5FP7 project starting with the focusing coil.

[ppppenguin]

The transformer inductances are not very important. What's of interest is the scan coil inductance, multiplied by the square of the transformer ratio that's then presented to the valve.

[AdrianPH]

Sorry Jeffrey, no email of post.

OK That is interesting, I have not come across that, so that means 4.94 H? Is that true for all frequencies?

Got to start the tidy up for the night.

Adrian

[AdrianPH]

OK worked it out assuming a perfect inductor with no resistance and can see where it comes from.
Night all.

Adrian

[ppppenguin]

No transformer is perfect so there will be various 2nd order effects at different frequencies. If the transformer is badly designed for the application then it can cause bigger problems.

I hope Ed DInning or Kalee20 will comment in this thread as they are our local transformer experts.

[AdrianPH]

I would hazard a guess that the DC resistance of the scan coils and secondary windings together with the resistance of the primary windings will have a part to play, the resistance of the scan coils would also be similar multiplied and added to that of the primary, so in the end the applied voltage at the primary has a lot to overcome?

Adrian

[ppppenguin]

The resistance certainly matters. At field scan frequencies the resistance is dominant, though as you can see from my photos, inductance certainly matters during flyback. At line scan frequencies the inductance is dominant, resistance causes losses and non-linearity. At intermediate frequencies, in the low kHz range, there must be a range where both matter equally.

For line scan, as you use higher EHT and wider angle deflection, efficiency really matters. At the end of each forward stroke you have 0.5 * L * I^2 of energy stored in the scan coil. So multiply by scan rate to get total power that might be used. This energy has got to go somewhere during flyback. It can be disspated in a resistor (and lossy elements of the circuit) or recovered, at least in part. That's what efficiency diodes are about. I once did a calculation for first generation UK colour TVs and reckoned the line timebase would use over 2kW of power if there was no energy recovery.

The same rules apply to field scan too, but because the scan rate is so much lower, the total power is too. Hence it's rarely worth trying to recover the scanning energy. It's probably been done, but I can't cite any examples.

[AdrianPH]

I stuck the EHT  section and added a mains power supply to an old chassis from a UKVRRR forum member, the chassis was part of a BC1151 display section off a radar unit.  With the CRT and line transformer fitted, I could progress  with more tests on the line deflection,  It meant a bit more planning and plotting response curves with a used EL84.  The project is certainly bring up topics I have probably never dealt with before with electromagnetic deflection and is definitely interesting to see things work even if not perfect.  Just takes me time to cram things on the chassis.

5FP7 part 4

Just wondering if I can find a CRT with a similar footprint to fit the chassis, one that is electrostatic deflection for later so I do not have to listen to the transformer, perhaps it will end up getting varnished.

Adrian

[AdrianPH]

The project has been tidied up slightly, fitted the scan pcb within the front of the case, protected fingers by using some aluminium panels to keep finger tips out of HT.  A small 50Volt mains transformer, bridge rectifier and cap used to supply the focus coil, fed via a variable resistor.

Lastly started with some horizontal scan coils for the frame side of NBTV and a bit of testing deflection with the bench power supply.


5FP7 part 5 video

Still a long way to go.

Adrian

[AdrianPH]

I am just thinking via the forum here and would appreciate your thought's on the matter.

I am building the Horizontal frame output for the NBTV project, I have a couple of EF91's providing a god 12.5Hz sawtooth with a reasonably quick flyback. To driving the scan coils the only way that comes to mind, is as before with an EL84 driving a transformer to connect with the scan coils. The issue is the 12.5Hz frequency.

I need 5 watts max to drive the scan coils, 50 Volts swing at a peak of 100mA. So I am thinking of trying a minimum 25 VA mains transformer as it may not become saturated at 12.5Hz.
Ideally I need a 3:1 transformer with just over 50 Volts output, but trying to see if the idea works first with something a bit easier to get hold of. Would appreciate thoughts?

Adrian

[ppppenguin]

Saturation depends on the frequency and applied voltage. Not on the VA rating. So if you want to apply 12.5Hz to a nominal 50Hz transformer you need to derate the winding voltages to a quarter of those stated. The voltages are RMS, whereas I think you're thinking of peak-peak. 50V p-p is 50/2.8 = 18V RMS. So you would need a winding specified for at least 72V RMS to work at 12.5Hz.