Re: Strange DC Flyback output waveform?
Thomas W, Thu Aug 08 2013, 04:00PM

I could be wrong, but that looks in my mind like a low pulsewidth signal with unreal amounts of ringing.

Re: Strange DC Flyback output waveform?
leviterande, Thu Aug 08 2013, 04:48PM

Indeed you are correct, Being afraid of damaging my Oscilloscope, I used a very low input, that is.. the transistor base current is very limited and the duty cycle as well as the frequency being not the optimal one. The spark length this way is only 1-3mm. Maybe I should increase power and the wave should change Il try but I am not sure about that.. Also where does the ringing come from I wonder. Thanks

EDIT: Ok, I have tried higher power levels now for both the 555 and feedback versions and the waveforms are basically still the same as you can see in the pics. The feedback version oscillates as I could see now at 43khz . The 555 version is set to oscillate at 27khz and 59%duty cycle.. I tried to vary the frequency and duty cycle but the waveform is basically the same hmm...

For a check, I just took a small ferrite HF transformer that has no rectifier or anything. hooked it to my 555 timer circuit. The output on that one was as expected a clean AC since wave. So it must be something with the flyback ??

Thanks

Re: Strange DC Flyback output waveform?
Sigurthr, Fri Aug 09 2013, 12:54AM

It may help to know that the secondary "ground" of most commercial flybacks will actually put out a "low" HV ac signal. I was able to wirelessly ignite several gas discharge tubes by capacitive coupling to the HV return from a standard DC Flyback.

I think you are seeing ringing and the AC signal from the HV return. Try placing a forward biased HV diode between the HV Return and your scope ground probe.

Re: Strange DC Flyback output waveform?
leviterande, Fri Aug 09 2013, 03:04AM

Thanks, Il have to use higher voltage diode. Anyway can I be sure that the flyback puts out dc, I mean it is , or it would be able to charge any caps

Re: Strange DC Flyback output waveform?
Alex M, Fri Aug 09 2013, 10:51AM


Is this why in most television circuits I have seen the pin we use to draw arcs from (ABL) seems to bugger off to some IC, and the actual HV ground point seems to go through the potential divider resistors?

Re: Strange DC Flyback output waveform?
Proud Mary, Fri Aug 09 2013, 03:58PM

At the moment, you have what is effectively a transformer secondary with half wave rectification. Clearly, a reservoir or smoothing capacitor is needed to produce DC. (I say 'effectively' because of the multi-winding series-diode construction of the modern 'diode-split' LOPT)

In TV CRTs, this capacitance is sometimes provided by the mu metal screen inside the tube alone, and sometimes this capacitance is added to with an additional smoothing capacitor.

If you strap a capacitor of about 1000 pF of suitable HV rating across the output, the waveform will be corrected for the small currents drawn by CRT anodes. The optimum size of this smoothing capacitor will depend on both the frequency and the load.

Google half-wave rectifier and peak detector to learn more.

Re: Strange DC Flyback output waveform?
Erlend^SE, Fri Aug 09 2013, 08:24PM

Alex1M6:
ABL is actually current sense.
ABL(Auto Beam Limit) is used to control the max beam current to avoid burning the screen and overloading the HV supply.

The screen/focus divider gnd can be made into a potential divider by having some resistor toward gnd(or abl) and messuring the HV that way if wanted.

Any of the other windings + a diode should give some clue about the output waveform(may need to turn the diode)

Re: Strange DC Flyback output waveform?
leviterande, Sat Aug 10 2013, 03:02AM


Thanks for your help. hmmm it is then all maybe because of "multi-winding series-diode",
Anyway I have a HV 850pf cap so I will try it.
I am wondering though, shouldn't the output of the flyback be simply pulsed dc. in other words: I thought it should look like this .
From What I already understand all a smoothing capacitor does is taking the half wave rectification pulsing and make it flat or near flat with a ripple of course if there is any low resistance connected

Re: Strange DC Flyback output waveform?
Proud Mary, Sat Aug 10 2013, 10:37AM


The half-wave pulses shown in the diagram you link to are those we would get from an ideal rectifier being fed with a sinewave - such as the mains supply in your house

An LOPT is not fed by a sinewave, but by sharp pulses derived from the almost on/off switching action of a high voltage power transistor, or in olden times, a thermionic valve.

An LOPT will not work properly if fed by a sinewave. The heart of LOPT action is the sudden collapse of the magnetic field at the end of a pulse of electricity in the primary. As it collapses, the energy stored in the magnetic field cannot just go nowhere and disappear (which would violate the The Law of Conservation of Energy), so it is converted back into electricty in the secondary, producing the EHT (Extra High Tension - an old term still in use) spikes you can see on your oscilloscope.

A sinewave does not fall fast enough from its peak back down to zero to work effectively in this way.

Also remember that modern LOPTs have lots of other duties to perform in a TV besides providing EHT for the CRT anode.

Here is a basic paper on the operation of LOPTs in TVs which I think you will find helpful: It uses American terminology, so that what I call 'line output' it calls 'horizontal output', and what I call a Line Output Transformer (LOPT) it calls a 'flyback', as most on 4HV do, but the way it works is just the same!

Good luck with it all!

Re: Strange DC Flyback output waveform?
leviterande, Sat Aug 10 2013, 02:54PM

Hi again Proud Mary and thank you for the paper. I have read lots of papers about the LOPT and my brain doesn't have enough ghz to still grasp how exactly it in details works but I am going to deep analyze that useful paper:) I don't know but it seems to me that tesla coil basic operation theory is so much easier to understand IMO.. I could be wrong(I mean the traditional tesla coil.., the SSTC technical side could be much different though)


Alright:) I have now tried 3 things across the flyback with moderate power 3.5vPSU input(probe is 1ft from +terminal):

1- A diode between 0v and ground(both diode sides tested)
2- A capacitor
3 A diode and a capacitor(diode hooked to 0v of flyback)

In short it seems that the waveform didnt change much.

1- First 2 pics.Amplitude decreased dramatically when hooking the diode. Only thing that happened here almost as it should was: the damped ringings turned only almost into one directed DC directed pulses as you can see in pictures. However there was still AC(perhaps its because the diode isnt perfect...ha?)

2- Third pic. Tried both 850pf and 4700pf ceramic disc caps, with one terminal end of cap and flyback 0v to the ground. Other end of cap hooked to + of flyback. Both caps caused the amplitude decrease a lot. With the 4700pf ceramic disc test as you can from the picture the 4700pfcap only smoothed very little of the ringings but amplitude was towards the negative side?? I am confused to what to make out of this. Is this how it suppose to look like?

3- When hooking both the diode(tried both sides) and 4700pf together, I get no result at all at the oscilloscope.

I am a little puzzled, the diode situation could be explained by imperfection of diode but what a about the smoothing capacitor that didnt smooth the out put?

Re: Strange DC Flyback output waveform?
Proud Mary, Sat Aug 10 2013, 04:22PM

Modern LOPTs are almost all of the 'diode split' type, which produce a DC output, so I'm not clear why you are adding an extra rectifier to its output. It doesn't need one.

If you could post a picture of your circuit arrangements, together with a circuit diagram, it would reduce the amount of guesswork in helping you sort it out.

Re: Strange DC Flyback output waveform?
leviterande, Sat Aug 10 2013, 06:55PM

Proud Mary, hehe no no, oh yes I know it is a DC dst flyback type and it charges my capacitors when I tested so it is DC certainly. However, I put the diode in accordance to the sugestion made by Sigurthr
"I think you are seeing ringing and the AC signal from the HV return. Try placing a forward biased HV diode between the HV Return and your scope ground probe."....to see why the output wave in the first place have AC components
I put the capacitor here to see if the output would be smoothed but as you can see from the waveform, it wasnt so smoothed ..

The driver circuit is a basic typical feedback one. the probe is 1ft from the + of flyback. Here are the 4 arrangements I made in my testing. The capacitor was a 4700pf ceramic disc type. The diode connected to ground was a HV fast diode and I tried both ways, with cathode to ground and with anode to ground. So now I wonder why the waveform was as it was and was never smoothed with the 4700pf cap connected, even with different size caps, all that happened was higher or lower amplitude of the same waveform

Re: Strange DC Flyback output waveform?
Proud Mary, Sat Aug 10 2013, 08:26PM

Good, now I think we can understand what is going on.

1. Forget what I said about the waveforms to be expected in TV service, and we will assume that the primary waveform is crudely sinusoidal.

2. Once every cycle, the voltage across the series diode-split secondaries climbs high enough to break down the air at the end of your EHT lead, probably as a corona discharge with streamers.

3. Until the voltage needed to break down the air is reached, no electromagnetic energy is generated that can cross the one foot gap to your probe.

4. As soon as the voltage falls below the ionisation voltage, the radiated electromagnetic energy will stop.

5. This is why you are seeing pulses at your probe. It would look very different if you were able to monitor the voltage across a load, such as a resistor, when you would see much more like the true waveform. I do not recommend you try this until you have got more experience, as an oscilloscope can very easily be ruined beyond repair by voltages of this order.

6. I have been a bit vague about 'electromagnetic energy', because I think that at a distance of one foot the signal arriving at your probe will have a mixed content. Some of it may come from the differential voltage gradient between the EHT terminal and your probe - thinking of the air being a resistor - a potential divider - in this case. And some of it will come from the detection of electromagnetic waves - i.e. radio waves - which no doubt are causing interference across a wide band of frequencies from VLF to UHF.

Your probe setup is completely unable to remotely detect DC appearing at the LOPT output. This is why the signal disappears when you put a larger capacitor across the output - the pulses have been smoothed out.

To sum up, the air around your EHT terminal is acting a bit like a zener diode. No current can flow until a certain (high) voltage is reached, and it will stop again at once when the voltage falls below the level needed to ionise the air. This is what is giving rise to the pulse wave form.

Thank you for showing us a very interesting experiment!

Re: Strange DC Flyback output waveform?
leviterande, Sat Aug 10 2013, 10:39PM

Aaah, thank you, finally some solving to make me sane. I made sure to minimize any corona discharges at the terminals by adding big spheres but obviously that isnt near enough then. I was nearly pulling my hair into baldness:) I suspected what you said right before I did the testing experimentation but I had no other way to test. Best would be to directly connect the probe across the terminals but the oscilloscope would go to heaven ofcourse.

(BTW when I connected the capacitors the waveforms still were there, and if I connect a diode the waveforms were still there but when I connected the diode and capacitor.. thats when all signal is 100% lost :) )

So the only way now is to measure directly somehow which would need at least a very high resistance added to the probe( to make the probe a 1000x and lots of other variables...).. BUT! Hmmm isn't this "remote probe testing" I just used, also used successfully in reading HV devices like Tesla coils or am I wrong, That's why I thought it would be good to use this remote probing in the first place hmm.., the method seems to be working fine with many Tesla coils if not all I have seen .. yet a tesla coil isn't exactly "electromagnetically silent" if you know what I mean.

Thanks

Re: Strange DC Flyback output waveform?
Antonio, Sun Aug 11 2013, 02:14AM

If you don't have anything connected to the flyback transformer output, the waveform there is almost not affected by the internal diodes, that charge just the small distributed capacitance of the transformer and external cables. The coupling to the oscilloscope probe is capacitive, so no DC can be seen anyway. The transformer rings with its distributed capacitance after the main pulse, and this is what you see. Connections as shown with external diodes or capacitors should not change this, but the extra wiring can change the coupling to the oscilloscope probe and change what is seen.

Re: Strange DC Flyback output waveform?
leviterande, Sun Aug 11 2013, 03:44AM



Thanks for the good explanation. But what about Mary´s explanation that its actually the pickup of the corona discharges EMF at the leads that are affecting the probe, maybe it is both but which do you think is dominating?

Re: Strange DC Flyback output waveform?
Sigurthr, Sun Aug 11 2013, 06:23AM

Without a direct connection you're just getting capacitive coupling, which only passes AC signals (most of which are radiated from the HV return lead, external components won't change this as they will appear to be in parallel and not in series with the "dc-block air capacitor").

Re: Strange DC Flyback output waveform?
leviterande, Sun Aug 11 2013, 02:58PM

I now also decided to confirm the source of waves.
I picked up the grounded probe with my hand and carefully wiped half a ft above and across the entire circuit all way from the insulated big spherical terminals of flyback, over the cables and above the bad boy flyback itself. Indeed the signal was much stronger and absolutely strongest exactly across and above the flyback with its big secondary. As I start now to move further away from the center of flyback and move along the grounded 2ft HV return cable all the way the signal diminishes extremely to a very small amplitude. I tried now the other way. I started now to move away from the flyback center along the red + terminal wire of the flyback and also indeed the signal diminishes the further along the wire I move away from flyback center. But the diminishing here is not nearly extreme as that of the HV return. However as I reach the end of the + terminal the signal picks up a little more. But the signal is 10 times stronger exactly above the flyback. So from all what I understand now the signal is the original ac source before the inbuilt connected diodes, so the signal must therefor come from either or both:
1:the entire secondary winding itself,
2: the lead that hasnt the diode connected across it directly.

Re: Strange DC Flyback output waveform?
Patrick, Sun Aug 18 2013, 07:14PM

I just recently saw this thread, been in north Dakota flying drones for academic purposes, but know im back.

exactly what is your goal here? are you wanting to measure the magnetic waveform from the core, or the electrical waveform from the HV output?

there are many factors that can muck-up what appears to be just another transformer. Flybacks are wonderfully complicated, with internal diodes, multiple windings with inter-winding capacities coupled all over, stray inductance, and so forth.

once you include a airgap to your probe, your effectively getting all kinds of bogus data appearing, and real data goes missing.

really, if you want an accurate electrical waveform, youll need an HV oscope probe (good luck getting one ). but if you want just the magnetic conditions of the core, you could wind your own coil, or find a good one to hi-jack just for measurement purposes.

EDIT: a step below the cost and difficulty of an HV probe, would be a resistive voltage divider, maybe tuned maybe not, depends on your needs. As others have said, that low side, isn't really a faithful ground, there's a whole bunch of garbage on it too around 1-3kv, at a blur of frequencies too.

you can find several of my related threads too, I have a benchtop HV PS using a feedback winding, and external resistors. and ive posted numerous threads on HV measurement... youll be able to learn from my mistakes if you like...