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 Posted: Mar 18, 2012 - 10:27 PM |
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Joined: Oct 30, 2002
Posts: 5715
Location: The Netherlands
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Transformer design is quite a black art, maybe even magic some would say. Boatloads of variables and one big compromise. All the different unit systems don't help either for the novice.
The spikes could be due to leakage inductance. Try a snubber network. |
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Posted: Mar 19, 2012 - 04:34 PM |
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Joined: Apr 25, 2006
Posts: 2296
Location: Germany
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After some digging in my paper-storage I found it:
In my scope-clock I operate a DG 7-74A tube using the
following SMPS design (EFD30 core):
primary: 6+6 turns push pull from 12V DC
secondary 1: 3 turns tube heater
(The heater must be isolated because cathode usually
is negative with respect to ground and voltage
difference between heater and cathode must be small.
secondary 2: 20 turns + voltage doubler(2 diode)
supply cathode and grid1 (negative) voltage.
secondary 3: 100 turns+2 diode doubler
generates -400 Volts for catode
secondary 4: 50 turns+doubler generates +200 Volts
for deflection amplifier
secondary 5: 100 turns + doubbler
stacks onto +200V gives +600V
And it works ! |
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Posted: Mar 19, 2012 - 04:48 PM |
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Joined: Nov 01, 2005
Posts: 6323
Location: Hilversum - the Netherlands
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| Jayjay wrote:
Quote:
Transformer design is quite a black art, maybe even magic some would say
O yes, it is black magic 
And I love it.
@ossi: Hello Professor Albus Percival Wulfric Brian Dumbledore, Head of the Academy for Magic Black Art. Always a pleasure to see you and/or one of Thou Art 
We (me and my fellow students) would love to see a picture of that SMPS++ design.
Nard |
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Posted: Mar 19, 2012 - 05:11 PM |
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Joined: Apr 25, 2006
Posts: 2296
Location: Germany
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The master of perf-board and magic smoke responds:
Attached see my scope-clock. The EFD30 transformer
is surrounded by HV fast rectifier diodes and HV elcaps.
The 4 small transistors are the deflection amps. The two TO250 transistors are the push-pulls. The
many resistors form two R2R DACs driven by an ATmega32.
Transformer design needs much magic smoke. You take it
out of many transformers and put it back into one
good design. Some call it try and error..... |
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Posted: Mar 19, 2012 - 08:17 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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| Good point about the cathode voltage, I didn't think about this. |
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Posted: Mar 19, 2012 - 08:24 PM |
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Joined: Nov 01, 2005
Posts: 6323
Location: Hilversum - the Netherlands
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And the pots at the top are for setting the levels for the specific grids ? Is there need for stabilization ?
Nice work, ossi.
@svo: with a core as ossi used it is easier to make high count windings. Your toroid is great for lower voltage SMPS's. But be carefull with the mu. Scavenged from an inputfilter with 2 equal windings: those have a high mu but saturate fast.
The cores that I use, apart from the ones I buy, are scavenged from old powersupplies: PC and other. Great source |
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Posted: Mar 19, 2012 - 11:44 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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@Nard: I don't notice any problems with my core, I think it's fine for this purpose. 150VAC is low voltage :)
Anyway, an update:
0) yesterday i measured ~300Vp-p with a lot of transients to which I did not pay too much attention;
1) I assembled a half-wave doubler. I expected to see +300VDC on its output, but I saw 600. Wow I thought.
2) Soon I noticed that not all is going well, the transistors were heating up quickly;
3) After some experimentation, I added 10nF caps between the drains of the MOSFETs and ground and increased deadzone between pushy and pully pulses (it was 0, now it's like 10%). This made the transients go a great deal. The pulses on the drain now look pretty flat and I'm seeing expected 300V on the output of half-wave doubler. No extra 300V for free, but also no heat anywhere: I like it.
4) added a 1 winding secondary for the heater. Tried it with a 6.3 bulb.. To be honest, I'd feel safer if the voltage was lower. I added a 0.033 cap in series.
@ossi: i browsed through the epic inductor thread and I saw many messages about your scope clock but the images did not load. Do you have an album somewhere? I guess I love scope clocks :) |
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Posted: Mar 20, 2012 - 12:03 AM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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Mmm one another: I tinkered a lot in the circuitlab and came up with this layout:
https://www.circuitlab.com/circuit/a5u3p3/multiplier3/
So far I only tried the top part of it and it appears to work absolutely as expected (after killing the excess of transients). I'm not sure why, but it worries me a little bit that in this configuration neither of the secondary ends is tied directly to ground (currently, with only the top part implemented, the bottom end is grounded). Shouldn't be a problem, but I learned to expect unexpected things here. What should I expect?
Also, the ripple on the output seems negligible. Would you recommend adding an RC filter anyway, or it should do just fine without? As for cathode/brightness/focusing/acceleration, they seem to not really care and as for deflection, I'm planning to use the longtailed differential configuration, like everyone else does, and it too should be pretty insensitive to ripple, no? |
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Posted: Mar 20, 2012 - 10:04 AM |
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Joined: Nov 01, 2005
Posts: 6323
Location: Hilversum - the Netherlands
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Quote:
I'm not sure why, but it worries me a little bit that in this configuration neither of the secondary ends is tied directly to ground (currently, with only the top part implemented, the bottom end is grounded). Shouldn't be a problem, but I learned to expect unexpected things here. What should I expect?
Nothing serious. Just be aware that the floating secundary will be at a high potential. So good isolation is necessary. And what potential depends on the doubler configuration.
For the ripple: I don't know. Never built a scope clock  |
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Posted: Mar 20, 2012 - 10:26 AM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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Okay that's cool! I wouldn't say that when one of the ends is grounded there's nothing to isolate either.
A picture due from the last night
Those little square thingies on the right are 2220 0.1/1kV X7R caps. It's crazy how tiny and seemingly harmless they are for such huge charges.
BTW. What do we count for "a winding" on a torus? How many "ties" does a wire make around the core, or how many times the wire passes through the hole in the middle? It never bothered me before, but now in the days of 5 windings on the primary and 1 on secondary, one has to ask himself such questions.
Quote:
Never built a scope clock
I'm also quite intrigued by hydrogen dekatrons, they require around 600V. Enough to sparkle the interest. |
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Posted: Apr 08, 2012 - 02:53 AM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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Okay, so I have advanced a bit. I have all the high voltages, adjustable and everything in HV is like I expected (full span is -450..+300). However 6.3V is giving me trouble. As you can see in the picture above, I had lighted the tiny bulb. But when I tried doing the same with the CRT heater, one of the trusty IRF630Ns in the push-pull died almost instantly. I'm trying to analyze the problem before I fry another MOSFET.. The heater resistance that I see is 1.7 ohm, which is ridiculously low: at rated 6.3V that would mean that it eats 4A: I would never be able to afford that. But I guess in a normal situation it heats up very quickly and its resistance then goes up. The datasheet says the nominal current is about 0.5A. Somehow it didn't happen, or didn't happen soon enough.
Another thing that bothers me is the mode of failure of IRF630N. It's rated up to almost 10A, my 12V source is able to provide 2A maximum. What went wrong? With just the resistive ladder and a lightbulb as a load they were running completely cold.
Until i figure it out I guess I can try heating the cathode from a set of 4 AA's. |
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Posted: Apr 08, 2012 - 01:45 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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I don't know, this thing is so unstable. The only thing it doesn't react to is me blinking the eyes, everything else is instantly displayed and in the most unpredictable manner. The deflection amps are not yet done. Is this normal that there's no way to get a focused dot in the centre of the screen? I either get it blurred, or rather far away from the middle. |
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Posted: Apr 08, 2012 - 03:09 PM |
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Joined: Jul 02, 2005
Posts: 5922
Location: Melbourne, Australia
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Svo,
Are you really using 1N4148 diodes? I thought that they had a PIV rating of 100 volts.
Cheers,
Ross |
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Ross McKenzie
ValuSoft
Melbourne Australia
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Posted: Apr 08, 2012 - 03:23 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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@Ross, I'm using SM4937, they are rated to 600V and have 200nS recovery time. The caps are 0.1µF/1kV. Here's a closeup, sorry for the excess of flux :)
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Posted: Apr 08, 2012 - 06:52 PM |
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Joined: Apr 25, 2006
Posts: 2296
Location: Germany
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Hi Svo,
bringing up the 6.3 volts of the heater is a real pain.
inmy scope-clock it takes I think more than 30 seconds
to heat up. Sometime its good to limit the supply
current. Getting a sharp dot can also need some
experimentation as I remember. I remember it was
important to have the deflection grids at the right
potential with respect to cathode and anodes. Then
having too much AC on the voltages modulates the
focus and may blurr the picture. If you have a
magnetic field near to the tube, that will also blurr.
Good luck, in the end you will win ! |
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Posted: Apr 08, 2012 - 10:23 PM |
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Joined: Aug 04, 2002
Posts: 1704
Location: Brisbane, Australia
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| Heater resistance will change with temperature and eventually the current will drop. |
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Posted: Apr 09, 2012 - 09:43 AM |
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Joined: Apr 25, 2006
Posts: 2296
Location: Germany
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Here you can see a short video of my scope-clock
http://www.youtube.com/watch?v=hokWAQaQ ... ItYiXKtx8=
The clk gets its time from DCF77 time signal
transmitter. Below the center you see a "rolling"
display of the incoming pulses.
Current consumption at 12V is 300mA. During heatup
it needs more than 600mA. |
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Posted: Apr 09, 2012 - 01:00 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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@ossi what a lovely sight! It's really hard to believe that mine is going to be like that one day.
Do you have some sort of feedback in your power converter, or do you just let it pump happily? I have noticed that with time the voltages tend to climb up a bit. OTOH, I have also noticed that my tube is happy enough with much lower voltages too, so I'm feeding the circuit with 7.5V, just barely enough for a 7805 to work. It appears that some +/- 100V don't really make a lot of difference for such a tiny tube. |
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Posted: Apr 09, 2012 - 01:56 PM |
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Joined: Apr 25, 2006
Posts: 2296
Location: Germany
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I have no regulation/feedback. The load seems fairly
constant so I simply let it pump. Do you have the data
of your tube ? Old scope schematics are also interesting
to get ideas how to built up th supply/defelection
and what voltages to apply to which electrodes. |
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Posted: Apr 09, 2012 - 09:23 PM |
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Joined: Jun 27, 2005
Posts: 3411
Location: St.Petersburg, Russia
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Enjoy with me a perfect wireframe of just one of the dark boxes which are about to come:
Those darker green patches are not highlights or reflections on the glass, they're some nasty electrons flying sideways.
Now with the deflection amps working, the transistors in the pushpull are getting rather hot. Uh oh, no me gusta. |
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Last edited by svofski on Apr 09, 2012 - 10:07 PM; edited 1 time in total
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