Another mr 16 led transformer question

[aveatry]

I am looking to replace some 12v 20w mr16 bulbs in my kitchen with 5w led ones
from what I have read I think I have a wire wound one so I should be ok ? it is very heavy. The transformer is running 3 x 20w mr16s in the ceiling now.
These are the ones that I am looking at getting http://www.screwfix.com/p/lap-mr16-led-lamp-gu5-3-346lm-5w-cool-white-pack-of-5/2454g
But another question is - are the screwfix ones AC or DC or both, the spec only says "truefit Replacement for 35W Halogen", on one of the Q&A someone has replied with this
I wasn't sure but tested one across a 11.1v lipo battery and it worked connected in either polarity suggesting they have a bridge rectifier inside. Connected to a 12v AC transformer and after 5 min it is warm to touch (NOT hot like halogen) and working.
So I thought that I would ask the proper experts here
Here is a picture of my transformer (which is the same as the one in the ceiling )
View media item 96926

 

[ban-all-sheds]

I think I have a wire wound one so I should be ok ?

You haven't, so you may not be. The big unknown is what minimum load does the power supply need?

 

[Mikefromlondon]

I changed some 20watt 12v halogen lights to LED 5w lights, there was a very tiny amount of flicker which came and went and so on, so went back to the electrical seller to ask him his opinion on flicker, and he said I could do with a dedicated 12v power supply, one that can run up to 4 x 5w LED lights, so yes your original one is not a wire wound transformer and it is a switched mode power supply, and try it first it may work, if you see any flicker then you will need ones intended for LEDs, and they are cheap as dirt for £4.00. All went well and Ok, and yes the LED lights are not polarised you could them on ac or dc.

 

[stillp]

I think I have a wire wound one so I should be ok ?

You haven't, so you may not be.

The term "safety transformer" usually means a wire-wound transformer with separated windings.

 

[bernardgreen]

With the unit disconnected from mains and lamps measure the DC resistance between the output terminals.

If it is a wound magnetic transformer then this resistance will be a few ohms at most.

Reverse the meter leads and measure again. If the resistance is the same as the first one then 99% certain it is a wound transformer

Do the same with the mains input terminals which may give a resistance of several ohms but if the same resistance in both directions then again 99% it is a wound transformer.

A final check is a medium wave radio held close to the device when it is powering lamps. If the radio emits strange noises when close to the device then 80% certian it is a switch mode power supply and not a wound transformer

 

[aveatry]

Wow thanks everyone.
I have got the meter out and on the 12v side it says 00.5 and it reads the same when I swap the + and - over
And on the 240v side it is 37.5 and it reads the same when I swap the + and - over
I actually don't have a radio to test with

I think I have a wire wound one so I should be ok ?

You haven't, so you may not be.

The term "safety transformer" usually means a wire-wound transformer with separated windings.

Google tells me that the symbol on the transformer is Short-circuit-proof symbol used in conjunction with the Safety isolating transformer symbol.
It also weighs 1.5kg which I thought was a sign it was wire wound
It is also is about 13 years old which--- is that before the next gen ones that don't work came out ?
But you lot are the experts and I dont know so I am just putting here what I do know for you to help me

There is no indication on it as to what the min load - if it has one would be.
The kitchen is the only room in the house with mr16s everywhere else in the house is gu10 and all leds so I have to buy some mr16 leds to try.
Yes I could put in a dedicated led driver BUT that involves a lot of work due to where the transformer is located.

 

[Mikefromlondon]

With the unit disconnected from mains and lamps measure the DC resistance between the output terminals.

If it is a wound magnetic transformer then this resistance will be a few ohms at most.

Reverse the meter leads and measure again. If the resistance is the same as the first one then 99% certain it is a wound transformer

Do the same with the mains input terminals which may give a resistance of several ohms but if the same resistance in both directions then again 99% it is a wound transformer.

A final check is a medium wave radio held close to the device when it is powering lamps. If the radio emits strange noises when close to the device then 80% certian it is a switch mode power supply and not a wound transformer

Modern generation young man will say "Medium Wave Radio" whats that dad?

 

[bernardgreen]

There is no indication on it as to what the min load - if it has one would be.

That is a wire wound transformer, and as such has no minimum load requirement.

"Medium Wave Radio" whats that dad?

The way we listened to Radio Luxemburg on 208 metres on a crystal set, wth a long wire aerial stretched along the garden.

 

[aveatry]

Just checking --- can everyone see the picture of it i posted in my first post

 

[Mikefromlondon]

There is no indication on it as to what the min load - if it has one would be.

That is a wire wound transformer, and as such has no minimum load requirement.

"Medium Wave Radio" whats that dad?

The way we listened to Radio Luxemburg on 208 metres on a crystal set, wth a long wire aerial stretched along the garden.

And not forgetting Radio Caroline, I also needed an earth wire to get a stronger signal as well as a long wire stretched in the garden, lol those days! I can remember, and I used to purchase 3 or 4 different monthly magazines called Practical wireless, Practical Electronics, Everyday Electronics, Elector and used to build projects, I spend my own money on my Education, I never received any state funded education!

 

[JohnW2]

I think I have a wire wound one so I should be ok ?

You haven't ....

Are you sure about that?

Kind Regards, John

 

[Mikefromlondon]

There is no indication on it as to what the min load - if it has one would be.

That is a wire wound transformer, and as such has no minimum load requirement.

"Medium Wave Radio" whats that dad?

The way we listened to Radio Luxemburg on 208 metres on a crystal set, wth a long wire aerial stretched along the garden.

And not forgetting Radio Caroline, I also needed an earth wire to get a stronger signal as well as a long wire stretched in the garden, lol those days! I can remember, and I used to purchase 3 or 4 different monthly magazines called Practical wireless, Practical Electronics, Everyday Electronics, Elector and used to build projects, I spend my own money on my Education, I never received any state funded education!

Just checking --- can everyone see the picture of it i posted in my first post

Yes that is not a proper step down transformer, a real step down transformer with 65VA (which roughly means just over 5Amps at 12v) would weigh a ton!

These so called switch mode power supplies, they use mains rectified from 230V ac to 330V DC and smoothed into a high voltage capacitor rated to at least 400V and anything from 47uf to to about 220uf and then this DC voltage is chopped at high frequency like 50Khz to 100Khz into a very small wound transformer, that gives you the safety isolation and the output of this small transformer is then rectified and levelled to give an output around 12v on full load rated for that PSU. Some feedback from the output is sent to the chopper circuit to shut down momentarily using an opto-isolator to regulate the output whilst still maintaining full isolation from the deadly mains 230v.

Some cheap crude ones do not use feedback to regulate the output, so when they are used on low load they will flicker, when on maximum load they are more stable and hence why LED dedicated transformers (actual switch mode power supplies ) are more elaborately controlled with feedback and also use a regulator in their output to further smooth out any ripple or fluctuation in output voltage.

Oh and all the while all modern switch mode power supplies have to have RF Filtering incorporated so as to reduce or completely eliminate the chopping frequency noise back into the mains and on to the equipment in use, so they need extensive filtering to meet latest electromagnetic interference requirements under the EEC directives, etc etc, but despite which one can still tune to the 2nd or 3rd harmonic of the chopping frequency on a medium wave radio held close to one of these power supplies, some of this interference is radiated as in electromagnetic waves, some is through induced magnetic fields from the iron core of the small transformer that can induce interference into adjacent equipment.

 

[aveatry]

It measures 5inch long plus extra 2 inch each side for the connections, x 2inch high x 2 1/4 wide and weighs 1.5kg. When I turn it over the insde seems to be filled with a solid plastic resin and not any kind of cover -- does that help because so far people cannot seem to identify it


Edit -- that is with the two leads you see attached I couldn't be bothered to take them off but have just now kind of weighed them separately whilst still attached so I would estimate the actual weight to be about 1480 grams

 

[bernardgreen]

a real step down transformer with 65VA (which roughly means just over 5Amps at 12v) would weigh a ton!

Sorry Mike, not a ton.... I have just specified a 75 VA transformer ( a proper wound one ) and the weight is given as 1550 grams

 

[JohnW2]

It measures 5inch long plus extra 2 inch each side for the connections, x 2inch high x 2 1/4 wide and weighs 1.5kg. When I turn it over the insde seems to be filled with a solid plastic resin and not any kind of cover -- does that help because so far people cannot seem to identify it

In that case, I would say that is, as you originally suggested, almost certainly a 'real' wire-wound transformer - quite apart from the labelling (which suggests that), what you describe is, IMO, too big and heavy to be a 65W 'electronic' SMPSU.

Kind Regards, John