This home-brew opto-isolator has been further simplified as shown below in the circuit diagram.
- 5ADP1v2 Defn & Grid Mod 3-Model.gif (28.05 KiB) Viewed 16235 times
The input/low-voltage side is a straight-forward current driver for two very bright red LEDs. There is a bleed current of about 3mA through the LEDs at all times which also serves to ensure no semis are ever cut-off. (R107) More on this soon. The input voltage is 4.0V p-p provided by preceeding circuits, the plug-in boards mentioned previously.
The output/high-voltage side consists of a moderate speed photo-diode feeding a grounded-base stage, about 3V p-p appears across R121. This is boosted to a suitable voltage by TR122-3, as shown around 35V p-p.
R122 along with the bleed current through the LEDs ensures that at 0V input TR122-3 are just slightly conducting such that there's only a few standing volts appear across R126, again this is to ensure nothing gets cut-off. The items shown in blue colour will need to be adjusted for the particular CRT type as well as each individual specimen. Until I get the CRT run-up in it's final configuration and take some voltage measurements these could well change for myself. The optical arrangements and component types play a large part in the gain of this, this will have to be adjusted in each case.
Viewing the waveforms below and the rise/fall times gives a bandwidth af about 2.5MHz. (Rise-Time waveform was the same.) In this case I used the scope probe with its 15pF of input capacitance to simulate the CRT and strays in the final application.
I have changed the semis to European-friendly versions, TR101-3 will be changed to 2N3904s in the final version just to ensure they're OK, I don't anticipate any problems though. These are not 500MHz (Ft) devices, 300MHz, but they seem OK.
R107 serves to provide the bleed current through the LEDs, varying the voltage appied will shift the black-level at the gid, currently it's simply tied to -12V.
Photo shows rough-and-ready nature of this many times rebuilt prototype.
More to come for sure...
Steve A.