Corrections/suggestions: sam@repairfaq.org
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+Vcc Q1 +----------------+
o | )::
| B |/ C )::
| +------| 2N3055 )::
| | |\ E 5 T ):: +------|>|----------o +HV
| | | )::( HV Diode, usually
| | -_- )::( built in.
| | )::(
+--|-------------------------+ ::(
| | Q2 _-_ )::(
| | | )::( Secondary (HV) winding,
| | B |/ E 5 T )::( intact.
| | ----| 2N3055 )::(
| | | |\ C )::(
| | | | )::(
| | | +----------------+ ::(
| | | ::(
| | -----------------------+ :: +------------------o -HV
| | 2 T )::
| | +---------+ ::
| | | 2 T ):: T1 - Flyback transformer from B/W or
| +-------------------------+ color TV or computer monitor.
| |
| R1 | R2
+----------/\/\/\--+--/\/\/\--+
110 27 _|_
5 W 5 W -
Pinout for TO3 metal can transistor:
_
/ O \ View from bottom (pin side).
/ o o \
( B E ) B = Base, E = Emitter, C = Collector.
\ /
\ O / C The metal case is the Collector.
Pinout for the TO220 or TOP3 plastic case transistor:
TO3Pn
TO220 _____
_____ / \
| o | | O | View from front (label side).
|-----| |-------|
|Label| | | B = Base, E = Emitter, C = Collector.
|_____| | Label |
| | | |_______| If there is an exposed metal tab, that is
| | | | | | the Collector as well.
B C E | | |
B C E
A slightly modified version of this basic circuit which I use as an RF source
to excite a glow discharge in helium-neon laser and other gas discharge tubes
is shown in: Flyback Based RF Source. This one uses
a flyback transformer without a high voltage rectifier (or with the rectifier
removed). The inductor, L1, is an addition that should reduce the stress on
the transistors and power supply by limiting current at the time each of the
transistors go into saturation just before the base drive switches to the
opposite side. I have not specifically tested this circuit with the inductor
but have used it with similar inverters.
These designs are similar to circuits found in: "Build your own working Fiberoptic, Infrared, and Laser Space-Age Projects", Robert E. Iannini, TAB books, 1987, ISBN 0-8306-2724-3 and many other places.
For larger (e.g., color TV or monitor) flybacks, or use with more than 12 VDC in, transistors with higher power ratings may be needed for sustained operation in addition to a good heat sink. An alternative is to parallel more than one power transistor along with small (e.g., .05 ohm, 2 W) current balancing resistors in series with their emitters.
I built one from a dead Mac-Plus flyback from which I removed the (dead) primary windings.
HOWEVER, if you add a high voltage capacitor to store the charge, don't even think about going near the HV!
Some experimenting with different value resistors and even the number of turns on each winding may improve performance for your particular flyback.
Most modern flybacks include built-in HV rectifier diode(s) and/or voltage multiplier (tripler) so output without additional components will be high voltage positive or somewhat smoothed HV DC.
Note: this kind of flyback transformer drives the CRT directly and uses its glass envelope as the main high voltage filter capacitor. (A foot square piece of 1/8 inch Plexiglas with Aluminum foil plates makes an filter capacitor.)
During initial testing at least, a Variac on the input (or variable voltage power supply) is highly desirable to avoid blowing anything should your wiring or parts not be quite right and to gain a feel for the capabilities of your circuit before it is too late! In neither of these is available, use a 10 ohm 25 W power resistor or 100 W light bulb in series with the load (inverter) to limit current to a safe value - one that won't fry too many things too quickly.
A typical circuit is shown below:
_ T1
H o-----o/ o---- _------+ 5 A diodes
S1 Power F1 Fuse )|| or bridge
1 A )|| +---------+----|>|-------+-------+-----o +Vcc
)||( ~| D1 |+ |
)||( +----|<|----+ | +_|_ C1
115 VAC )||( 12 VAC D2 | | ___ 20,000 uF
)||( +----|>|----|--+ - | 25 V
)||( | D3 | |
)|| +---------+----|<|----+----------+--+--o Gnd
)|| ~ D4 - _|_
N o---------------------+ -
All of these parts should be readily available:
The high voltage section on the right may actually be constructed as a voltage multiplier rather than a single winding with multiple HV diodes. The rectifiers or multiplier, and/or focus/screen divider may be external to the flyback transformer in some models.
Flyback transformers used in black-and-white TVs and monochrome computer monitors do not have a focus and screen divider network.
The ferrite core of a flyback transformer is constructed with a precision gap usually formed by some plastic spacers or pieces of tape. Don't lose them if you need to disassemble the core. The ferrite core is also relatively fragile, so take care.
The focus and screen divider network uses potentiometers and resistors (not shown) with values in the 10s to 100s of M ohms so they may not register at all on your multimeter. The high voltage rectifiers (CR1 to CR3 on this diagram) are composed of many silicon diodes in series and will read open on a typical VOM or DMM.
Note that there is no standardization to the color code. However, the fat wire to the CRT is most often red but could also be black. Of course, you cannot miss it with the suction cup-like insulator at the CRT anode end.
+--|>|-----------o HV to CRT
_ 1 ::( CR1 (25 to 30 kV,
| B+ o-------------+ ::( suction cup on
Drive | )::( fat red wire)
winding < ):: +-------+
| 1.32 ):: |
| 2 ):: +--|>|--+
|_ HOT o-------------+ ::( CR2
_ 3 ::(
| 50 o-------------+ ::(
| ):: +-------+
| .11 4 ):: |
| 35 o-------------+ :: +--|>|--+
Various | )::( CR3 |
auxiliary < .28 )::( /
windings | 5 )::( \<-------o Focus
| 16 o-------------+ ::( / (3 to 10 kV,
| )::( \ orange wire)
| .12 6 )::( |
|_ 0 o----------+--+ ::( 9 |
_ | 7 :: +--+ /
| H1 o----------)--+ :: | \<-------o Screen
CRT Heater < .08 | 8 ):: | / (200 to 800 V,
|_ H2 o----------+--+ | \ brown wire)
| | |
| +----|--------o To CRT DAG
| | ground
+---------------+
-- end V1.23 --