12V halogen downlight and transformer question

[wilburwilbur]

I would like to install 5 x 12V Halogen lights using a DAIS DA-105 transformer. Each light is 20W. The Transformer states the MAX is 105W/VA. The instructions are not very good, so want some advice.

1. Can the transformer take this load ( 100W )
2. The transformer has 3 outputs for lights and i want to fit 5

3. Can i take one single output from the light to a single Joint Box and connect the 5 lights up at this point. The distance between each light and transformer will be within the maximum stated in the instructions.

4. Everything will be fitted as per the present wiring regulations. I do not intend the lights to be dimmed. Thanks for any concern in advance.

5. I do not like LED lighting and I prefer to use the transformer option in this case.

 

[Scoby_Beasley]

I think you need a safety margin by using max 80% of the transformer output. This means you'll be looking for a 125W/VA+ transformer.

Then again I could be talking out of me harris!!

 

[wilburwilbur]

The instructions are not very good with this transformer, but i could not find anything about safety margins stated.

 

[Taylortwocities]

You do not need a saefty margin.
The transformer can deliver 105VA (watts) your load is 5x20=100 so you are good to go.

And you can take one cable to a JB (accessible) and then 5 separate cables to the 5 lights, or daisy chain them, or wire all 5 back to the transformer.

 

[bernardgreen]

Halogen lamps take a lot more power when switched on. The resistance of the filament when cold is a lot lower than when the filament is white hot.

Hence a 105 watt "transformer" may have to supply 200 watts ( or more ) until the lamp filaments are at normal operating temperature. That might put the "transformer" into overload and shut down before the lamps can reach normal operating temperature.

 

[RF Lighting]

Don't be silly.

Surprisingly halogen transformers are desgned to work with...

Wait for it...

Halogen lamps. Shock horror

 

[Iggifer]

Halogen lamps take a lot more power when switched on. The resistance of the filament when cold is a lot lower than when the filament is white hot.

Hence a 105 watt "transformer" may have to supply 200 watts ( or more ) until the lamp filaments are at normal operating temperature. That might put the "transformer" into overload and shut down before the lamps can reach normal operating temperature.

Sounds more like Metal Halide to me than Halogen, if that was the case manufacturers would de-rate their own transformers accordingly

 

[bernardgreen]

Surprisingly halogen transformers are desgned to work with... Wait for it...Halogen lamps. Shock horror

True and when supplied as a unit, lamp and transformer combined, the manufacturer will ( should ) ensure the transformer ( if it is a switch mode PSU ) can cope with the cold current of the lamp(s).

But when mix and matching lamps and transformers ( PSUs ) from different sources there is no garantee that the transformer will cope with the cold current unless it has a stated short term overload ability that exceeds the start up load of the cold lamp(s).

 

[securespark]

If it is a 105 and has three outputs, it looks like it's been designed to run 3x 35W lamps.

You have to watch your volt drop on the secondary side. Admittedly, with 20W lamps, it's not as horrendous, but I remember doing shopfitting with large toroidals in the early 90's when these things were in their infancy and we were running 6 milli for a few metres....

 

[flameport]

You have 100W of lighting load.
You have a lighting transformer rated to 105W.

Just wire the thing up and be done with it.

 

[securespark]

Bit flippant...

 

[333rocky333]

I remember doing shopfitting with large toroidals in the early 90's when these things were in their infancy and we were running 6 milli for a few metres....

Me too, also boots romford catching fire due to long runs of 1mm flex, however 105 va is electronic and his instructions would state to limit wiring length to about 2 metres so volt drop as you say less of an issue.

However you may find each of the 3 outputs are limited maybe to 50 Va and internally overload monitored, thus scuppering your idea of bunching, you may have to do 2+2+1 or get a single output transformer

 

[winston1]

However you may find each of the 3 outputs are limited maybe to 50 Va and internally overload monitored.

No, it is not like that. There is one output connected to 3 sets of terminals, simple as that.

 

[wilburwilbur]

Thanks for the replies here - good advice. I connected the 5 x 20W Halogen 12V to the transformer (single output from one of the transformers 3 outputs to a JB). The lights DID NOT work. I took one of the bulbs out and tried again, after about 5 seconds or so, the lights came on. I tried again with all 5 in place and again nothing. Then repeated, with only 4 lights in place - took brief time for the lights to come on after switching.

Looks like i will have to get a bigger transformer- or put a single on each light, cannot be doing with waiting 5 seconds for the lights to come on after switching them on !!

I might try to put 2 x 20W on 2 outputs and a single 20W on the remainder - then use another dedicated single transformer for the 5th 20W 12V light. But would i still have this ' lag ' when switching the circuit on ?

 

[foxhole]

I always use a trany for each bulb, gives access to each easily when they need changing or upgrading.
Your trany may be designed to give 3 outputs of 35 watts so inappropriate for your wiring method.