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Got a dud dimmer? The fault is bound to be a blown Triac. Fix it for good with a higher-rated Triac. How to repair light dimmers Do you have light dimmers in your home? Has one or more of them failed? Are you cheesed off with the thought of buying another one? Well, don’t. Repair it with a more rugged Triac and it should be fixed for good. By LEO SIMPSON Light dimmers are a great accessory in lounge and dining rooms, to subdue the lights and set the mood. The same comment goes for lights in bedrooms. They are handy too when there are young children in the household. You can set the dimmer low to check on them without disturbing their sleep and they can also help a child go to sleep if he or she becomes anxious in the dark. But while they have their good points, dimmers can fail. Usually they 42  Silicon Chip fail when the lamp filament blows and this is par­ticularly the case if the lamp fitting has the bulb upright. What often happens is that when the filament blows, a loose section of it flails around and makes contact with one of the filament supports. The resulting arc blows the Triac and from then on all you have is a light switch – it’s either on or off. In our article entitled “Power Control With A Light Dimmer” in last month’s issue, we mentioned that it was generally possi­ble to repair a failed light dimmer by replacing the Triac with an SC141D. That is true but for long term reliability it is better to take the approach outlined in this article. This whole subject was brought into focus once again when one of the SILICON CHIP staff recently had a dimmer blow in his home. We decided to bite the bullet and see how difficult it was to repair. Removing the dimmer The first step is to have the dimmer removed and temporari­ ly replaced with a standard light switch on the same size switch plate. Don’t even think about working on the dimmer while it is still connected. Having had the dimmer removed, you can inspect the small rectangular module itself. This will have an end panel which is generally secured with integral clips and plastic tape. Peel off the tape and then pop out the end panel by carefully pushing on the integral clips – it is more difficult to describe than actu­ally do it. Now remove the knob – it just pulls off. Make a drawing of how the two wires from the dimmer module connect to the switch. Disconnect the two wires and then it is simply a matter of push­ing on the knob shaft to remove the small PC board assembly from the plastic housing. The Triac is mounted at one end of the board and usually has a small aluminium heatsink. On some dimmers this heatsink is pop-riveted to the metal tab on the Triac but on the one shown in the photos in this article it was merely placed in contact with the Triac tab by the pressure from the end plate of the plastic module. This is hardly an effective method but it does make it easy to replace the Triac. Now why did the Triac fail? We have already described the mechanism of failure but since it is such a common hazard you wonder why the dimmer manufacturers don’t simply use more rugged Triacs. In the dimmer shown in the photos, the original Triac fitted was a Philips BT137 series. This is rated at 8A which ostensibly is more than adequate considering that the dimmer was rated for a maximum load of only 300 watts. The problem is that the BT137 series Table 1: Triac Ratings Type BT137/500 BT138/500 BT139/500 SC141D SC146D SC151D MAC320A8FP BTA10-600B BTB16-400B Current Rating 8A 12A 16A 6A 10A 15A 20A 10A 16A Triacs have a non-repetitive peak surge current (ITSM) rating of only 55A. This is insufficient to cope with the arc currents mentioned above. The obvious solution is to replace the Triac with a higher-rated unit and there are quite a few to choose from, all costing $6 or less. Table 1 shows a list of Triacs which are widely available from electronic parts retailers. Looking at Table 1, you will see that there are several devices with at least double the surge current ratings of the BT137. All have similar packages and the same pinouts so they are drop-in replacements for the BT137. However, it is good practice to go for one with the same or a higher voltage rating as well. Therefore, if you can get the alternatives, you can reject the 400V SC146D, SC151D and the BTB16-400B Voltage Rating 500V 500V 500V 400V 400V 400V 600V 600V 400V Surge Rating 55A <at> 50Hz 90A <at> 50Hz 140A <at> 50Hz 74A <at> 50Hz 110A <at> 50Hz 110A <at> 50Hz 150A <at> 50Hz 100A <at> 50Hz 170A <at> 50Hz although the last-named device does have a massive surge rating of 170A. Because we had them on hand, we plunked for the Motorola MAC­320­ A8FP, a 20A device with a surge rating of 150A (at 60Hz) and an insulated tab. While the insulated tab is a good feature in some equipment it is of no advantage in domestic dimmers and there is a slight drawback in that the insulated tab is a little taller than the metal tab on TO-220 packages. The next step is to use your soldering iron to remove the failed Triac from the PC board. Take care not to overheat the copper tracks otherwise there is a risk that they may lift off the board substrate. If the heatsink has been pop-riveted on, you will need to drill out the rivet. Clean off any burrs or metal swarf Popping out the end panel on the dimmer module reveals the PC board assembly and the Triac’s aluminium heatsink. Before the PC assembly can be removed, you must remove the knob from the front panel. November 1996  43 Replacing the Triac is merely a matter of unsoldering the dud one and soldering in the new. The heatsink should be attached to the Triac’s tab using a screw, lockwasher and nut. Make sure that the finished PC assembly will still slide easily into its plastic case. around the hole when you have finished drilling. Next, solder in the new Triac, making sure that you don’t have any solder bridges between the Triac pads. The Triac must be oriented the same way as the original device, so that the heat­ sink can be refitted. Don’t use a pop rivet though; use a machine screw, lockwasher and nut. It’s also a good idea to smear a little heatsink com­pound on the Triac mount­ing tab before fitting the heatsink. Note that the heatsink will be live and there is no need for such niceties as mica washers. Before you reassemble the PC board assembly into the dimmer module, use your multimeter to check that the Triac is open circuit. If you switch your multimeter to the highest resistance range you will find that the resistance across the two dimmer module wires should be above 30MΩ. Do the same resistance check across the Diac. This is a small glass package with one lead connecting to the gate of the Triac. The Diac should also appear to be open circuit. Now reassemble the dimmer module. You can either have it reinstalled or you can use it as a soldering iron temperature controller, as outlined in last month’s issue of SILICON CHIP. SC 20 Electronic Projects For Cars $8.9s5 plu $3 p&p Yes! 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