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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
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