Low Voltage Lighting transformer problem

In article , Ben Mack writes

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.

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

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.

tronic TFs are high frequency, toroidals are 50Hz.

NT

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

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.

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.

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

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

The data I looked at was for the Intram Barwell models on this page:

I see, he said looking surprised, thanks for the details. But one critical detail isnt mentioned there: 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

This may be horribly simplistic, but as these are 12V Halogens, would a possible approach simply be to rectify the current, so instead of

Sid

It's the self-inductance in the two-wire circuit between the source the load. The current in the wires is in opposite directions - differential mode, if you want to call it that. Common-mode issues are pretty irrelevant here (other than from the POV of assessing EMC).

You'd have to rectify *and* smooth *and* maintain the same RMS voltage into the load.

Skin effect and inductance are two different effects.

At 12V the volt drop across the diodes would be significant. Is there any good reason for using (low voltage) halogens, apart from as a heating device?

In article , snipped-for-privacy@mail.com writes

There's certainly some scope for filtering to smooth out the high frequency components but adding components could upset the stability of the switching transformer. If I had the measurement gear to check the results I'd certainly have a play but I don't at the mo.

Straight rectification would still leave you with choppy dc. Adding smoothing would even out the bumps but now we're in the area of risking destabilising the switcher, they are pretty robust but I couldn't find a spec for how much (say) capacitance they could stand on the output.

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

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

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..

Of course shoving a power factor correction capacitor on the end might assist boosting the output..