Cheap transformer?

[Joseph_Pestell]

I have been pulling apart a bar that has halogen lamps.

 

Each lamp has it's own transformer rated at 11.6V and maximum 4.9A. The label does not say whether AC or DC but I am assuming the former. On the face of it, great for running the point motors, accessories, etc on a layout. Anyone tried this?

 

Halogens usually replaced these days as so energy-inefficient. So there should be lots of these transformers about.

[KeithMacdonald]

1 hour ago, Joseph_Pestell said:

I have been pulling apart a bar that has halogen lamps. ... The label does not say whether AC or DC but I am assuming the former.

 

Have you considered the idea of putting a multimeter across the light to see whether it is AC or DC?

[WIMorrison]

Transformers only work with AC ...

[KeithMacdonald]

Just now, WIMorrison said:

Transformers only work with AC ...

 

Correct. And very often coupled with rectifiers to output DC.

[newbryford]

Just now, WIMorrison said:

Transformers only work with AC ...

 

Problem is many people still call any low voltage supply - AC or DC - "transformers".

 

I've lost count of the number of people that I know,  say that they have a 12vDC transformer.........................

[WIMorrison]

1 hour ago, KeithMacdonald said:

 

Correct. And very often coupled with rectifiers to output DC.

That’s not a transformer then, plus you missed out the capacitor that normally goes with a rectifier - making the circuit even less like a transformer which can still only work with AC.

 

[KeithMacdonald]

Looks like the Rivet Police have caught up with us again...

[WIMorrison]

Hmm, all you had to say was that you were wrong by suggesting you can get DC out of a transformer ...

[KeithMacdonald]

20 minutes ago, WIMorrison said:

Hmm, all you had to say was that you were wrong by suggesting you can get DC out of a transformer ...

 

You're perfectly welcome to be a pedant. You just have to be accurate and able to quote correctly.

[Joseph_Pestell]

11 hours ago, KeithMacdonald said:

 

Have you considered the idea of putting a multimeter across the light to see whether it is AC or DC?

 

Does one not risk damaging the meter by putting the "wrong" current across it?

 

Anyway, there lies a sad story. I own several multimeters. But the wife is being difficult (understatement) about me taking any tools from the marital home.

 

Today's first d-i-y job is to restore power to the central heating pump. I had not allowed for someone having wired it into the kitchen ring main that I took out on Thursday. I would always have the heating pump on its own supply.

[Joseph_Pestell]

9 hours ago, WIMorrison said:

Hmm, all you had to say was that you were wrong by suggesting you can get DC out of a transformer …

 

You can get DC out of a transformer if you put DC in.

 

In this case, I know that we have 230V AC going in but no indication if the exit current is AC or rectified to DC, What would a halogen lamp usually work on? I will look later to see if there is any indication on the bulbs.

[sharris]

Halogen lights are happy to work off either - e.g.  car halogens will be running on dc, domestic lighting halogens will be on ac.

 

A transformer for a halogen lamp is unlikely to have a rectifier on the output - why would the manufacturer go to the effort of including unnecessary components?

[Crosland]

Lighting "transformers" are often more than just a transformer, especially if described as an "electronic transformer". A picture is worth a thousand words and would aid identification...

 

2 hours ago, Joseph_Pestell said:

 

You can get DC out of a transformer if you put DC in.

 

Being another pedant, no you can't. What you are describing might be commonly referred to as a transformer but it is a whole lot more complicated. It's one of the reasons our mains distribution system is AC and not DC.

Edited December 21, 2019 by Crosland

[Nearholmer]

Strange thread!

 

Im pretty sure that JP is trying to bring a helpful suggestion to the table.

 

Yes, he got the terminology and physics wrong, but that doesn’t detract from it being a potentially good idea: reuse a redundant lamp power supply for model railway purposes.

 

Just make sure first that you know whether it is a power supply that provides ac, or dc.

[Terra-Nova2]

I usually take the simple way out and read the input output ratings marked on the item. For people like me you need your best glasses and maybe even a magnifying glass. The ac or dc bit is usually in symbol .ie sine symbol ac, and parallel lines dc. It’s quite rare for this not to be there.

[JohnGi]

There are a lot of these on ebay at the moment. Search for "used halogen transformer". Those that I saw are all labelled 11.6V 4.9A AC.

[grriff]

I'm a little concerned. I'm assuming the input to the transformer is mains with an output of 11.6v. As has been said above, a transformer only works because it uses an alternating current. If you don't understand that, you shouldn't be messing with mains electricity. Sorry to be so brutal but lives are literally at stake here.

[WIMorrison]

3 hours ago, Joseph_Pestell said:

 

You can get DC out of a transformer if you put DC in.

 

In this case, I know that we have 230V AC going in but no indication if the exit current is AC or rectified to DC, What would a halogen lamp usually work on? I will look later to see if there is any indication on the bulbs.

 

I suggest that you do some reading on the fundamental properties of electricity, then you might realise how totally incorrect and impossible your statement is.

[Joseph_Pestell]

2 hours ago, Nearholmer said:

Strange thread!

 

Im pretty sure that JP is trying to bring a helpful suggestion to the table.

 

Yes, he got the terminology and physics wrong, but that doesn’t detract from it being a potentially good idea: reuse a redundant lamp power supply for model railway purposes.

 

Just make sure first that you know whether it is a power supply that provides ac, or dc.

 

Thank you for that positive reply. I do get tired of so much negativity on here.

 

My O level physics was a long time ago but I don't see why a transformer with a DC input could not produce a DC output. I will look it up later.

 

Anyway, back to the main idea which is to provide a cheap source of transformers for railway modellers having been somewhat shocked at the prices proposed by some well-known trade suppliers. It would seem that the idea is valid. As I recall, most standalone controllers are for 12v or 16v AC supply. 4.9A gives plenty of grunt.

 

These lighting units are quite compact and I am a bit surprised that they don't get too hot.

[Nearholmer]

A transformer consists of two coils of wire, usually would onto an iron core.

 

Current flowing in one coil produces a magnetic field, and while that current is changing (not while it is constant) the magnetic field induces a current to flow in the second coil.

 

If you connect direct current to the first coil, current will flow for the very tiniest moment (long complicated explanation available) In the second coil, then stop.
 

If you connect alternating current to the first coil, alternating current will begin and continue to flow in the second coil, because the current in the first coil is always changing.
 

 

Edited December 21, 2019 by Nearholmer

[Dave John]

Hang on. The "transformers" used for halogen lighting are very often PWM switched mode power supples and that rating of 11.6 V at 4.9 A assumes that it is connected to a 50 W resistive load. Even then it is usually a PWM output equivalent to 11. 6 V , not true  DC. There is usually a bit that says Load "20 W to 50W" on them. 

 

If you connect them to a load of less than 20 W they don't generate any output at all, or worse they put out a string of higher voltage spikes until a resistive load is connected to the output. Very nasty if the load is inductive , such as a point motor. 

 

My advice, bin them before you damage the model railway stuff. Yes there are loads about, I have removed and scrapped hundreds of them. If you are replacing the lamps with leds then use 240 V Gu10 lamps or MR 11 12V Leds  through a proper led driver. 

 

 

[sharris]

I opened up an Ikea halogen reading lamp a while ago when it gave up (the sort with a pair of telescopic arms between the base and the lamp) - the base unit just had a standard iron cored wound transformer. 

 

The transformer served two purposes, bringing mains down to 12V ac for the bulb and giving the base a bit of mass to keep the lamp from tipping over.

[Hamburger]

37 minutes ago, Dave John said:

If you connect them to a load of less than 20 W they don't generate any output at all, or worse they put out a string of higher voltage spikes until a resistive load is connected to the output.

Absolutely agree.

On a cheap chinaware 12V module I measured spikes up to 36 volts.

[sharris]

Does it look like this?

 

The output says AC on this and similar units.

 

However, it does look from the outside more like a SMPS than a linear transformer, so YMMV as to the quality of the AC output.

 

[kevinlms]

1 hour ago, Joseph_Pestell said:

 

Thank you for that positive reply. I do get tired of so much negativity on here.

 

My O level physics was a long time ago but I don't see why a transformer with a DC input could not produce a DC output. I will look it up later.

 

Anyway, back to the main idea which is to provide a cheap source of transformers for railway modellers having been somewhat shocked at the prices proposed by some well-known trade suppliers. It would seem that the idea is valid. As I recall, most standalone controllers are for 12v or 16v AC supply. 4.9A gives plenty of grunt.

 

These lighting units are quite compact and I am a bit surprised that they don't get too hot.

What you need to know is that these transformers are designed for a specific purpose - that of providing a power source for quartz halogen lights and so they are optimised for that. This means that they are designed to put out a constant supply for 50 Watt lights.

 

You could use them for other functions, with a similar load characteristics, but they would be unsuitable for loads significantly less than 50 Watts, because as others have stated, the voltage will float much higher.

 

Sorry, but it is not 'being negative' to inform you why they won't work. Nor should it be considered as wrong to tell you that no transformer will work on DC. It's a fact of physics.