Lighting transformers

[Mallard]

I think your diagnoses is right. I had a look inside one of my ' low voltage transformers' (That is what is written on the top) and although I didn't really know what I was looking at it sort of tallies with your description. The interesting thing is that it also says on the transformer SECONDARY 12volts-200 WATTS MAXIMUM. Now these transformers are at least 8 years old although they have never been used. So I checked one or two websites selling these things and they all quote figures like 30-150 watts. So it looks as if modern practise is to qoute a minimum load.

 

I am now in the process of sourcing a 12v 40 watt bulb. Although this is in one sense a big waste of power I have plenty to spare so is not a major problem

[melmerby]

I think your diagnoses is right. I had a look inside one of my ' low voltage transformers' (That is what is written on the top) and although I didn't really know what I was looking at it sort of tallies with your description. The interesting thing is that it also says on the transformer SECONDARY 12volts-200 WATTS MAXIMUM. Now these transformers are at least 8 years old although they have never been used. So I checked one or two websites selling these things and they all quote figures like 30-150 watts. So it looks as if modern practise is to qoute a minimum load.

 

I am now in the process of sourcing a 12v 40 watt bulb. Although this is in one sense a big waste of power I have plenty to spare so is not a major problem

 

Following on from my last post I have been able to investigate further.

I have two of these devices of different makes, they are both labled "Dimmable Electronic Transformer" Both are rated 105VA max with a minimum of 20VA or 30VA.

The output from both is a similar high frequency AC waveform which can only be described as pulsed or modulated up and down. The peak to peak output is about 35V which would tally with a quoted 12v RMS output.

The difference between the two is when the load is removed. On one the output voltage drops to about half and the shape of the waveform changes a little.

On the other the voltage increases about 25% and the frequency and waveform shape change dramatically.

My conclusion is that we should not assume they will work reliably below the minimum load although they may!

 

As an aside I cannot understand why anybody would want 12v halogen spotlights anyway, the light beam is too controlled and the lamps are extremely short lived.

In our previous house there was a light fitting in the kitchen and one day one of the lamps exploded showering hot glass splinters all over the kitchen work surfaces - extremely dangerous. You don't get that with fluorescents!

 

Keith

[older]

Very confusing to a lay person, all i want is to power the lights on my layout as simply as possible.

Any suggestons as to what 12v supply to use.

Thanks,

Richard.

[Brian]

Hi Richard

Unfortunately it not quite that simple!

It depends on whether you're using filament lamps or LEDs or a mix of both?

LEDS will consume roughly 1/8th to 1/6th of the current of a filament lamp.

12 to 16 volt GoW or GoR filament lamps will often be rated at around 60 to 80 milliamp each. You need to add together each lamps current draw to get the total loading on the power supply.

 

This is quite a nice PSU from Maplin that offers various output voltages up to 12 volts at 1.2Amp and its Regulated too. 'Regulated' means the output volts remain the same regardless of the load placed upon it (up to its maximum). Maplin 12 v Regulated PSU

 

Example....It will power 15 x 80ma filament lamps or more lamps if their ma rating is lower. Or about 120 LEDs with a series resistors (1K0 1/4watt would be fine at the 12 volt setting equalling approx. 10ma/LED) or even 12 volt rated LEDs etc.