Most SMPS are just outside the audio range
but well below RF. About 30 kHz being typical.


cost cutting exercise, higher frequency equals less inductor. Would work
if short leads used. If they marked the transformers clearly as only for
use with say 2' leads they could avoid a lot of returns and complaints.


I'd rather not have a device with an output in the RF spectrum unless
really necessary.

--
*Why isn't there mouse-flavoured cat food?

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