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/ |
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 |
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. |
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 |
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 |
Low Voltage Lighting transformer problem
|
Low Voltage Lighting transformer problem
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 |
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 |
Low Voltage Lighting transformer problem
|
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 |
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 |
Low Voltage Lighting transformer problem
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 |
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 |
Low Voltage Lighting transformer problem
|
Low Voltage Lighting transformer problem
|
Low Voltage Lighting transformer problem
|
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 |
Low Voltage Lighting transformer problem
Andy Wade wrote:
wrote: 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? You'd have to rectify *and* smooth *and* maintain the same RMS voltage into the load. 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. ? |
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 |
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 |
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/ |
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 |
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. |
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 |
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/ |
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 |
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. |
All times are GMT +1. The time now is 12:41 PM. |
Powered by vBulletin® Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004 - 2014 DIYbanter