An earlier construction attempt (c 2015) of a CRT Greenscreen A-V monitor design
didn't entirely succeed because of some difficulty in generating the Extra High Voltages
needed to drive a 3JP1 cathode ray tube,
i.e. 1500 Volt for the deflection anodes and 3000 volt for the PDA (post deflection anode)
I tried the well known "cockcroft-walton" diode multiplier circuit, but voltages tended to drop away
under the load of the CRT and its associated circuitry.
The project therefore, has been in limbo for some time.
In 2019 I found some small/cheap self contained EHT (EHV) modules on Ebay
that may be able to generate the required Extra High Voltage levels needed.
My initial setup looks promising in bench tests.
Two modules are connected so: DC supply inputs paralleled, outputs in series aiding.
Output is "fully floating" and in no way electrically connected to the input ccts.
Variable voltage input is from LM350AT 3 terminal adjustable regulator. (see schematic)
DC supply input was optimum at 5 volt, the heatsink then being only warm with the 550mA current.
Higher inputs i.e. 12V meant the heatsink getting hot in wasting power across the LM350T.
EHT voltage output measured on (high input-impedance) DMM by using 100:1 ratio resistive voltage divider.
With 2 modules connected as shown in schematic a 2.5 volt supply will give 1500V x 2 = 3000V
A variable pot' in the voltage supply regulator allows control up/down the high-tension output
Manufacturers spec'
DC 3.7V to 1800V "boost module"
Can work continuously (100% duty cycle)
Max recommended input voltage 4.2V
Input current may be up to 2A
Exercise safe working practice because of high-voltage electrical shock hazard
Output capacitor (22nF 2000V rating) may retain hazardous charge after switchoff !
Size 41 x 26 x 20mm
Cost AUD$5 ea (3Q 2019)
The circuit is tradionally known as a "blocking oscillator"
The transistor is initially biassed 'on' by current flow thru its base resistor
Collector current steadily rises due to transistor amplifying action.
When the transistor (and transformer core) saturates (current value levels out)
The rate of change of current flow is such, that by transformer action a pulse is induced in feedback winding
Because the pulse polarity; transistor is biassed-off (this is 'blocking')
Collector current drops to zero.
Collapsing field in primary produces a potential in the secondary by transformer action
obeying the law of electromagnetic induction. Cycle then repeats (ad nauseum) i.e. oscillation
Frequency of oscillation measured at 37 - 60KHz (depending on supply voltage)
Secondary voltage is rectified in a dual "peak-clamp" type, voltage quadrupler circuit.
The voltage quadrupler circuit gets the most from the least,
I have sucessfully used it in a CRT test jig to obtain the necessary high voltage.
© 3Q 2019