Welcome To Lets Fix It

   //   

Step One

If your touch lamp is stuck permanently ON, or will not switch on, then the power control transistor (called a THYRISTOR, or TRIAC), is shorted, or open circuit (blown).What usually happens is that when one of the lamps blows, the element in the lamp momentarily creates a plasma arc before it completely breaks, and will often blow the 3 Amp fuse. However, fuses are usually too slow reacting to stop the THYRISTOR from being damaged by the overcurrent. I have only known the little IC (chip) or diode get damaged once (see other pictures below), it is nearly always the THYRISTOR/TRIAC.


Step Two

If you are confident with soldering and general electrical work, this CAN be fixed, provided the electronic unit that contains this part is not sealed with a potting compound. They are usually just a 2 part plastic box, that unclips to reveal a small circuit board, with a small chip, and the THYRISTOR on it. The first problem is getting into the base, to take the box apart. These are sometimes riveted on, so you will have to drill out the rivets, and glue the base back on later ( a glue-gun is good for this).


Step Three

When you locate the module, take care not to break any crimped wires, such as the earth wire, or sensor wire. You may need to spread the sides with a small screwdriver to defeat the plastic clips holding the two halves together. When apart, locate the largest component on the board, with a metal tab with a hole in it. It will most likely be a BT134 or BT136, or a string of numbers with BT134 or BT136 in as part of it, stamped on this part. If it is through-hole, use a solder-sucker (aspirator) to remove the solder from around each leg of the THYRISTOR, and remove it. If it is on an SM (Surface-Mount) board, then it might be a bit trickier, as you will have to melt the solder from each “tab”, and lift it with a sharp knife, until all three transistor legs a clear of the board. It may have the heat-sink tab soldered too, and you may have to apply more heat for that to part.If it is screwed down with a small screw, and a mica insulator, save those bits, in case the screw supplied with the new THYRISTOR is a bit larger than the original. I believe that the majority do not get warm enough to require a heat-sink, and are “free-air” mounted as in the picture. NOTE: some newer touch lamp modules may have an internal soldered-in fuse, so check this is OK with a meter before proceeding. Replace with the same rating if it has blown. MAPLIN have a good range of fuses. Also see additional information below, regarding a fusible resistor in the negative return on the PCB, that can protect the TRIAC and IC (FAQ information below kindly supplied by BW).


Step Four

HINT: replace the BT134 or BT136 Triac with a HIGHER CURRENT RATINGone, such as the BT137, BT138, BT139, or BT140. Most of those should have the -600E suffix, denoting a MAX 600 VOLT device, but if you can only get the -800E, that will be OK (the 800E or F will actually withstand higher spike voltages). NOTE that the E means “sensitive gate”, and some touch lamp drive circuits need this or equivalent. I believe these have been replaced by the F and G suffix devices, where the F has a 25mA gate current for a fully ON state. Any one of those above should work in this simple circuit. Note that the BT136 series can handle UP TO 4AMPS normal load, but will sometimes fail when the bulb goes, as it will only handle a maximum peak current of 25A for 20ms. The BT139 series will handle an RMS ON-STATE (normal load) current of up to 16 amps, and should withstand a blowing bulb, giving time for the fuse to blow if needed. The peak current of that device is 140 AMPS! I have had the lamp above blow a few lamps, blow the 3 AMP fuse ONCE, but the BT139 TRIAC replaced a couple of years ago has been just fine.


Step Five

EXTRA PICTURES BELOW FROM 2011: To ensure you do not damage the lamp base and stem when drilling out any base cover rivets, invert it and place in a waste bin with bubble-wrap to protect it.Unfortunately, the module pictured below was too badly damaged to repair due to the control IC being damaged by the gate-to-anode short in the TRIAC after the lamp blew (the IC gate was blown when the resistor was burnt - see the black part in the centre of the picture. Luckily I had a spare module to replace it with. The wire colours are often a one-to-one swap over. Ensure all wires are sound, safe (no nicks) and all protected by the plastic module box after repairing. The base can be stuck back with a glue-gun.


Step Six

Additional information below kindly supplied by BW of BEDS (about yet another variation of touch-lamp circuits, where a fusible resistor is used to protect the circuit): “This specific touch lamp had a low ohms resistor between the neutral and the rest of the circuit. It was 0.22 Ohms (Red, Red, Silver bands). There were no markings for the power rating. Being such a low resistance and its position in the circuit, it’s a fair guess that this was acting as a fuse. When I opened the dimmer module, there was a small black mark on the resistor indicating overheating and my meter showed it was open circuit. I also found the TRIAC had gone short. I bought a TRIAC of the same type from Cricklewood electronics (I think it was a BT136 800E from memory).


Step Seven

Calculating that a 75W lamp at 240 Volts would only dissipate 0.025 Watts. Although counter-intuitive, driving a 75W bulb through a 0.22 Ohm resistor at 240v only dissipates 0.0215 Watts in the resistor! I ordered two types of .22 OHM fusible resistor just in case: armed with this knowledge, Cricklewood electronics sell two low-power 0.22 fusible resistors: Part No.: G0R22 Metal Film Fusible Resistor ¼ Watt. 0.22 ohms (cost 30p) Part No.: J0R22 Metal Film Fusible 5% Resistor ¾ Watt 0.22 ohms (cost 35p)


Step Eight

Now size is a fair indicator of the power rating of a resistor. I found the ¾ Watt was twice the size of the blown part and the ¼ Watt was 2/3 the size. Since my calculations indicated the ¼ Watt should be ample, plus the fact that it fitted better in the dimmer black box and that I’d rather fit the lowest power rating that works, I fitted the ¼ Watt. My touch lamp now works perfectly again. I’ve yet to see if the resistor blows when the next bulb fails, but I’d rather replace a cheap part than blow the triac again or worse. Certainly those two parts (fusible resistor & TRIAC) are easy to replace, and if the fusible resistor protects the control IC CHIP that is a good benefit, as they are hard to replace and source. If you need to replace a blown fusible resistor of unknown power rating, where the resistance and/or wattage of the bulb being driven are different, you can just plug in the different values into the equation as follows to work out the minimum required power rating: Minimum required power rating for the fusible resistor = (Bulb wattage / mains voltage) squared, multiplied by the resistance of the fusible resistor. Give a healthy margin to avoid overheating. In my case the ¼ Watt resistor had more than ten times the calculated power dissipation of 0.0215 Watts.”




Fix a touch lamp by following this straight forward guide.

  • Author:
  • Date posted: 10th August 2012
  • Posted In Household
  • Viewed 75430 Times
  • Most Viewed Guides This Week