DIY Touch Lamp FAQ

Q: How do I fix a touch lamp?

 

A: 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 never known the little IC (chip) get damaged, or the diode, it is nearly always the THYRISTOR/TRIAC.

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 rivetted on, so you will have to drill out the rivets, and glue the base back on later ( a glue-gun is good for this).

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.

HINT: replace the BT134 or BT136 Triac with a HIGHER CURRENT RATING one, 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 beleive 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! See HERE for a very good transistor specifications site. NOTE: I have recently replaced a failed BT136 with a generic BT139-800 (no suffix) and it worked just fine. I got these for £1 each from CRICKLEWOOD ELECTRONICS, currently on this page HERE. Thanks to AM and CB for feedback on that, plus other useful information. CB tells me that he successfully replaced a BT136-600D with a BT139-600 from FARNELL at £1.49 each. They have a minimum order value of £25.00, but if you need other items, such as computer media or parts, then that is not too bad. Order a few if you have more than 1 touch lamp. You may be able to source a BTA16 Triac for about £2 from there or other links on the right too if they are out of stock. However, you must use the C or CW suffix to obtain a 35mA gate turn-on current, otherwise the standard part (50mA Igt) might not turn the lamp to full on, with some touch modules.

UPDATE OCT 2007: I have had the lamp above blow a few lamps, blow the 3 MP fuse ONCE, but the BT139 TRIAC replaced a couple of years ago has been just fine.


Q: What are the common lamp bases?

 

A: In the UK, the 4 most common are:

Bayonet Cap, or BC, often shown as a B22 (26 MM across the pins)
Small Bayonet Cap, or SBC, (B15, 17 MM across the pins)
Edison Screw (ES, number E27, 27 MM across the base)
Small Edison Screw (SES, number E14, 17 MM across the base)
The small E14 and B14 lamps are popular with modern lamp fittings and chandeliers, because the fittings can be smaller and lend themselves toward slimmer, more elegant designs. It can be confusing though. See the graphics on the right.


Q: Why does a Circuit Breaker trip when a lamp fails?

 

A: This is a very common problem, and nothing to worry about. You may even find that a couple will go in a multi-light fitting in the same week. This because if you bought all the lamps together when it was new, they will tend to fail fairly close to the expected 1000 hour life, since modern manufacturing techniques produce lamps with the same internal characteristics time after time. The reasons for the CB tripping out, are due to the fact that when the filament fails, for a very brief milli-second or so, it draws a “plasma” like arc (spark), in the vacuum of the glass, as the 2 ends of the filaments part. Because this is a high current, relative to the CB tripping current, it will trip as it is designed to do that. There is supposed to be a little glass fusible element one one of the lead-out wires in the neck of the bulb, but I have never known these blow fast enough before the CB detects the high current. Sometimes the wires vapourise during this high current state, and I have known the glass envelope part company with the metal base with some force! Again, nothing to worry about, but it can make quite a bang in a glass ceiling fitting! To remove the broken bulb base, see LAMP HINTS


Q: Can I replace an oven lamp with a normal E14 appliance lamp?

 

A: NO. These MUST be replaced with high temperature lamps, specifically designed to withstand the high temperatures inside ovens and grills. They have a quartz glass envelope, and will be marked for use as an oven or grill lamp.


Q: What are the Low Energy lamp equivalent wattages?

 

A: There are slight variations with these Compact Flourescent Lamps (CFL for short), because of differences in folded-tube design, efficiency, and in starting (ballast) types. The electronic ballast types (versus magnetic) are usually slightly more efficient. However, here is a basic list:

Tungsten Lamp Energy saving CFL Light Output(Lumens)

25W 5-6W 220 – 300
40W 8-11W 400 – 550
60W 13-18W 850 – 1160*
100W 20-25W 1200- 1750*

* The 1160 (18W) figure is roughly equivalent to a 75W tungsten lamp, and a 1750 (25W) CFL is roughly equivalent to a 120W Tungsten lamp.


Q: How Can I remove a Halogen (Dichroic) lamp from a ceiling fitting?

 

A: This is normally very easy to do, either with a “lamp removal tool”, or just use one of those hooks that have a suction cup at the back. Ensure your lamp is COOL FIRST – then stick on the hook and pull down. Obviously choose the best size for the lamp. Most are either 35mm (MR16), and 50mm (MR11). Some of these may have a thin wire ring that pulls down from inside the fitting, that you can release, and just pull it out with. The OTHER common types are the GZ10 and GU10. These have rear contacts similar to a flourescent starter, so twist anti-clockwise a few MM before pulling out. You may find a DIY shop that stocks a removal tool (to avoid P&P on ordering a low-value part). However, check out part SCP250 at: http://www.work-play.co.uk/catalogue/index_diy.asp?rName=Bulbs