If you just rectify, you dont have to smooth, and the RMS voltage into the load will be the same..although there is still an HF component..of reduced amplitude.
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.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:
38701 87529 6000hr bulbs: 21975strip:
22239 98307 68183 95961 23953 17886 15608and 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.
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