DC-DC 12V-1kV

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I have a little cute CRT tube named 3ЛО1И and I feel guilty of not giving it enough love. From what I know, it needs whereabouts of 500-800V on the accelerating anode, I don't think the current will exceed 100uA. I know how to make 800V from 220V with a regular isolating transformer and a diode multiplier. But I would really like to keep it small and wallwartsome, potentially nixieclockable, so I'm shooting for 12VDC->800VDC conversion. Maybe someone has done something of the sort recently, care to share your experience? I'm thinking of using an AVR to switch a trusty IRF630, like in a buck-boost converter, but with a switchmode transformer in place of the coil and feeding a 2x multiplier from the secondary. Worth trying, or there's a better way to do it?

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Push-pull or flyback converter with an appropriate transformer? But with a ratio of 1:66 this is going to be a big transformer with boatloads of windings on the secondary. Maybe 1:33 with a 2x multiplier.

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Re: the amount of windings, we can talk pretty high frequencies here so maybe fewer windings. I know of no good way of estimating them other than by experimentation. Say, how do you calculate how many do you need on the primary on a toroid with such dimensions and such permeability at such frequency, like, why don't we just make it 1 winding on the primary and 33 on secondary? I guess at some frequency 1 winding will make enough Z for the power supply not to burn down? I remember the past full of sparks when I tried to make a switchmode power supply and my secondary was 1.5 windings. It worked, until it exploded, but I don't think the transformer was at fault.

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I think you first need to determine at what frequency you want to operate it, and the amount current ripple allowed and from that an inductance can be calculated. From inductance you can determine the amount of windings needed on the primary side.

You might find good information in the datasheet of the LT3439 and the AN118 application note.

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I have generated high voltages using a xenon trigger transformer. Very small. Not sure how well it will work at high repetition.

It all starts with a mental vision.

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Usually the inverters for the bulbs are incapable of producing sustainged voltages. They need some HV to ionize the gas and then the voltage is pretty low.

Thanks for the pointers jayjay, reading.

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1:33? Why use such high ratios? If, for example, you create 160V in the primary, the secondary will only need to have five times the primary turns.

Driving a flyback is easy: We turn the power stage fully on until the inductor's current exceeds a certain, pre-calculated value. Right after that we turn the power stage off, making the core nuts until it returns all the magnetic energy it has stored in its air-gap. The core will try to do anything to return the stored energy back: It will try to find the easier way, creating an opposite current either in the primary coil destroying the power stage or in the secondary one lighting up our load. When the core is fully (Discontinuous Mode) or partially (Continuous Mode) discharged, we turn the power stage on again and we repeat the above indefinitely until the generated voltage in the secondary reaches the desired value; then we stop and wait for that voltage to drop below another threshold in order to repeat everything. This is the secondary voltage regulation. We can monitor the secondary current if we need a constant current source. For safety reasons we should monitor both! In a more elegant way, we can avoid the so-called "hiccup" of the switching power supply if we try to regulate the output by changing on the fly the driving pulse width, or the frequency, or both, instead of brutally turning the whole thing on and off. Then, we have a so-called "green" SMPS! (I know how much you love that term :P)

Now, windings of a few only turns give the inductor not much inductance, which requires higher switching frequencies in order to avoid saturating the primary coil and melt everything by skyrocketing the primary current; multi-turn windings have larger inductance and require lower frequencies to give the core the time to store the energy. Actually, we are not talking about a transformer because we do not collect the energy as we are pushing it to the primary winding; this is the Forward Topology. Instead, we collect the energy stored in the magnetic medium, which is the ferritic core. So, this is two coils strongly coupled magnetically together using the same ferritic core, where the primary creates energy that is stored in the visible air-gap of the ferrite core (or in the invisible countless tiny gaps created by the bonding material of the ferritic powder the ferrite cores are made of), and we discharge the coil's energy using the second coil (the "secondary"). This is the so-called Flyback Topology that energised the electron beam of the TVs, where it was firstly used commercially and, thus, became known; the electron beam that "flew back" to the next line of the field/frame during the blanking time after rendering the end of the previous line. :)

So, we should know the power the flyback will need handle in order to choose the ferritic core material, properties and size. Knowing the type of the ferritic material, the maximum and the ripple current (I_max-I_min) of the primary inductor and the time of the current transitions (in other words, the switching frequency), and we can calculate the primary coil inductance per turn. An LCR meter will be proved to be valuable, if it exists, to verify the inductance of the masterpiece we created in order to calculate the exact frequency needed to drive it. For one-off's you might even need to trim the core gap with a fine file!

Anyway, regarding the boring details about formulas, snubbers and MOSFET driving, you can find almost everything you need to know in the manufacturer's application notes and data sheet of their power switching products. If I remember correctly, TopSwitch offers guides for the transformer design.

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

Last Edited: Wed. Mar 7, 2012 - 03:47 PM
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Flyback or push/pull. Don't drive the transistor too fast otherwise you'll get huge spikes on your switch due to the transformer's leakage inductance. I'm a big fan of a high ratio transformer so that you can keep your primary switch's voltage rating low.

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Check out the little inverter modules used for florescent LCD backlight tubes. The high AC voltage is there, just add a rectifier!

Tom Pappano
Tulsa, Oklahoma

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Tom Pappano has about the best idea of the lot in terms of ease of implementation. One caution for this approach is that these LCD CCFL drivers have high output impedances. The output voltage is designed to foldback after the CCFL tube "strikes". This may or may not effect you at 100 uAmp draw, it depends on the specific module. These inverter modules generally have two output voltages specified: the "starting" voltage and the "running" voltage. The starting voltage is always higher and is essentially the no-load voltage - and probably the specification you should be looking at for your application. The running voltage is the voltage the output will pull back to as it is loaded to the specified output current (after the bulbs "strike"). Generally these CCFL backlights take around 5-10 mA running current per bulb, so your 100 uA load will fall into the "no-load" or "starting voltage" mode of operation.

If you still want to build your own DC-kiloV converter, I'd recommend the diode multiplier approach. There is a little trick to getting the traditional diode multipliers to work from a DC supply rather than an AC one. If you want to pursue this path, let me know & I'll give you some guidance.

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Thanks for all the advice. I'm a little bit worried that hacking a ready-made CCFL inverter may take me longer than building my own from scratch. I have experience making buck-boosts for the nixies and they are only a step away from flyback converters so I'm rather looking in that direction. Probably even push-pull or half-bridge to fully utilize the core. Anyway, at the moment I'm only lazily researching the options, any input is appreciated.

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Especially when the current consumption is low, a boost converter is IMO the way to go. Finally found time for the little scope-tube ?

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I just want it to glow :)
Do you think a boost converter will do? Somehow I have never seen a boost converter that goes over 200V, but I do not know why. Do you?

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The 1000V+ schottky diode for the boost converter will be the hard part to find.
How about a boost converter to 200V (nixie-wise) followed by a multiplier: a push-pull-stage driven by a squarewave generator feeding a diode-capacitor matrix. Of course those should be flux-capacitors :lol: But the diodes can be ordinary 1N4007's or so.

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I am surprised no one has suggested a Cockroft-Walton multiplier http://en.wikipedia.org/wiki/Cockcroft%E2%80%93Walton_generator. That is easy to get working.

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If you want wall-wart style, 120V AC to 800V DC is a lot easier to accomplish than 12V DC to 800V DC. If you're gonna plug it in the wall anyways why would you reduce voltage before amplifying it?

From 120V 60Hz AC, feed into a 2-stages Villard doubler with 68uF caps to get ~580V AC, and rectify to ~820V DC, 100mA. Wear gloves and shoes.

Also because each stage of those ladders only operate at twice the input voltage, and since regular 120V peaks at ~170V, all components before the rectifier only need to be rated for 400V.

Last Edited: Thu. Mar 8, 2012 - 03:04 AM
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Hmm funny how similar Villard cascades are to CW... Same? Was there a battle for a name that was won somewhere?

EDIT ok so from reading a bit more....

CW = Greinacher <> Villard Cascade, but are sometimes incorrectly refered to as.... Phew...

A CW cascade is a Villard cascade with an added peak detector on each stage...

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Plons wrote:
The 1000V+ schottky diode

I don't think you need a Schottky diode , the 0.4V vs 0.7V drop difference is negligible at these voltages.

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Hi, I did not read all the posts, but I have done a 5V to 800V DC/DC converter recently.

I used the LT3580 and schematics according to datasheet for -350V. I use dual LDT565630T-011 transformers.

I think 800V might be out of spec. Im running at 350V. But I have tested up to 800V, and it worked at least for a while. And I have a little higher current consumption than you.

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@hugoboss: because I want to use store-bought 12V (or 9V, or something) wallwart.

@dak664: lol

@AgwanII: good to know that those appnotes are scalable and thanks for the transformer reference.

@Plons: that could be a very neat idea, given that a good part of this circuit would be tried and true for me. Let's see, my typical nixie converter can probably put out ~200V at 30-40mA. Sounds like enough juice to quadruplify it and have enough for the second anode and deflection system.

But it bothers me that I could do all that in one stage by using a flyback or pushpull. And unknown transformer. Say, to begin the experimentation, I have a nice ferrite ring which is about 22mm OD. I could start experimenting with some random number, e.g. 5 windings on the primary and 350 on secondary and see how it goes. I would start right away but I do not have a wire that would physically fit 350 windings in that ring. Not to mention the questionnability of excitement of threading a loop of wire 350 times through a tiny hole. So 2-stage could win :)

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jayjay1974 wrote:
Plons wrote:
The 1000V+ schottky diode

I don't think you need a Schottky diode , the 0.4V vs 0.7V drop difference is negligible at these voltages.


Agree. But the switching speed requires the use of a schottky. IMO. A fast recovery type is also an option.

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In a boost converter straight to 1000, shouldn't the Vds of transistor also be >1000? I don't know, sounds weird to me.

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svofski wrote:
In a boost converter straight to 1000, shouldn't the Vds of transistor also be >1000? I don't know, sounds weird to me.
Yep, the transistor as well.

@dak664: that's what I meant with "teh matrix"
I should stop posting after midnight.

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svofski wrote:
@hugoboss: because I want to use store-bought 12V (or 9V, or something) wallwart.

But.... Why? Because they look good?

You can still use multiple stage multipliers from 9V but loss is MUCH higher, and there are a LOT more components involved...

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I think going from 12v to 1000v directly with a boost
is not so easy, you have an extreme dutycycle.

I also have experienced difficulties in making HV
transformers since the parasitic capacitance of the secondary together with high inductance results
in a low resonance frequency, so you cant operate
at high frequency.

I would suggest a push-pull (two logic-level
MOS transistors primary driven by two non-overlapping PWM outputs from the AVR. This effectively doubles the input voltage. Then I would transform to 250V and use a voltage quadrupler.
(Been there, done that....)

Another remark: It may be that you need a 800V diode
for 400V rectification, depends on the configuration.

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If I can go from 5V (USB) to 800V with my small solution, it shouldn't be that hard to go from 12V to 1000V?

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The problem is not that you CAN go from 5V to 800V, it's mostly that you probably shouldn't, when there is a perfectly good 120V waiting in the outlet.

WHY WHY WHY would you want to take 120V, make 5V from it, then boost it back up to 800V, using 10x as many components??!?!

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You could make it portable then. How did they do this on camcorders that use a tiny CRT for the viewfinder?

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Probably as I have done. My solution takes 10 components or so, and takes up 2-3 square cm of PCB. Could be smaller if I wasn't interested in having the voltage settable and mesurable.

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@hugoboss: because I don't like dealing with stuff in the wall, I prefer it when it's dealt with by certified professionals in the area.

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svofski wrote:
@hugoboss: because I don't like dealing with stuff in the wall, I prefer it when it's dealt with by certified professionals in the area.

Hmmm.. Well 800V DC isn't much safer TBH... 800V burns through skin, 120V AC just jolts you (unless you somehow decide to hold on to the wires)...

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hugoboss,

In the OP svofski wrote:
I know how to make 800V from 220V with a regular isolating transformer...

220V is available, not 120V, all more the reason to avoid direct conversion from the line for the minmal power needed.

Stan

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I'm carrying on with the project slowly. So here's the pushpull transformer driver:
https://www.circuitlab.com/circu...
Yesterday I tried it in real life. It worked nicely, up to some point. The MOSFETs that I had handy didn't have too much stamina, only rated for 20V BRVds. At 3.5 to 5V of input DC I had something like 100-140Vp-p on the output with my random-wounded toroid transformer, loaded on a 15K resistor. At 6V of input voltage the magic smoke was finally released. I imagined that the transformer would kickback at ~ double of input voltage, but what I have observed must have been at least a quadruple. Curious. The simulation doesn't show anything suspicious.

I'm waiting for some more durable MOSFET and I ponder meanwhile. Maybe I'll burn a few more of those puny ones just to see what exactly kills them.

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You sometimes can get some nice MOSFETs from old
motherboards. In a push-pull it is important
thet the primaty is very "symmetric", best
is to wind two wires bifilar (together) and the
connect them accordingly. Its also important to
drive the mosfets with enough voltage if they arent
logic level ones.

I am sure we had a thread here about that stuff
some years ago...

Good luck !

Here is the old thread, make 12 Volts from 5 was the application. There is some info about push-pulls
there.

https://www.avrfreaks.net/index.p...

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The symmetry indeed appears to be an issue. I now have a pair of good old IRF630Ns pushpulling my rather nasty looking coil. Here it is so you could have a laugh:

The primary is 5+5, the secondary is hell knows how many. I picked a coil that I used in some of my previous experiments.

Anyway, running at 80kHz, the smoke does not get released immediately. One of the transistors always gets rather hot pretty quickly, while another tends to stay cool.

Such is the output:

And such is the the drain of the hot transistor:

And the cold one:

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Update: bifilar-rewound the primary. Running at 80kHz now, both transistors are lukewarm. Awesome! Thanks ossi for the bifilar idea.

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As I'm moving on to the next step and trying to make the current design repeatable. Any hints on reducing the transients without dramatic redesign efforts? For the reference, after making the primary symmetric, both transistors see the former "hot" picture. A lot of energy seems to be wasted there.

I studied Peret's clock circuit a little bit. There are a lot of secondaries in the design. Initially I planned on getting what I have now, feeding it to a voltage multiplier and then just use a series of resistors to divide the resulting voltages into everything that I need. I'm not really comfortable with transformer design, so I'm trying to minimize the windings parts of the project. The currents in a CRT appear to be ridiculously small, so this plan should work, but what do I know.

Will the regular 4007 type diodes work fine in a multiplier at ~80kHz?

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Quote:
Will the regular 4007 type diodes work fine in a multiplier at ~80kHz?

I am afraid not, Svo. The 30us recoverytime is the spoiler.

Nard

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Yeah, I was looking at the same figure, Nard and it was worrying me. So I'm going to get some that have "Maximum Reverse Recovery Time: 200nS". I hope those will perform well. Oh dear!

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It is nice to see that you sorted things out! Now, Nard is right about the 1N40xx/1N54xx series; these diodes are too slow for such tasks. For Ultra-Fast solutions there are the UF40xx/UF54xx series at 1.0A/3.0A as well as the MURxxx family, all at 50ns/75ns/100ns. For lower or low voltage rectification with recovery times down to 1ns(!) I prefer the MBRxxx family Schottky diodes.

-George

I hope for nothing; I fear nothing; I am free. (Nikos Kazantzakis)

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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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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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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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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.....

Attachment(s): 

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Good point about the cathode voltage, I didn't think about this.

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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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@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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Mmm one another: I tinkered a lot in the circuitlab and came up with this layout:
https://www.circuitlab.com/circu...
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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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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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.

The Dark Boxes are coming.

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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.

The Dark Boxes are coming.

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