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If the replacement of the bulb or the socket is not enough to put a halogen lamp back into service, it is certainly the transformer which is at the origin of the breakdown... Explanations!

Repair a halogen desk lamp

Find the fault

This lamp design purified has stopped working after a few years of use... Its significant price (about 70 euros) pushes to find the fault.

Transformer: not only to ballast!

Although "burned", the halogen bulb (low voltage type) is not the cause of the failure.

It is therefore necessary to check the electrical circuit as well as the transformer that converts the 220 V sector into 12 V AC.
Housed in the base, it is also used to ballast the lamp. To access the transformer (the centerpiece of the mechanism), it is necessary to disassemble the base (simply screwed), whose sole is provided with a crown that makes it possible to rotate the lamp.

Transformer: how does it work?

The transformer has an iron magnetic circuit on which are wound two coils: the primary joined to the sector (input coil) and the secondary connected to the bulb (output coil).
There is no electrical contact between the two (only a magnetic link) and it is the ratio between the number of turns (turns of wire) primary and secondary which determines the output voltage (12 V AC).
When the lamp is turned on (220 V) with the switch closed, a measurement by the multimeter indicates that the voltage exists across the primary but is nil to the secondary. Other checks: the transformer does not heat and the welds of the wires on the terminals are in good condition...
We can therefore deduce that there is a break in a winding (usually on the primary).
Of small section, the primary thread is indeed subject to breakage. It can also short circuit in case of overheating. To ensure this, an ohmmeter is placed between the two ends of the winding.
If the device indicates infinity is that there is internal cut.

Find spare parts

Continuing the research, we find that the incoming son in the bushing have "charbonné", and they easily separate from the pod.
One may then be tempted to put the lamp away, since it is necessary to change the transformer and the socket. But desktop halogen lamps look the same and have the same components.
Recovered on a lamp found in an attic, the new transformer is a little longer, but the same power 50 VA (identical to 50 W). To introduce and block in the base, it takes a little adaptation.
But the result is conclusive!

Proceed to the repair

Proceed to the repair

Transformer composition:

Two yellow wires feed the lamp in 12 V and two white the transformer and the double control switch.
From it, two wires (black and red) go back to the transformer.

Connect the lamp

Connect the lamp

Connect the lamp.
The multimeter in the "alternating" position, measure the voltage at the terminals of the domino.
The display indicates that the plug and the cord are in good condition.

Cut winding

Cut winding

Lamp connected, place the test points on the secondary outputs.
The measurement is 0 V instead of 12 V.
The transformer certainly has a cut winding.

Check the voltage

Check the voltage

To check the transformer voltage, place a jumper between the voltage wire (white) and an intensity wire (black or red).
The output measurement is always at 0 V.

Test the resistance

Test the resistance

Test (ohmmeter) the continuity of the two wires passing in the arm of the lamp.
A touch of touch makes contact with each end of a wire.
The resistance must be low, here 0.27 ohm.

Replace the damaged part

Replace the damaged part

The halogen socket heated and a wire broke off (right).
Retrieve a socket and a reflector portion (middle) that will be bolted to the original reflector (left).

Refix the reflector on the structure

Refix the reflector on the structure

The wires of the arm end with flat pods.
Refit the reflector on the structure, equip the wires with appropriate lugs insulated by thermo sheath and make the connection.

Change the transformer

Change the transformer

The transformer recovered here is of the same power and has exactly the same inputs and outputs.
It is held under a flange by a plastic wedge.

PRACTICAL ADVICE

• Avoid touching the lamp with your fingers by interposing a paper towel or a soft cloth. This is also true for the elongated halogen tubes of garden floodlights.
• For live measurements, such as on the transformer, use a multimeter with well insulated test probes.
• Unscrew the fasteners carefully. Some parts of the lamp tend to heat up, which makes the plastic brittle.


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