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Registered Member #1215
Joined: Sat Jan 05 2008, 02:23PM
Location:
Posts: 3
Hi Guys,The EHT supply in my CRO has failed due to a shorted turn in the EHT converter transformer.They are no longer avaliable.Original design used a ferrite E core,self oscillating at 30Khz.Two independent 2KV secondaries (couple of milliamps,with some stabilization feedback), one feeds a trippler.Runs on 10 volts.Searching the net finds many low voltage boost and buck designs also very high voltage designs.Unable to find a suitable CRO design.Downloaded a lot of switching supply info but i am now totally confused as to how to go about this design.Other than picking a suitable E core what are the important parameters that i should work on ?? (the order and importance of these parameters appears to vary with all the examples that i have downloaded).... in other words this circuit has a fixed voltage input,relatively small variations of the small output current, relatively low operating frequency, what are the important variables that effect the design ?? All suggestions and links greatly appreciated Regards....George
Registered Member #1215
Joined: Sat Jan 05 2008, 02:23PM
Location:
Posts: 3
Hi Steve,Thought of trying this but the wire is microscopic (40 gauge ??) and the winding is a difficult shape ie. perhaps 10mm wide and perhaps 50 layers high (very wide flat shaped E core).Thought my chances of winding this without collapsing were zero. Thanks for the reply George
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Well, unfortunately you'll find that most ways of building a similar switching supply will involve the pain of making pie windings out of 40 gauge wire.
Maybe you could get a flyback from an old B&W TV to fit in there? Do you know how many kV and mA you actually need, and how tightly regulated it has to be?
Registered Member #1215
Joined: Sat Jan 05 2008, 02:23PM
Location:
Posts: 3
I Have previosly joined both the HP Group and Tekscopes....however nobody could suggest a source for the transformers that i had not already tried.The other suggestion was,the same as Steves, to try rewinding it. Steve, the only info on the circuit was the two secondary windings DC votages (after diodes and some filtering, of around 2,500 volts each.I would suspect the positive supply (which also runs the trippler) would be no more than one milliamp and the negative supply perhaps two milliamps) from past experience.I have collected a pile of old TV and monitor flybacks but was unsure how to use them as regards the much lower supply voltage (10 volts compared to around 90 plus as in Teslar coil ccts etc).I have no idea how much the voltages would vary with increases in Tube brightness but trippler current is only normally in the order of microamps at full brightness.If i sense the output voltage,using high value resistors do you think i could use this to feed one of the specialised IC's,for regulation (as i often see on this site). Cheers...George
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
What about a cold cathode (CCFL) inverter? A suitable transformer can be extracted from a dead 22"+ LCD panel (I have several) and they are often dual winding so generate +/-HV.
One basic C/W doubler and voila! :)
In fact most of these PCBs can be reverse engineered with minimal effort, the manufacturers make it easy for troubleshooting.
Regards, -A
email me, i have many CCFL boards... :) will trade for one of those dual winding beasties to fit this LCD TV with eight of them.
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Faulty HP oscilloscope EHT supply
