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Fluorescent Lamp Ballast Project.
What happened?
The ballast was part of a 6 volt flashlight which had a broken lens cover on the
Fluorescent bulb (I figured what the heck it’s broke anyway). The circuit board is about
1 inch square and is just the right size for my project (build a solar powered porch light).
At first I thought (hoped) that perhaps the circuit was somewhat tolerant and could accept
13.6 volts. I connected the input leads to a battery and within a few moments the
transistor began to smoke, the circuit was drawing about 1.2 amps. I immediately
disconnected the power and figured I had overloaded the power transistor. Next I looked
at the circuit components and deciphered the circuit diagram and concluded that I would
at least have to change the electrolytic capacitor C1, from a 10vdc, 45 uF to a higher
voltage value. I replaced this cap with a 30 vdc, 47 uF. Next, I figured that I should
increase the resistance of R1 from 1200 ohm since I was doubling the voltage. I replaced
R1 with a 2600 ohm resistor. I reconnected the battery and it worked! The original
current flow with 6 volts connected was .19 amp and the new current flow was .23 amp
(with 13.6 volts connected). The lamp was brighter but I figured that was OK, the power
transistor did not fell hot and I figured the current flow was within limits. After about 30
minutes, the fluorescent lamp began to flicker wildly and the power transistor had
become very hot again. Back to the drawing board!
Now I have to think. I searched the Internet for the part numbers I could see and
downloaded data sheets. The transistor is an NEC 2SD882 (NPN Silicon power
transistor). The identity of the capacitors is somewhat less certain and the transformer
has no markings. The capacitor numbers I “found” on the Digikey and manufacture
websites. The C2 looks somewhat like a Panasonic ECU-S 2A103K which is a 100v,
10KpF ceramic multilayer radial leaded capacitor. TheC3 has the markings 2A562K and
looks exactly like the 2A103K but comes up in a Murata capacitor part number
GRM219R72A562KA01D which is a 100v, 5600 pF capacitor but the Murata style is
different. The Murata part number looks very like a little (very little) brick that you
solder to a circuit board.
Next I needed to find how these components behaved when the circuit was running and
see if I could tell what was going on when I turned up the voltage. I poked around on the
circuit board with an oscilloscope I bought several years ago from a TV shop that was
selling out. As shown in figure 2, I sketched the traces of the voltages I encountered and
tried to line up the timing as best I could given my limited understanding of how these
components work. The trace patterns repeat across the scope which is scanning
somewhere between 10 and 100 MHz. I lined up the traces by guess based on how the
RC circuit probably controls the transistor Q1.
Here is what I think is happening. The RC circuit builds up voltage until it goes
positive which turns on the power transistor. I can’t tell much after this occurs but I
suspect that the transformer takes the current and creates a back emf that turns off the
transistor. The switching with the inductance and capacitance creates a pulsing through
the transformer (somewhat shown in the curves and squiggles in the traces). This
pulsing, as shown in the traces, created about 40 volts AC (56 volts at 13.6 volt input)
across the fluorescent bulb. The LC circuit has a resonate frequency but I can only guess
what it is.
Here is what I don’t know.
What holds the R1 voltage to -3 volts?
What kind of transformer is this and how are the taps connected to the windings?
Who manufactures these so that I might get a datasheet?
I found a switching transformer from Rhombus Industries Inc (PN T-1165) and looked at
the winding connections but I can’t tell if this is what I have. I suspect the frequency has
a lot to do with the power and efficiency but I don’t understand enough about this kind of
electrical circuit to be able to determine how to modify it to operate at different voltages
or power levels. I wouldn’t mind fiddling around a bit to learn more about the circuit and
if nothing else, by trial and error, to find a 12 volt version of this circuit (which was the
original plan).
Figure 1.
Fluorescent Light Ballast circuit diagram
R1 – 2600 Ohm (Original 1200 ohm)
Q1 – NEC 2SD882 NPN silicon power transistor
C1 – 47uF, 30vdc (Original 45uF, 10vdc)
C2 – 10KpF (A rectangular flat green cap marked A103K and looks somewhat like a
Panasonic ECUS 2A103KBA)
C3 – A rectangular flat green cap marked A562K but slightly bigger than C2 and I found
a similar number from Murata GRM219R72A562KA01D
Marking on base of circuit board 1657-1 and 221.
11
Collector
7
Collector conducting current
0v
Base goes positive
0v
Base
-5
R1
0v
-3
RC time constant
Time
Ballast Component Voltage Trace
(Sketch)
Figure 2.
Voltage Trace from Collector and Base of Q1 and R1 at Transformer X3.
(Vertical time line implied as traces were measured independently).