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Registered Member #14432
Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18
Hi everyone, I am happy to be part of this forum. I have been playing and charging various HV capacitors with both feedback and 555 oscillated DC DST flyback taken from a PC CRT. I decided to try and monitor the waveform from the flyback with either 555 or feedback oscillation by my Pc USb oscilloscope. This is what I got: Pic1 Feedback oscillated, Pic2 555 oscillated
As you can see from the two pictures, I am not so sure what to make out of it but it seems that the flyback puts damped AC? Sholdnt the output be strictly pulsed/time varying DC since the flyback has a rectifier?
I simply connected the ground of the probe to earth ground and the 0v of flyback also to earth ground. The probe is haning about 1ft from the + terminal of the flyback. Ignore the number reading as it fluctuated a lot.
Registered Member #14432
Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18
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 ??
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.
Registered Member #3943
Joined: Sun Jun 12 2011, 05:24PM
Location: The Shire, UK
Posts: 552
Sigurthr wrote ...
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.
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?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
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.
Registered Member #1565
Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159
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)
Registered Member #14432
Joined: Sat Apr 20 2013, 01:18PM
Location:
Posts: 18
Proud Mary wrote ...
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.
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
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
leviterande wrote ...
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
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!
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Strange DC Flyback output waveform?
