these are starters used in conventional fluorescent fittings certainly here in the uk Europe and
Someone was inquiring. How does an electronic starter work so I've got one here that I've opened up
And I have reversed engineered it, but let's take a look at the principle first of how these work
when you have a fitting
with a fluorescent tube in it
Then the typical circuit arrangement here in the UK it may differ depending on voltages around the world
But the typical circuit is typically
a/c in
a choke or current-limiting device. This is before electronic
drivers came out and the choke is looks something like this a big lump of
Metal with the windings all concealed inside, and this is a low-profile unit designed to use in fluorescent fittings this in particular
This is not actually a choke. This is a transformer for halogen lighting, but made by a choke manufacturer
So you get the current limiting device and then you get the tube itself
so the
Current is limited goes into the Tube through a heated cathode back out to a starter
let's just write S for starter and
from the other side that goes into the other end of the tube through another heated cathode and
the starter again and
The reason for the heated cathodes is because if it wasn't for these heated cathodes
It'd be very hard to start fluorescent tubes because the voltage across and be very high. They'd basically be neon tubes
they'd be cold cathode Tubes and
this heating element at the end actually has a
thermally
emission
Thermally emissive coating on it that when it's hot emits electrons
and it lowers the voltage drop [get] across the end of Tube and
because of that it means that these tubes can operate at conventional voltages, but
to start them up you have to preheat those and the way to do that is with these starters
Now typically when you power it on the voltage across the tube will be the open circuit voltage
But the full mains voltage limited by this choke
But because there's no load it will appear across the starter as a full high voltage
and
Because of that these devices use that higher voltage as an indicator that they're to operate and the simplest form is
The glow starter. The glow starter is a little glass globe
with two electrodes in it
and
a bimetallic strip which in this case is just tagged across coming down and then just
a wee dimple out
there's also a capacitor across these just to reduce interference suppression from the switching and
Also to probably quench arcing on the contacts
When you turn the tube on the full mains voltage is present across this and it glows
Internally because it's a glow discharge lamp and the voltage is high enough it glows and it heats up and this bimetallic strip in
here gradually bends over until it touches this electrical and shorts it out, when it shorts out
These electrodes then start heating up the currents flowing through the choke through the heating elements through the starter through the other heating element it
heats up, and then once this is cooled down it breaks the circuit and
Sometimes a tube'll strike sometimes it doesn't if the tube doesn't strike
you'll get a brief pulse usually but then the voltage will fly up to the
full mains voltage again the cycle repeats and these starters tend to just keep going [tink, tink, tink]
and trying to start the trip until it strikes or
Doesn't strike and as happens to some case which they'll just keep trying. It is really annoying
However, there's another way to start these tubes, and that is electronically, so if you put a bridge rectifier
across here
A/c A/c plus minus and then you bridge the output of that bridge rectifier with in this case a thyristor and
There's a bit more to it than this as you'll see. then you can use a bit of electronic circuitry to detect from the voltages
open circuit that tube is not lit and the voltage flies up to the full mains voltage and
you can use that to signal to the circuitry that it's to shunt. So this thyristor turns on it shunts the
Bridge rectifier which then presents just a few diode
drop voltages and it effectively turns the heating elements on the cathodes and
the difference between
the traditional glass
starter with the
Gas in it and the electronic one is the electronic ones usually just have one go and there are two advantages to this
Each time you start an electrode a fluorescent tube if it doesn't strike properly it sputters metal off and the tube gradually gets blacker
or this one's gone very black at the end suggesting that it's already failed because another thing when the
emissive coating fails in these and it just does gradually wear out
The voltage Drop across them will get higher and it will cause it to sputter more in they go dark at the end
So the single strike is better for them in the sense that the number of strikes sputters more metal off
So the less the better it like makes the electrodes last longer another advantage
is that because it only tries once and
You don't get that sort of tink tink tink tink tink of a dead tube trying to light
It will just basically have a time delay it'll warm the ends
break the circuit and if the tube doesn't light the tube doesn't light and that's it. it's not going to have another go
Some more advanced ones may have multiple goes at lighting it, but in the case of this one
I think it's just a single go, but anyway let me demonstrate these
and the best way to demonstrate them is to put an
ordinary lamp
In series first of all with a conventional starter, which is the glass globe here
it's got the gas discharge the electrodes, and it's got the little ... the capacitor across the back the
Metal film capacitor for suppression, so this is just in series with this lamp
Do not put them directly across the mains, they go bang. I know this from experience when I was younger, and yeah
I took out the fuse on the circuit in the house
so it was very exciting and blew the tip off my pen which was
Disappointing because it was quite a nice pen but not to worry these things happen
so I've got the starter in series with an ordinary lamp here and if I turn this on it's going to
basically
Flash the lamp. That's how it would be trying to start the tube
And I don't know if you can see that that this but this is glowing
It's not very bright, but you may be able to see that
glowing in there
So it just keep it will just keep pounding away trying to start with that cycle
If however and I'm going to short this capacitor out. just in case it does hold a modest charge
I don't think it's a very high value capacitor
There's one way to find out the value of that capacitor, and that's to test it with a capacitance tester. Let's do that
And I'm not sure what value this is going to be
Let's put it to two hundred nano farad
And put it across there
5 nano farad, okay, it's not a terribly high value
This will just be a dedicated capacitor made for these
there's no labeling on anything that it's just it looks like it's got resin up the end of it and
Really it's just dedicated to this task, so lets try one of the electronic starters now
So I'm going to twist these wires around noting that this will be a live connection there
I've got the lid of the quick test open so it's safe
But when I close that down, they will be live
and
if I
plug this in the lamp should light briefly and then it should go out so let's see what happens
it lights and
Then it goes out now the first ones had thermal compensation
I have reverse-engineered this unit, and it doesn't seem to have that
Facility but some of them would cater for cold environments, and I can recall when these first came out. Oh blimey, decades ago, and
When it first came out they were they were novel enough to be featured in Tomorrow's World
Which is a BBC science program, and when I saw them for real we were using them in
I think it was Cummins Engine Co in Shotts , and we just finished building some new office sections of the building and
we used these the first time and
it was winter it was ice cold the tubes were cold, the tubes were brand new they hadn't really had a chance to
Basically burn in yet, and we turned them on. a few of them lit
But a lot of them just didn't light and it was only a couple of turning the whole lot off not again
They worked, but that was only because they were new tubes, and it was freezing cold
So let's say look at the schematic of this because as I say I have reversed engineered it
and it's a bit odd it's different from the
manufacturers recommendation for this component here
Which is a dedicated ST component for lighting, for use in starters.
So let's get these wires
i don't think there's anything particularly high voltage on this
And I'll get my test unit out of the way there
So I took a picture of this
Not a very in-focus picture, so haven't bothered...
I've just printed it onto ordinary paper and not glossy photo paper, but it shows [that] you know
it's just a scattering of standard quarter watt resistors a
Transistor this Thyristor type device and some diodes and underneath is a thermal fuse and and I
also took a back picture of the back of [the] print circuit board and
just process a little bit to make it sort of easier to overlay the components on and
it turns out that the
Triac tape type device, it looks like a a large transistor
It's actually a thyristor but it's a tn221 I've got a datasheet for that somewhere. Have I got the datasheet? I have
It's a thyristor. Which is optimized for use in this application
Its main features are it's got quite a low triggering current
So the circuitry doesn't have to drive it too hard to actually make it turn on. It's quite a high holding current
not sure the exact purpose of that
And it's got what's they call a high clamping voltage structure which means that it will withstand quite a high voltage across it before it actually
gets driven into just conducting on anyway and the reason for that is that part of the way a
fluorescent tube is struck is that when the starter opens circuit and the field on the
inductor
Collapses you get a sort of a transient it gives a high voltage spike
so and the Manufacturer also provides a
Basic circuit for this and it's quite interesting
It's got the mains coming in, it's got the inductance ballast they call it. The choke we'd call it. The fluorescent tubes
So the current has been limited through the heaters at the end of the fluorescent Tube
it's going through a bridge rectifier and as I say if you use a standard bridge rectifier across that then when you bridge the
positive and negative of the output the bridge rectifier with a
Thyristor which they're doing in this case
Then effectively it's like a switch it just shunts out completely
the other thing here that isn't in that circuit when that thyristor's on is this diode and the reason for that is the
diode is providing a small voltage to power the circuitry
Now in this case when you turn it on if the voltage is high enough current will flow through this
zener diode and the reason that that they've got the zener diode here is so this circuit doesn't try and
operate if the tube has struck
Once the tube has struck the voltage across it drops to closer to about 90 volts, though the voltage will depend on
the
age of the tube, the length the tube, damp to the tube, gas in the tube, so many variables
but the voltage will be in the range of say 90 -100 volts and
if it is
below the threshold voltage of this zener then the zener won't conduct and the circuit will just be out
out of circuit if the voltage is high enough current flow through this resistor through the zener and charges this capacitor and
it then drives the gate of the thyristor which turns on so initially when you turn it on, the
Thyristor is turned on and it shunts the
shunts the starter pins and
Also creates a slight voltage across this diode
The voltage is going to only going to be about point six to One volts dependent current this flow
It's going to be a modest current and see I don't think that's a specific Diode
I don't think it's got any sort of increased voltage, so it's all operating at very low level and
When that sort of one volt you're going to get across that will slowly charge up this capacitor
until it reaches a threshold of this
thyristor and when it reaches the threshold of the thyristor that will then turn on or latch on and
That will effectively shunt this thyristor
it effectively puts it slightly below it's eh it's cathode via this Diode
And that turns this thyristor off the Thyristors normally latch on there
even if it doesn't turn off [straightaway] it will turn off the next zero point of the mains cycle and
that's the point that the tube will then try and strike and
I'm guessing. I mean I look at this and the way it implies and then struck the actual on this guide. I
Don't know it implies that it could reattempt to restrike it, but I don't hundred percent see how that's going to work particularly I
Would have thought this thyristor would well I would have thought it would latch but having said that then the voltage
Could it could go through the start cycle again hmm tricky, but anyway, I reverse engineered
this actual pulse starter, and it was different
The circuitry was more or less textbook
Identical to this one, but the difference is [that] this here is not a thyristor. This is a darlington transistor
So let me let me bring a notepad in again
so here's the circuit and
It compares almost identically to this one except for this component here. [which] is shown as a thyristor here, but in the case of
This unit here. It's actually a darlington transistor. I've drawn it as an ordinary transistor. Which it's not. I shouldn't have
drawn it as an ordinary transister a darlington transistor actually looks like a this. It looks like the
Ordinary Transistor
But it's also got another transistor
Driving the base of the second transistor, and what happens here is that because you get two transistors cascaded
the gain of them
C200 a gain of 100 it will be [a] hundred times 100 it means that they very very high gain
Is they're very sensitive transistors. So that's what this one is it is a Darlington apparently and
MPSA14
so here we get the same situation that it's identical circuit, but in this instance it turns on that transistor then shunts the
Thyristor and turns it off
so um
yeah, interesting and once
That it does just fire once. I'm not sure how what keeps this transistor on after that's fired
It's kind of I don't think a less a very small move current flows through
The gate of this to actually [feed] this part of the circuitry, and it's just enough to keep this section circuitry active
but it seems a bit of a
Strange way to do things, but I could see that if the oh no because the gate currents quite sensitive. It's very odd
I'm not sure why.
Why it doesn't just keep trying. It does literally just try striking once and that's it
and then it seems to just power this transistor for long enough that it just holds it in that state and
very odd
Unless it's the fact that it
it may actually
fire this just long enough to actually spike that just to increase the charge in that capacitor again. It's odd, I'm not 100% sure there
I'm not sure. It's quite an odd arrangement, but it does work and
It's got the thermal fuse. Just underneath this component here. That's presumably in case
Something goes wrong, and it just passes current continually. And to stop it getting too hot that's also in the vicinity of those diodes and
So any sort of problem in that area
That caused it to conduct continually it would result in it theoretically just
Ending its own life and bring that tripping that little thermal fuse, which is a [one-shot], and it just means it kill the unit
But these are available as conventional starters or with potted leads, you know
The whole the circuit board with leads attached in one of these cases, then filled with a potting compound to make it completely water tight
Presumably for outdoor applications and for situations where you just want to use this
inside a fitting because they are pretty reliable they they do last for a very, very long time
If you've got any thoughts on what makes this
circuit stay
latched so to speak
what keeps this transistor conducting to stop this
turning on again
Leave a message in the comments if you can guess what's actually making it do that. It seems it seems
non-intuitive, interesting nonetheless