it's been a hell of a week on call already and i hope i can get this finished before i get called
out again so today i am going to just do a quick uh curiosity teardown on this super common at
least in north america this is a really common style and brand of a lighting dimmer this one
was in service in my house for well over the 25 years that i've owned the place so it doesn't
really owe me anything but the mechanical on off switch part of it has died so i just went
to the store and bought a replacement oh sure now it works um anyway i'm gonna tear into this
guy and just see how much different it is from the absolutely most basic dimmer circuit which
is this little kit that i built oh i don't know a while ago anyways i'll put a link to it up there
and it is basically at its core no more than a capacitor variable resistor a fixed resistor
a not a dual diode what are those things called a diac i think
and a triac to do the actual switching of the current
i'm going to guess that this thing being a commercial product from a professional company
is going to have a little bit more going on inside i'll probably have some rf filtering emi kind of
filtering um and mov may be fusing maybe not sure whether you can see it through the paint or not
but it's rated at 600 watts 120 volts ac
which you'd expect for a north american device so that 600 watts is going to be more than enough for
most residential applications unless you're doing something really funky you can get higher current
ones too anyways without any further ado let's just open it up surprisingly it has screws holding
together i was expecting rivets or come on get out i was expecting rivets or tabs or something
like that and there are a couple of tabs but that's what's holding the potentiometer on
this is much easier to get inside of than i expected oh yeah the three wires
so there's the ac line in ac line out to the load and just a ground the ground as you can
see is riveted right onto the case which is also where i'm assuming the triac is riveted
to the case as well uh providing some heat sinking that's good so there is the potentiometer with its
little switch on the back of it push on push off switch which you know like i said is working now
the line in line out don't really matter which is in and which is out they'll work the same but
let's just call this one the line in just for fun that comes in through the switch the switch has a
couple of capacitors on its output side going down to looks like one of them goes down to one lug
of the variable resistor the wiper of the variable resistor the other one goes to the
one end of the variable resistor the other end of the variable resistor goes nowhere so it's not
set up as a voltage divider it's just set up as a series resistance then from the wiper to one tab
of the triac we have an inductor just a series inductor and then that goes down to one where of
that where is the output coming off oh the output is also coming off that center wiper okay i'm
going to draw this out it's even simpler actually it has less components than this thing does
that surprises me yeah let me just draw this out and i'll come back
well that didn't take too long to reverse engineer um let me just zoom in on the schematic and show
you what i found it's not too surprising so the ac line comes in either in and out
is kind of irrelevant in this circuit um supply and load doesn't really matter which side it's
on but let's just assume for arbitrary reasons that this is a supply side so power comes in
through the switch uh smoothing capacitor 0.01 microfarad or 0.1 microfarads uh 250 volts
and then across to another of the same type of capacitor which goes down to the one end of the
variable resistor now that variable resistor it is a potentiometer technically i mean that's what
the component actually is but only two of the terminals are used the wiper on one end
so it's just a variable series resistance so the power also carries on to the one of
the two main terminals of this triac and it's not exactly a triac but we'll get to that in a second
the power from the other side comes in goes through that series choke or series inductance
this guy here which is also going to be filtering and
and cleaning up switching spikes and stuff like that that goes to main terminal number two
and the gate is just controlled between this capacitor and this resistor here
very very simple and as i said more simple than this guy even this guy has a second resistor in
there and it has two components instead of just this one now originally i was calling that a triac
and it kind of is but it kind of isn't what it really is is a quadrack which is a triac
that and a diac that in the same package the diac doesn't conduct until it gets to a
high enough voltage to trigger the gate of the triac so that basically allows them to combine
two components into one save on part count all the rest of that i had a real bear of a time finding
the data sheet on this thing and the part number is not a part number that i could
find a data sheet on so i had to go on a wild goose chase i actually ended up using
the wayback machine a couple of different times and finally landed on a modern equivalent let's
say from that is still in production which makes sense because these things are still in production
meet the quadrac they are as i said a special type of thyristor combining a
diet and a triac in a single package i will link to this wikipedia article
which will take you through to more theory on the diacon triac than you ever wanted to know
but as i said the diac is for triggering the triac gate diax are essentially a bi-directional diode
that switched the ac voltage and trigger the triac or scrs or other similar things so they don't
conduct until they hit their breakover voltage so that gives the triac a hard on at a certain level
of voltage rather than just slowly soft coming on maybe on maybe not on as voltage ramps up on
the afc waveform and here is a data sheet for a modern quadrack from the little fuse company
i'd forgotten that they actually make silicone i thought they just made fuses as the name suggests
so there is what it looks like oh i label the gate it's actually trigger because the gate is really
this connection here okay regardless and there in the data sheet is their sample test circuit
that look just a little bit familiar add in another capacitor there for filtering add in a
series inductor and you've got what i've got sitting on my workbench wow so the timing as as
i said is basically a combination of this resistor and this capacitor that just selects how quickly
this thing gets up to its trigger voltage on each half cycle or each half of the ac waveform
as shown down here and there from wikipedia is just the voltage uh
voltage versus current uh trace of a diac in both directions so positive voltage
going up up up the current is not really rising until you hit that 30-ish volt break over then
all of a sudden boom it starts conducting same thing in the negative direction boom there it goes
that was surprisingly simple the hardest part was trying to figure out what that device was
i had never encountered that one before but now that i have i know a little bit more
i suppose i could if i was really brave take that potentiometer or that switch off the potentiometer
apart and try and figure out why it's mechanically jamming up and being intermittent
but like i said it was less than 10 bucks to replace it and this thing gave me
you know 25 plus years of service so it certainly doesn't owe me anything i'm sure
it's just a mechanical fault in there it's just worn out from thousands and thousands of uses so
i'm not going to try and fix it any further that was never my intention tonight anyways i just was
curious and i hope if you were curious that your curiosity has been satisfied at least a little bit
mine has i know that um thanks for watching i know this is just a quickie but
uh sometimes that's what you get comments and questions down below i'll talk to you later