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Modifying the Lighting Transformer

Posted by Triode_Kingdom (A) on December 7, 2016 at 14:25:24

The discussion about these transformers is slowly getting buried, so I decided to open a new thread. Here are the basic steps I use to rework the oscillator transformer to produce a higher frequency. I want to stress again that this area of the circuit is not isolated whatsoever from the AC mains. Under no circumstances should it be touched or contacted while power is applied!

Here's the 60W lighting transformer as it comes from the factory:

Looking underneath, the oscillator transformer is connected to both power transistors. One leg is in series with a resistor. I pull the transformer from the PCB using solder wick to remove solder from the pins and board. These boards aren't through-plated, so once the solder is removed from the backside, the transformer should wiggle free.

Here's a top view of the transformer surrounded by a schematic of the windings. Note that a toroid is considered to have one complete turn by simply passing a wire through the hole. The wire does not need to come back around or do anything else. Looking at the photo, the oscillator coils are on the left and right. Each has four turns. The single-turn coil at the bottom is a feedback winding.

Viewed from underneath, it can be seen how each wire is wrapped around and soldered to its post.

To remove turns from the transformer, first snip the two main windings above the base.

Next, carefully heat each attachment point and slide the coil off its pin. Exercise care here not to overheat the pin and soften the plastic base. This job is easiest if you have a rubber-jawed bench vise that can hold the transformer while you work, but it can be done without that if care is used.

When you're done, it should look like this...

Now unwrap all but two turns of the two main windings. I use tweezers for this.

The next step is to tin the ends of the wires. This requires a very hot iron, such as the 100W Weller, or a gun. I use Weller 100/140W on the high setting. You might need to puddle a little solder in the crook of the tip and immerse the wire in it. It takes 10-20 seconds for the solder to break through the enamel coating. Once it does, you'll be able to tin the lead quickly by sliding it through the puddle of molten solder.

Now wrap each lead around its pin one or two turns and snip off the extra length. Remove any excess solder by quickly touching the joints with solder wick. Again, be careful not to overheat the pins.

This is the transformer after being modified and reinstalled on the board. Note that in the transformer pictured above, I brought the wires down along the sides of the base. The transformer below has them positioned down the front. Either method will work fine.

I did try this with only one turn on each main winding, but the oscillator wouldn't run. I suspect a single turn doesn't create enough flux in the core to saturate the transformer. With two windings as shown, the no-load frequency is about 66 kHz.

Edits: 12/07/16

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Analog Engineering Associates

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