electronic transformer re-design

[caga]

hi to all,
does anyone dealed with these tiny 12vAC electronic halogen lamp transformers? I am trying to make its output voltage ~24vAC but could not managed so far. I have found some schematics but did not help. The common idea these are step-down AC converters with a half bridge connected to the output transformer, but I could not guess where the voltage is set?

 

[Nigel Goodwin]

Don't they use a switch-mode transformer?, you would probably need to rewind the transformer?.

Try posting the circuit!.

 

[caga]

not exactly I think, there is a transformer but I think it is only a step down, and I try to re-wind its output part but something blow out! Tyring to find which part is?

 

[Nigel Goodwin]

not exactly I think, there is a transformer but I think it is only a step down, and I try to re-wind its output part but something blow out! Tyring to find which part is?

Obviously it's a step-down switchmode transformer, but can you post the circuit?.

 

[caga]

I did not draw a schematic yet but it is very much like:
https://www.st.com/stonline/products/literature/an/3707.pdf
The one that I am dealing with do not have short circuit protection transistor.

 

[Nigel Goodwin]

I did not draw a schematic yet but it is very much like:
https://www.st.com/stonline/products/literature/an/3707.pdf
The one that I am dealing with do not have short circuit protection transistor.

There's no feedback or regulation in the circuit, so you would need to rewind the transformer to increase the voltage. The winding feeding the lamp requires twice as many turns as it does for 12V, and would generally use a thinner wire so it fits in the same space.

 

[caga]

I tested exactly the same idea, there are 8 turn at the lamp side of the Tr, and I made it 16 but I dont know why some components blew out and it is not working now. I will continue to examine, and will post back if I can find different.

 

[Nigel Goodwin]

One thought! - what wattage is the 24V lamp you're trying to feed?, and what wattage was the 12V one the transformer was designed for?.

If things have blown, it will need repairing before you can go further (or even just buy a 24V one?).

 

[caga]

Your right! I will buy a new one!
It was a standart 12vAC halogen lamp max power is 50W, when I am testing I conncet a 10ohm 10W resistor just to see the result. (As I am expecting a 24v AC, I measured the resistance of 50W halogen lamp, it was ~0.5, so I chose 10ohm to be safe but...)

 

[Nigel Goodwin]

Your right! I will buy a new one!
It was a standart 12vAC halogen lamp max power is 50W, when I am testing I conncet a 10ohm 10W resistor just to see the result. (As I am expecting a 24v AC, I measured the resistance of 50W halogen lamp, it was ~0.5, so I chose 10ohm to be safe but...)

You can't measure the resistance of a cold lamp, they are greatly lower when cold - calculate it from the voltage and power instead.

For a rough estimate, 50W at 25V is 2A, so it's hot resistance is about 12.5 ohms, and you will need a MUCH larger resistor than 10W, it will need to dissipate 50W without glowing like a light bulb :lol:

Your 10 ohm will probably have over loaded it?.

 

[caga]

absolutely, I thought as same, and was ready to shut down the power after few seconds, only trying to see the result. since the electronic tranformer did not work, it blew(??) up as soon as I give the power, lets say at t=0.5s, so it could not find a time to glow. But of course the reason of blowing, by somehow, can be the load!