Question about a transformer

[carlsbergy]

I want to run 3 x 35w low voltage lights off a transformer. I have purchased a transformer that runs up to 105w. It has 4 x terminals on the outlet. Would I run 2 x lights into two of the terminals and the last light into the other two terminals?

 

[Softus]

I want to run 3 x 35w low voltage lights off a transformer. I have purchased a transformer that runs up to 105w.

In that case you'll be running the transformer at its maximum rated output. Could you not get a unit with more capacity?

 

[securespark]

105's are designed to run either 2 x 50W, 3 x 35W or 5 x 20W.

70's are designed to run 1 x 50W, 2 x 35W or 3 x 20W.

As long as you make sure you keep within the operating band of the tranny (most have a minimum of 20W) you'll be OK.

When you say "4 terminals", do you mean 2 for one side of the lamp & two for the other?

 

[carlsbergy]

Can swap for a larger transformer, no problem. There are the two terminals on the input, and four terminals on the output side in a line

 

[Lectrician]

I would go for a 150VA tranny - These will have 3 pairs of output terminals rather than two pairs. You are also safer should anyone up the lamp wattage to 50watt - People do this all the time in my experience - expecially in shops/pubs etc.

 

[electronicsuk]

I wouldn't bother replacing the transformer myself, it is fine for the loads you have specified. If someone does choose to uprate to 50w lamps then the most likely outcome is that the transformers thermal cut out will go open circuit, or you'll drag the voltage on the secondary side down so much that 50w lamps become dimmer than their 35w counterparts anyway. Make sure you use suitable gauge cable, remember a 35w load @ 12v is nearly 3A, and any voltage drop across the cable will be quite noticeable too.

 

[carlsbergy]

Thanks for your replies, much appreciated, if I do decide to keep the transformer I have, do I connect two of the lights into two terminals of the output and the remaining lamp into the other two terminals of the output?

 

[breezer]

your choice, you could even daisy chain form 1st to 2nd to 3rd

 

[electronicsuk]

Thanks for your replies, much appreciated, if I do decide to keep the transformer I have, do I connect two of the lights into two terminals of the output and the remaining lamp into the other two terminals of the output?

Yes, if you can manage to fit two wires into the same terminal then that will be fine, although in my experience with lighting transformers this can sometimes be difficult. The other option is, of course, a larger terminal block outside the transformer, or to daisy chain as has already been suggested.

Just be careful with the latter as the load on the first and second sections of cable will be high, so you'll need heavy gauge cable to take the load and keep voltage drop to a minimum.

 

[fattony]

remember a 35w load @ 12v is nearly 3A, and any voltage drop across the cable will be quite noticeable too.

hmmm is that strictly true, I understand where you got that figure from but its a resistive load so the calculation would be different from P/V, think about 10 low voltage downlighters in a room would = 30Amp

Need to look into this one a little further, I always talk people into GU10's

 

[electronicsuk]

remember a 35w load @ 12v is nearly 3A, and any voltage drop across the cable will be quite noticeable too.

hmmm is that strictly true, I understand where you got that figure from but its a resistive load so the calculation would be different from P/V, think about 10 low voltage downlighters in a room would = 30Amp

P/V is designed for use in calculations with resistive loads, it's when you have inductive or capacitive loads that start causing leading/lagging and power factors other than 1.0 (or close to, because in reality you're never going to have a P.F. of exactly one) that you have to start changing the calculations you use.

Ten 12v 35w downlighters would be nearly 30A, but only at 12v. Not accounting for transformer losses, they would draw 35 * 10 = 350 / 230 = ~1.5A on the mains side.

 

[fattony]

I see your point, I've never really thought about a 12v light 35w bulb calculating to 3Amp, do the low voltage downloghters cost more to run then the 230v GU10's

 

[electronicsuk]

I see your point, I've never really thought about a 12v light 35w bulb calculating to 3Amp, do the low voltage downloghters cost more to run then the 230v GU10's

Not by a discernible amount, a 35w lamp is a 35w load regardless of the voltage it runs on. There will be more power lost in the cables and transformers in a low voltage lighting system, but it won't amount to a major difference in running costs.

 

[equitum]

Just had a similar scenario today where customer had originally wanted 10 x GU10 halogens in bedroom and I'd put in first fix for these in 1.5 mm T&E on the usual radial serving the 10 luminaires.

Problem was he bought selv instead - 10 x 20W "starlight" luminaires and a single 210W transformer.

Transformer instructions stated no more that 2Metres cable length on the secondary side, which was equiped with four pairs of terminals.

As it was a fairly large room, there was at least 1m of cable between each luminaire position, which meant I couldn't use my installed cabling, and would need to install cables from the primary of the transformer to several more transformers, more labour - more cost, and a sh*hole of a loft with room only for a contortionist to manoeuver around in.

I decided to buy 10 x 20-50W (£60) transformers and use my original wiring.
Only other problem was needed a JB on each position on the 240V side as the transformers had tinned 1mm prewired tails and then conn strip at SELV side for luminaire as again came with tinned tails . Oh and had to wire the whole lot from within aforementioned loft as only 25mm apertures !

Was this the most cost effective method ? Sorry to waffle had a crap day !

 

[carlsbergy]

Thanks for all your advice, went for a 150w transformer with 3 x pairs of outlets - works a treat. Thanks guys