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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 23rd Aug 2012, 7:09 pm   #1
Martin G7MRV
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Default Driving a Dekatron

Hi Guys, small question about powering these beasties,

Would the simple spinner circuit, as can be found on 'mikes electric stuff' webpage etc, work if fed from the sort of simple HV generator circuits as used to be at the back of radio-shack electronics books, ie the 555 timer and transformer type circuits?

I need to make a very simple test circuit but dont have a HV supply nor an isolation transformer, so im thinking one of these 555 timer/transformer circuits would be a safer idea for now?
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Old 24th Aug 2012, 10:01 pm   #2
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Default Re: Driving a Dekatron

Found this circuit http://threeneurons.files.wordpress..../sp393_sch.gif

which looks to be just the sort of thing i need! Not sure i have the FETs though. But most of it seems just to be a switching PSU. I can probably rejig it to use a 555/transformer/voltage multiplier circuit, with parts i have about (time to dig out my old copy of Forrest Mims 'Getting Started in Electronics'!)

Wonder if i can get it built before the Doncaster Show next weekend, would be a gimmick to have running on the desk at GB1LDS!
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Old 25th Aug 2012, 8:46 am   #3
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Default Re: Driving a Dekatron

Quote:
I can probably rejig it to use a 555/transformer/voltage multiplier circuit
I think that would be a bit too much of a step up of 12 to 450V, 38 times. The FET circuit looks OK to me.
 
Old 25th Aug 2012, 10:04 am   #4
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Default Re: Driving a Dekatron

The switching PSU in that circuit is quite well designed. L1 needs to be a high quality part, and it uses a Cockroft-Walton multiplier to go the rest of the way.

I *hate* 555's. There is a bug in their design, faithfully copied by all makers, which momentarily shorts Vcc to Gnd and has a short burst of HF oscillations on the output during the transitions. The CMOS variant is free from it. Using one to clock a PSU can result in strange failures. Decoupling is important.

I agree with Merlinmaxwell, the FET circuit looks like a good worker.

David
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Old 25th Aug 2012, 9:32 pm   #5
Martin G7MRV
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Default Re: Driving a Dekatron

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
I can probably rejig it to use a 555/transformer/voltage multiplier circuit
I think that would be a bit too much of a step up of 12 to 450V, 38 times. The FET circuit looks OK to me.
Yes, straight from the timer output, i agree that would be a bit much (not to mention exhaust my stock of high voltage caps). The circuit i was thinking of uses a 240v pri/12v sec transformer backwards, to give close to 240v out, i would then have used a doubler after it.

The trouble with that switching supply is that it uses several parts i know i dont have. Ive since found a 555 timer based switching supply circuit (almost identical, but uses the 555 instead of the LM393 and zener). The advantage of the 555 (probably its only advantage) is that i have a tube of them!

I have some good quality Toko and Murata inductors in suitable values, what im not certain of is the MOSFET and the diodes for the doubler. Im hoping a suitable MOSFET can be found in one or other of the scrap SMPSUs i have hanging about. The diodes seem to be fast recover types. Ive a whole bandolier of 1N4007s which have suitable ratings, i might try them. I'll knock the circuit up tomorrow if i can find a mosfet for the job.

Anyone suggest what parts i might salvage from an old SMPSU?
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Old 25th Aug 2012, 10:06 pm   #6
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Default Re: Driving a Dekatron

If the primary of the SMPSU is working you could take the transformer to bits and wind a 450 v winding onto it with just the old winding that is opto coupled for regulation.
Just count off the turns of a known winding to get the ratio.
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Old 26th Aug 2012, 11:06 pm   #7
Martin G7MRV
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Default Re: Driving a Dekatron

I dont think i currently have the patience for rewinding transformers!

I'd like to make the circuit in the link i posted, but im finding it hard to find a supplier that has all the parts needed (even using the listed alternatives) in one place and without an extortionate minimum order or shipping charge! None of my usual suppliers has all of them, and if i use several suppliers, well i may as well buy the kit from the states for £20!

How critical is the value of L1? This circuit its 220uH, another ive seen 100uH. Im not sure what value i'll have knocking about (i know i have a 470uH!)
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Old 27th Aug 2012, 12:06 am   #8
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Default Re: Driving a Dekatron

The 555 is a bad choice for driving transformers, as it cannot by design produce a 1:1 mark-space ratio. Instead I would recommend the HEF4047BE CMOS astable / monostable and bistable. This gives the required 1:1 MSR thanks to the divide-by-two action of the bistable; and it has complementary outputs, so would be ideal for driving a centre tapped transformer.
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Old 27th Aug 2012, 8:38 am   #9
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Default Re: Driving a Dekatron

L1 is a key part. Without doing all the calculations, I'd expect 270 and 330uH to be usable values of inductance. But inductance isn't the only important specification. his needs to be a reasonably low loss part so that the transient when Q1 turns off is big enough to make the wanted output voltage. Some inductors are really designed as EMC filters where lossiness is a good thing. L1 also has to take a peak current without saturating
A quick look at Farnell and their part 2062729 looks suitable at £1.10. There are lots of substitutes for the dual comparator, U1

The FET is cheap at 60p Q2 can be any high voltage small signal transistor with over 200v Vcoe rating.

NE2 you can nick out of an old socket or switch with a built-in neon pilot lamp, (but change the series resistor)

There are several indications that the designer of this circuit knew what he was doing. A lot of the parts could be found in junk boxes or in scrapped equipment. The author has taken the time to list a few substitutes for the interesting buts, but the potential range is far wider.

If the kit from the states includes a PCB, then I'd go for it.

Cheers
David
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Old 27th Aug 2012, 11:20 am   #10
Martin G7MRV
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Default Re: Driving a Dekatron

Cheers for that info. Ive decided to go with that circuit, having finally found somewhere that stocks all the parts i havent got! (and resigned myself to the postage charge!). I'll probably still have a go at a version with junk i have, but gettign all the special parts in one go means i can get one built quicker.

The kit from the states does have a PCB, and looks pretty nice. It does however have two disadvantages for me - 1, it uses a PCB valve socket, where i want to mount mine ona panel, and 2, ive decided to build two of the power supply sections, as theres another version spinner i'd like to try if i can find a 0A5 tube
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Old 27th Aug 2012, 11:45 am   #11
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Default Re: Driving a Dekatron

I have been using this 555 SMPS quite a bit recently. Not sure if its the same one you referred to.
http://www.turbokeu.com/myprojects/n...ixie%20psu.gif

The diode needs to be a fast type, and the MOSFET needs Rds(on) of <1 ohm. I used BA159 and STP11NK40, respectively, and a 150uH inductor. Seems to be fine up to 20mA output- haven't tried pushing it further.
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