Repairing Step-down Transformer Lamp

Pictured above is a repair project with a step-down transformer powering a desktop lamp. Visible in the picture are the components of the lamp (bulb, base, and transformer), a soldering iron, power strip, and a useful little multi-tool from SOG.

Looking Inside The Black Box

One of the deficiencies of the design of this lamp is the remote transformer inside a plastic box. The transformer is located half way between the ends of a power cord that is approximately 6 feet long. It’s such a heavy component that the manufacture might have had a better design with more robust packaging. When the plastic case was opened, this is what was observed:

  • Damaged plastic black case
  • Fractured output connector
  • Step-down coil with magnetic core
  • Primary side coil hardwired (soldered) to AC power cord

Primary Side Coil Repair & Test

Step 1 is removing the manufacturer’s potting material to access the leads of the primary side coil.

NOTE: Electrical component manufacturers frequently pot their products with non-conductive material. The effect of potting acts robustness to the product and for consumer safety. Electrical devices have sensitive components inside them and potting will immobilize them, making the devices more robust against mechanical shock and vibrations. High voltages can be a safety hazard, so the electrical isolation property of the potting material also prevents accidental electrical short circuits, which can cause fires, injuries or fatalities.

Step 2 is reconnecting the primary winding leads to the AC power cord. This was possibly an extra step in the troubleshooting process because the soldered connections may have been damaged by Step 1. Regardless of when the leads were damaged, the connections were clearly broken when viewed without the potting material around the primary winding.

Its worth mentioning that there is some type of leaded component wired across the leads of the primary winding. In the picture below the reader can make out the outline and some of the part’s black case beneath the potting tape.

Step 3 is to apply power to the transformer and verify proper operation.

Unfortunately, when the power cord was reattached to the leads of the primary winding, there was a miswire that created a problem when power was applied. When power was applied (the AC cord was plugged in) there was a loud pop that corresponded to a puff of white smoke coming from the little mystery component.

The figure below is a close up view of the damaged component and the related wiring. “Letting the smoke out of the box” sometimes happens. Thankfully, no one was injured and there was no catastrophic damage to the transformer that would prevent the lamp from being used.

With the wiring to the primary coil repaired and corrected, power was reapplied and verified. The screenshot below is the 110 VAC voltage waveform measured with Oscium's iPhone oscilloscope.

Repairing The Output Connector

Inspection of the lamp’s transformer showed damage to the output connector. From outside the transformer case the output connector appeared to be perfectly fine. However, as the reader can see in the following two images, the connector has been fractured in such a way that the electrical contacts from the transformer float free inside the case. Not only does this prevent the proper function of the transformer, but the exposed contacts could touch and create a short circuit that either damages something or hurts someone.

Secondary Side Coil Repair & Test

The steps used to repair the primary coil were implemented to repair and test the secondary coil and power connector.

The image below shows the repaired output connector and the electrical tape wrapped around the coil to stake the primary’s leads in place.

The last step in repairing the output connector is verifying the output waveform. The figure below is a screenshot from Oscium’s iMSO2 oscilloscope app. The waveform displayed here is approximately 40 VACpk-pk.

Reassembly And Test

The last part of repairing the lamp and returning it to service is reassembling the cords and transformer case, and then plugging the thing into the wall outlet and flipping the power switch to ON. The image below shows the successful completion of the repair. The bulb is lit and there aren’t any smoke or glowing red components.

Concept Of Operation

This may be a bit of review from the last blog post, but it’s repeated for reference.

The diagram below depicts the major components in the lamp’s circuit.

The AC Source represents the household 110 VAC from the wall outlet.

The bulb on the far right of the diagram represents the lamp. The actual bulb in the lamp itself is a low voltage (40 VAC) device that still produces high intensity bright light. The relatively lower voltage and apparent high power consumption implies that the bulb draws significant current.

The dotted line represents the plastic case around the transformer. This lamp uses a single winding step down transformer to change 110 VAC into the 40 VAC used by the bulb.

The last element in the lamp circuit is the switch. Mechanically, the switch style is a single pole-single throw (SPST); there is a single electrical contact and that moves to make or break the circuit when the switch is actuated.

The IMSO App works with iOS devices to display the iMSO-204L oscilloscope measurements on an intuitive touch screen interface. Users can use their two fingers to pinch-zoom to expand or contract the vertical and horizontal axes.