Low Voltage Lighting transformer problem

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

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

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

Excellent :-)

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

T&E will drop 0.4V driving 12V 40W

In article , fred writes

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

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.

other shallow lights: screwfix.com numbers

round:

strip:

and if youre vision is really going, 10383

NT

In article , Dave Plowman (News) writes

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.

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

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