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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
What surface would the laser be shooting at? a metal surface or glass? also if you wanted to add energy to just the hydrogen you could use a hyrogen source of light maybe not even a hydrogen laser.
Im thinking an infrared laser would be the better laser to use for the heating purpose, it could be a wavelength other than 1.3um since your shooting distance is short.
But it seems complicated, perhaps not worth it given the self heating and heat conduction youll get while its in operation.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
I do have a 100mW laser, that focused at close distance, produces heat, but it would be impossible to obtain better results than the filament already produces: it glows incandescent after a few minutes.
I am happy with the way it works by itself, the only drawback would be the 5 minutes heating time before any use.
If I would really need to change it, I do have access to the coil used to connect the reservoir filament to the primary filament, it is placed at the bottom of the tube. I could increase the voltage separately,or at least use a smaller inductor to allow more current to pass. But it would be too risky, so I'll just use it as it is.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
The TGi2-400/16 and the smaller TGi1-130/10 , I got them both as NOS, but the bigger one already shows signs of use from my first tests (thin metallic layer on the glass):
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi Radu, I'd strongly advise you not to use the circuit at the top of this thread, which looks like a simplification from a student textbook.
The problem with the circuit as it is drawn is that the heater-cathode insulation will probably fail when a high dv/dt voltage develops across the transformer primary during the leading edge rise time of the anode pulse.
In fact, everything possible must be done to reduce inductance in the cathode circuit when you have a rate of change of 800A/μs. Connect the cathode directly to the chassis with a thick copper strap as short as possible.
Below are two practical examples of the right way to couple the output.
If you are more concerned with power than pulse shape and timing, you may want to use a lower impedance PFN, and then use a wideband impedance matching transformer to connect to your load.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Some old news I didn't have the chance to post:
I did the change, and managed to get it working .
Strangely, the behavior of the tube changed while switching. I re-wired it in the old topology, and the behavior was consistent. Apparently I kept it on for a longer time, and probably there was more hydrogen released. The pink discharge was better placed now, spatially positioned just under the plate, as expected. Last time I did this (as in the video abovE), it was more diffused. I was unable to capture it on video, as the experiment was short lived:
Suddenly, the filament started to glow extremely bright. This happened before too, from time to time, but after cooling would get to normal functionality. Not this time. Also the reservoir secondary filament was too dim, so probably not much hydrogen produced either. Keeping the filament on in this condition , resulted in metallic layers deposited on the glass, with shadows cast by internal components, where the path from the filament to the glass was obstructed.
I assume the tube is now damaged, but what happened? Did I switch too late to the topology Proud Mary suggested ?
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Proud Mary wrote ...
Maximum average power could be exceeded when:
NO (1) anode voltage too high
NO (2) duty cycle too high
MAYBE (3) pulses too long
MAYBE (4) output impedance too low
NO (5) reservoir heater voltage too low (i.e. not enough hydrogen due to gas clean up)
NO (6) emissive material from the cathode has been deposited on the grid causing grid emission
LIKELY (7) inverse voltage limit exceeded so reverse conduction occurs - known as "arc-back"
Looks like 7) is probably the cause of this problem, and also I'm not that sure my commanding signal was in the right parameters, since I adjusted it a lot during my tests. So Steve, I will try the protection you've indicated for my next attempt. I do have replacements, but I'm not in a hurry to use them, not until I do my homework properly.
I must admit I'm surprised to see this tube is so "sensitive". My flyback power supply was not more than 200W, nothing compared to the RADAR installation where this tube was used.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
I must admit I'm surprised to see this tube is so "sensitive".
This little marvel is the famous 1258 miniature hydrogen thyratron and it stands just 52mm high. Its peak output power limit is 10kW, when it will draw a peak current of 20A - but its average current must not exceed 50mA - the figure we would use to calculate heat dissipation.
Its compact size made 1258 very popular in small marine and aircraft radar modulators - but if you ignore any of the absolute limits given in the data sheet of this or any other thyratron, it will fail quickly.
Many thyratrons have their own unique rules, so design from the data sheet is essential if disappointment is to be avoided.
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Thyratron Switched Tesla coil (THYTC)
