Small CRT Questions
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From: FyberOptic on 24 Jun 2010 20:51 Hey folks. I came across a portable 5" B&W TV which has some minor tuning problems, so I thought this could make a good platform to finally experiment with controlling a CRT on my own. It might be fun to try creating vector graphics and such from a microcontroller, or rigging up a simple oscilloscope since I don't own one yet. My questions are mostly regarding voltages. What kinds of voltages should I generally expect to drive deflection coils/plates? And would those be in DC or AC? It seems like DC would do it considering it's basically an electromagnet driving them as far as I know, but I don't know enough about it to be making any premature assumptions. I would also assume that the voltage needed would depend on the tube size, so a larger TV would require more perhaps? I would also need to be able to control the intensity of the electron beam. Or at least an on/off. So, again, I'm curious what type of voltage one should expect to apply, and AC or DC? The TV in question just runs off of 12v. So I'm also wondering how much voltage it might potentially generate at the flyback for this small kind of tube? I'm afraid to put my dinky multimeter to the task in case it goes beyond the 750v setting (not that I'd feel comfortable even putting that much to it). This is more just out of curiosity, since I'm sure it's still plenty enough to give me a good shock. I'm still not sure what connection is what yet either until I do more investigating. I'm assuming that neither the deflection nor the intensity requires very much voltage. But until I know more of what to expect, I'm afraid to go sticking a meter or anything on it just yet. Any info or pointers to a resource with information on such things would be much appreciated!
From: Tim Williams on 24 Jun 2010 21:27 Best start, trace the circuit board around the flyback transformer. Get an idea for its pinout and what voltages it made. You can drive it basically the same way to get the same voltages, then add your own signals (video and deflection). Video is typically applied to the cathode, in the 0-100V range, grid grounded. Focus will be in the 800V range, and 2nd anode, maybe 8kV for that size tube. You'll want at least 4kV. Current is under 1mA, so the cathode is quite easy to drive, though you'll want a fair bit of bias current if you want high bandwidth (NTSC is only a few MHz, not a big deal). The deflection coils will probably be low voltage, so you can drive vertical from a +/-15V constant current amp. I tried this myself, and discovered the horizontal coil is actually quite awful, much worse to drive than vertical. I think it's primarily intended for the circuit it's placed in, and doesn't really do good elsewhere. It's not a simple RLC component. Incidentially, don't let the fine wire and ferrite core fool you: the vertical deflection coil isn't high inductance. Take a close look, the windings are wired out of phase, hence producing a "total" fringing field. The ferrite core acts like two pole pieces, with zero magnetic field in the middle between the windings. I once made a simple setup, using a comparator for sync seperation, RC + comp for vsync seperation, and a pair of one-shot ramp generators for sweep. Boring linear amps with current feedback were driving vertical deflection, but as you can see, the slew rate was a bit weak at +/-15V supply. Video: http://www.youtube.com/watch?v=9-y8fXgMx3w Picture: http://myweb.msoe.edu/williamstm/Images/CRT_2.jpg HSweep waveform: http://myweb.msoe.edu/williamstm/Images/CRT_HSweep.jpg notice clipping during retrace (coincident with the bright edge on the picture), and the slow RLC slope in the ramp region. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms "FyberOptic" <fyberoptic(a)gmail.com> wrote in message news:6ea968a0-8d5e-4e27-a85c-4bdfee3f8346(a)j8g2000yqd.googlegroups.com... > Hey folks. I came across a portable 5" B&W TV which has some minor > tuning problems, so I thought this could make a good platform to > finally experiment with controlling a CRT on my own. It might be fun > to try creating vector graphics and such from a microcontroller, or > rigging up a simple oscilloscope since I don't own one yet. > > My questions are mostly regarding voltages. What kinds of voltages > should I generally expect to drive deflection coils/plates? And would > those be in DC or AC? It seems like DC would do it considering it's > basically an electromagnet driving them as far as I know, but I don't > know enough about it to be making any premature assumptions. I would > also assume that the voltage needed would depend on the tube size, so > a larger TV would require more perhaps? > > I would also need to be able to control the intensity of the electron > beam. Or at least an on/off. So, again, I'm curious what type of > voltage one should expect to apply, and AC or DC? > > The TV in question just runs off of 12v. So I'm also wondering how > much voltage it might potentially generate at the flyback for this > small kind of tube? I'm afraid to put my dinky multimeter to the task > in case it goes beyond the 750v setting (not that I'd feel comfortable > even putting that much to it). This is more just out of curiosity, > since I'm sure it's still plenty enough to give me a good shock. I'm > still not sure what connection is what yet either until I do more > investigating. > > I'm assuming that neither the deflection nor the intensity requires > very much voltage. But until I know more of what to expect, I'm > afraid to go sticking a meter or anything on it just yet. Any info or > pointers to a resource with information on such things would be much > appreciated! >
From: Phil Allison on 24 Jun 2010 23:07 "FyberOptic" > Hey folks. I came across a portable 5" B&W TV which has some minor > tuning problems, so I thought this could make a good platform to > finally experiment with controlling a CRT on my own. It might be fun > to try creating vector graphics and such from a microcontroller, or > rigging up a simple oscilloscope since I don't own one yet. ** First you need to realise that the " CRTs " used in TV sets are very different animals to the ones used in scopes. TV set tubes are " picture tubes" and use very high voltages (10 to 25 kV) for beam acceleration and focusing and have NO internal plates for beam deflection. That is done with magnetic fields produced by coils mounted on the neck. They whole design is intended to produce a "raster" on the screen with hundreds of horizontal lines. For playing around, you really need to get a small * scope tube * - the voltages used are far more manageable ( 500 to 1kV) and deflection is electrostatic and hence uses only low voltages and virtually zero current. This page has some interesting circuits for very simple scopes: http://www.electronixandmore.com/project/7.html ...... Phil
From: John Devereux on 25 Jun 2010 03:09 "Phil Allison" <phil_a(a)tpg.com.au> writes: > "FyberOptic" > >> Hey folks. I came across a portable 5" B&W TV which has some minor >> tuning problems, so I thought this could make a good platform to >> finally experiment with controlling a CRT on my own. It might be fun >> to try creating vector graphics and such from a microcontroller, or >> rigging up a simple oscilloscope since I don't own one yet. > > > ** First you need to realise that the " CRTs " used in TV sets are very > different animals to the ones used in scopes. > > TV set tubes are " picture tubes" and use very high voltages (10 to 25 kV) > for beam acceleration and focusing and have NO internal plates for beam > deflection. That is done with magnetic fields produced by coils mounted on > the neck. They whole design is intended to produce a "raster" on the screen > with hundreds of horizontal lines. > > For playing around, you really need to get a small * scope tube * - the > voltages used are far more manageable ( 500 to 1kV) and deflection is > electrostatic and hence uses only low voltages and virtually zero current. > > This page has some interesting circuits for very simple scopes: > > http://www.electronixandmore.com/project/7.html Nice! This was my first entry into electronics as a ~13 year old, trying to make a large black and white TV into such a "scope". It worked, sort of. Not very well of course due to the coil inductance. But I got a waveform displayed without killing myself. -- John Devereux
From: Phil Allison on 25 Jun 2010 08:31
"John Devereux" > > "Phil Allison" > >> ** First you need to realise that the " CRTs " used in TV sets are very >> different animals to the ones used in scopes. >> >> TV set tubes are " picture tubes" and use very high voltages (10 to 25 >> kV) >> for beam acceleration and focusing and have NO internal plates for beam >> deflection. That is done with magnetic fields produced by coils mounted >> on >> the neck. They whole design is intended to produce a "raster" on the >> screen >> with hundreds of horizontal lines. >> >> For playing around, you really need to get a small * scope tube * - the >> voltages used are far more manageable ( 500 to 1kV) and deflection is >> electrostatic and hence uses only low voltages and virtually zero >> current. >> >> This page has some interesting circuits for very simple scopes: >> >> http://www.electronixandmore.com/project/7.html > > > Nice! > > This was my first entry into electronics as a ~13 year old, trying to > make a large black and white TV into such a "scope". It worked, sort > of. Not very well of course due to the coil inductance. But I got a > waveform displayed without killing myself. ** Ha, ha !!! I did the exact reverse at age 16. Having built a very basic valve scope with a Philips 3 inch tube, http://members.chello.nl/h.dijkstra19/big/crt/CV2431-big.jpg I decided to connect it to the internals of my 17 inch B&W TV - after a few false starts and a few * sparks* I was very delighted to get a small green and black viewable pix. I still have the same scope and it still works. Got 5 more besides now. ...... Phil |