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/

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

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.

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

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

wrote:

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.

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 :-)

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

--
Andy

Andy Wade wrote:
wrote:

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.

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 :-)

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

if its 2 core cable the L shuold be vanishingly small, as i is flowing
in opposite directions in each core. How do you get your 0,9uH figure?


NT

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

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.

--
Andy

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