Making a very simple oscilloscope Pt 5: HV gen (380 V DC) via the mains & driving Hor-Vert plates
Part 5 of the series “How to make a very simple oscilloscope”. In this case (only) a VLOG with a schematic. Please read the description/textbox first. Next video (18 okt 2022) is here • Making a very simple oscilloscope Pt 6: th...
Important: the def. schematic of the simple oscilloscope is here (5 Dec 2022)
• Making a very simple oscilloscope Pt.15: d...
Disclaimer: don’t work with the high voltages in this circuit when you are a beginner or have no basic understanding of what happens here in terms of electricity and/or safety issues.
Circuit & video NOT made for children or minors (!). Risk of high voltage shock.
Part 5 of the video series: how to make a very simple oscilloscope. I have now (15 October 2022) made a new High Voltage supply, not with an (old school) TV line transformer (16-18 KC) but with 2 simple transformers that work on 230 V AC (50 Hz) primary, and 24 Volt (30 mA) AC on their secondaries (nominal).
CORRECTIONS: the DC output voltage is not 270 Volt ( I say that on 2.38 in the video) but 380 Volts DC.
On 6.50: the sloppy wiring gives a phase shift on the horizontal/vertical CRT plates, generating an open oval form on the screen.
On 14.25: perhaps this is not clear. The solution is to give the horizontal or the vertical plates a static voltage (out of the HV supply) via a potentiometer of 1 M (1 mega-Ohm) connected to both plates to move the dot to a specific position on the screen, the electron beam (negative) is repelled or attracted by static voltages, (+) or (-).
With all the secondary coils in series (video) we are talking about (say) 4 x 24 V = 120 Volt. The open voltage is much more, say 150 V AC. Doubled & rectified that AC (via the diodes, watch the video, voltage doubler schematic) we have an open voltage of approx. 380 Volts DC to drive the CRT tube.
That is the Anode-Cathode voltage that the DG 7-32 needs to work properly conform its datasheet.
Part zero : • Talking about a Cathode Ray tube with stat... (about the DG 7-32 CRT tube)
Part 1: • High Voltage circuit 1KV- 4KV for an oscil... (about HV circuits on 16-18 KC, line transf.)
Part 2 : • Making a very simple oscilloscope Part 2: ... (first setup, video named part 2)
Part 3 : • Making a very simple oscilloscope Part 3: ... (shielding the tube for EM influences)
Part 4: • Making a very simple oscilloscope Pt. 4: a... (a HV generator on 16-18 KC schematic)
Part 5: (this video, 15 October 2022)
In the meantime I made this video: “what to do with a sawtooth generator”. Such a sawtooth generator is the time base for an oscilloscope. Link is here • What to do with a sawtooth generator ? (A ...
Again: also with this proper (conform the datasheet of the DG 7-32) voltage I could not get the brightness grid work properly.
I know that the brightness grid must be made negative, in the order of (say) 100-120-160 Volts, etc. In fact it must be made more negative compared to the cathode of the CRT, making it possible to “shut off” the electron beam from the cathode to the anode completely.
The effect is than (normally) that the screen gets “black” (no electrons can hit the screen any longer, they are all repelled by the brightness “grid”). Perhaps this is the reason why this DG 7-32 was “dumped” on a flea market. Anyway, it works and I will not search any longer for better options.
I think I am going to use this HV circuit and not the circuits with the 16 KC HV (old TV sets) flyback coils. They were in earlier video’s.
But: feel free to use them, they also work good (!). Always use a 10-32 Ohm 10 Watt series resistor to supply them via the voltage rail.
Otherwise your transistors will burn out in these HV oscillator circuits that I have published. For 30 Volts: a 30 Ohms/10 Watt resistor, it safeguards the max. current that can flow. Use Ohms Law: U=IxR.
That 16-18 KC HV output via that line transformer waveform (though smoothed out via the end-capacitor) has a (tiny) effect on the dot on the screen, be it for the horizontal or vertical deflection line.
But that 50 Hertz (sine wave) of this new schematic (now, in this video 15 Oct 2022) can (in theory) also effect that dot on the screen, though there are only harmonics (in that case, because it is a sine wave) that don’t hamper the waveform.
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