5FP7 project starting with the focusing coil.

[ppppenguin]

The lighbulb trick is a variant on the lamp limiter. Use a low wattage bulb such as a 40W.

Definitely have to scale with frequnecy. Your constant of 42 (related to the "transformer equation" mentioned by Ed implies 50Hz.

[AdrianPH]

But before I go any further I could do with understanding why the line scan is double the voltages of the DC tests? Is this because of back EMF? Andy.

Here's hoping this is correct, but I would not know which inductance to use, just the scan coils or that of the transformer, or the scan coils through the transformer. I will go for the first. As I am seeing 26 Volts pk-pk across the scan coils and when measured they have an inductance of 63mH, I am pushing close to 204mA through them over 2.3 milliseconds Using L x di/dt I would only get an induced emf of 5.6 Volts so the 13.6 to 14 Volts would be closer to 19.6 Volts, so I still seem to be a bit out.

I will recheck the inductance of the coils as I have added frame coils over the top and there is also the focusing coil close by so i will check again what the inductance is in case it has increased.

Adrian

[ppppenguin]

The inductance is that of the scan coils multiplied by the square of the transformer ratio. Same as any matching transformer.

[AdrianPH]

Jeffrey, I can appreciate that the valve would see that inductance when looking through the transformer to the scan coils, but measuring the peak to peak voltage directly on the scan coil windings, I am wondering what inductance I should be using at that point.

I will get the box out and measure in case of increase.

Adrian

[AdrianPH]

To late for an edit.

The Line scan coils are still the same, this is measured, OK with my simple tester, at 63mH across the Line scan coils when not wired to anything else. If I try to measure the inductance with the line scan coils wired to the secondary of the transformer, I see 0.11mH.

Looking at the secondary of the transformer I see 0.1mH

Adrian

[ppppenguin]

Inductance measurements are notoriously tricky but let's take those figures at face value. Coils are 63mH. When measured from the primary of the transformer you see 0.11mH. The ratio is about 570. Take the squre root and the transformer ratio should be about 24:1. Is that correct? That also implies the voltage on the primary of the transformer should be 24x higher than that on the secondary.

The inductances of the transformer windings without the scan coils connected are not relevant.

[AdrianPH]

The measurements were from the secondary side of the transformer, the transformer has an 8.8:1 ratio primary:secondary.

In my mind I am assuming that the transformer secondary is providing the pd to drive the current through the scan coils, and when driving the scan coils to get the required deflection, 26 Volts peak to peak is what I am seeing on the scope, actually across the terminals of the scan coils. 

In an earlier post it was worked out that the valve would see an inductance around the 4.8 H mark. = 8.8 x 8.8 x 63x10^-3.  It was also said that the transformer inductance did not come into it because of the square of the ratio!

What I am trying to figure is why the transformer requires double the required DC voltage for the required deflection, which I calculated was 13.6 Volts, sticking back emf in there suggests and extra 5.6 Volts which would only take it to 19.6 Volts so I am still not understanding where I am going wrong in my thoughts.

The primary peak to peak on the scope is showing as 230 Volts and that is as I would expect with the 8.8:1 ratio seeing 26 volts at the secondary.

Adrian

[AdrianPH]

Another war and peace.

I had a free day, so decided to see if I have been making any measurement errors that I could find also to measure the current flowing from the secondary of the transformer to the line scan coils, so a 0.5 Ohm resistor was placed in series with the scan coils and a scope probe connected up across it. 
Another scope probe was connected across the scan coils to display the applied voltages.  Everything was DC coupled.
I set the vertical trace to give a reasonable height, 75mm and marked the CRT glass at the ends of the trace with a marker pen.
The scope display is here.
   

The straight line is my Reference Volts either the HT rail for the primary side or 0 Volts for the secondary.
The tallest trace is the voltage across the scan coils and was set to 5 Volts per division, the more rounded trace is the voltage across the 0.5 Ohm resister and was at 20mV per division.  The horizontal divisions are 1 mS, so the sweep rate is a bit slower than it should be at around 3.5 mS ,running at approx 285 Hz.

Looking at this suggests 28 Volts p-p across the scan coils, with 92mV across the 0.5 ohm, so current at 184mA p-p.  I think I have too much damping on the transformer, but was not sure if that could be the issue.

I re did measurements with just the DC power supply across the Line scan coils and used it to deflect the trace to the CRT marks and measured DC current and volts with a couple of multimeters. The change in DC to get the same height of deflection was 11.29 Volts and the current measured at 178.1 mA.  So the current appears to be the same as expected between a line scan deflection and DC deflections,  Line scan coils DC resistance at 63.3 Ohms, 63 mH.

After this I decided to play with the damping of the primary circuit, as the above was very heavily damped.

Horizontal trace now at 0.5mS per division, after resetting the scan rate to 400 Hz.

If I looked at the voltages without any damping this is what I got : -
   

If I damped what I would guess is still heavy but stopped any overshoot I got this with a 6K8 and 0.1uF across the primary: -
   
The tallest peak is the primary volts at 50 V per division, 290 V peak-peak. 
The secondary is the next sawtooth (smallest trace) at 20 Volts per division, 40 peak-peak
The rounded waveform is again through a 0.5 Ohm resistor at 20mV per division showing the scan coil current at 184 mA.
The primary and secondary volts provide for nice sawtooth waveforms, but the Scan coil current which I would say is what drives the trace is not good, like in the over damped picture above.

So starting again, this time just focusing on the Scan coil current, I have a 56K Ohm resister across the primary, this stops the ringing after the initial flyback spike.
   

This has a totally different look, but if I neglect the flyback pulses things are starting to make a lot more sense to me looking at this.

Both primary and secondary voltages have the big flyback spikes, still on 50 and 20 volts per division for them, with the secondary spike approx 68 Volts reference, the primary must be at least 600 Volts above HT.

But looking at the secondary voltage after the initial flyback it is now down to approx 10 volts swing.

So to check I moved the HT trace out of frame and just looked at the secondary scan voltage trace and the scan coil current trace.

   

The voltage trace is now 10 Volts per division and the current trace still at 20mV per division.
This is showing a 10 Volts peak -peak ignoring the flyback.  The current trace at 84mV across 0.5 Ohm resistor, so 168 mA Scan current for a trace of approx 75 mm. This being more what I would expect after doing the DC tests.

So the over damping in order to get good Voltage sawtooth traces was my error, it needed the pulse.  I guess the higher voltage at 40 Volts peak to peak I was seeing and confusing the hell out of me, was due to the secondary voltage having to build up the scan coil current to peak then having to reverse the current as I had damped out the flyback pulse which would normally take care of it.

Adrian

[AdrianPH]

I have recently taken a transformer to bits, as far as I can tell it is a EI-66 transformer with a 22mm centre limb measurement, there are 61 laminations in total with each having a thickness of 0.5mm so an area of 6.71 x 10^-4 M².

This next bit is me trying to get things correct, so a check of calcs and thinking would be appreciated.

It was a 240 Volt primary and I unwound the primary turns using the hand winder and got 1,100 turns, feeding this back into the formula from Ed rearranged for B, I got a value of 1.46.

Now if I am correct in my thinking  for the frame coils I would need a voltage across them of 51.3 Volts at 100mA peak to peak, I would like the primary impedance to be around 4K5 Ohms for the valve, I worked the secondary impedance to be 513 Ohm so an 8.779 ratio on that giving me a turns ratio of 2.96:1.

Stop working in pk-pk values as pointed out by Jeffrey, 51.3 Volts Pk to Pk is 18.14 Volts rms, so a primary voltage of 18.14 x 2.96 is 53.73 Volts rms.

Going back to the formula, this time using 4.0 rather than 4.44 as I am after a sawtooth? and at 12.5 Hz frequency.
Turns = 53.73/(4.0 x 12.5 x 1.46 x 6.71 x 10^-4 M²) means I need 1097  primary turns.

Secondary would be 1097 / 2.96 or 371 turns, I may increase this slightly by 1.05 to allow for losses to 390 turns on the secondary.

I am wondering if I could also treat the current similar with peak to rms when it comes to capacity, i.e. I need 100mA peak current so rms this would be 36 mA in the secondary, the primary would then be 12 mA. Or if it is better to rate the wire for the peak current,  34mA primary and 100mA secondary, or possibly somewhere in between to give some leeway.

The previous primary of 1100 turns was with 0.25 diameter wire, probably 34 awg and was quite full at that so my choice of wire could make the choice if I can go for push pull or not.

Adrian

[ppppenguin]

I haven't followed your calcs in detail but here's a quick sanity check. Primary voltage is 54V. If it was a 50Hz design that would allow 4x54=216V on the primary. Hence the number of turns you need should be a little less than the original 240V primary. You've got about the same number so that leaves a bit of margin which is good, provided you can fit all the wire on the former.

Heating in the copper is based on RMS current, the usual I*I*R, so you can rate the wire for that. Peak current not relevant. Allow a little margin.

The big unkmown is what the DC in the primary will do. This is beyond my knowledge. Best wait for Ed Dinning to give his opinion. Pushpull won't have DC in the primary so that's safe.