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question about ignitrons

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saturatEd

Sun Nov 16 2008, 11:03PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

This is kind of related to my previous thread, but I thought it will be better to start a new one, because this is after all a new topic, and might help in the search engine to have it under a new subject line.

Ok.
Plain old welding ignitrons... they all have average current ratings and max averaging time listed in datasheets, but I'm not so sure about the way I understood that.

They are tested under AC conditions for typical uses of welding and rectifying jobs, but if say there is a 30A average current rating, over say 10 seconds averaging period, does that mean you can pass 30A steady DC current for 10 seconds? Or is this rating limited to pulsed (half wave sine) operation?
And if this is a rating strictly for typical operation conditions (AC voltage) , is it at all possible under any circumstances to drive continuous DC current through an ingitron.

Proud Mary

Mon Nov 17 2008, 09:12AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The 'ignitron' is a Westinghouse trademark for their take on the mercury arc rectifier.

In the 'conventional' mercury arc valve, the arc is typically initiated by momentarily dipping the graphite anode into the mercury cathode pool by means of an electro-mechanical actuator, so that it pulls the arc away with it. In three or six phase systems, the arc transfers itself to the anode of highest voltage, but even so 'keep alive' electrodes drawing a few amps are usually installed so that a momentary interruption will not stop the device conducting, so switching itself off.

In the 'ignitron', the arc is initiated by a short high voltage trigger pulse, much like a photo flash tube.

In a single phase ignitron, the device either stays ON until the supply is interrupted, or switches itself OFF on negative half cycles so acting as a half-wave rectifier.

Operation involves the condensation of evaporated mercury in the upper part of the glass bulb, from whence it returns to the pool.

The 'ignitron' is an inherently low duty cycle device. You will see that were you to connect one to a DC supply it would first act almost as a dead short because of its very low conduction impedance, and then fail in one way or another, perhaps even explode if the heating-cooling cycle of the device prevented the mercury from condensing back into the pool.

Ignitrons of special design (i.e. built to order) are still used very occasionally in high pulse power applications - such as discharging capacitor banks in nuclear particle 'kickers' - but are otherwise obsolete.

I hope this helps.

PS: Aside from the obvious toxic hazard in the event of breakage, don't forget that mercury arcs emit strong UV that will quickly damage unprotected eyes and skin. Most definitely non-RoHS compliant!

saturatEd

Mon Nov 17 2008, 03:34PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

Ok thanks, so you are saying that average current rating over a specified period, is not literally average?

In other words 1s of 10 amps does not equal 10s of 1 amp in case of an ignitron

Proud Mary

Mon Nov 17 2008, 05:27PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I would really have to look at the data sheet of your particular device to understand quite what you are getting at here, son. Do you have the part number?

saturatEd

Mon Nov 17 2008, 05:54PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

Ok, you can take a look at NL1054....

I think I'm not suppose to put direct links, so you can find the pdf at this page:

Proud Mary

Mon Nov 17 2008, 08:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The graph for rectifier applications outlines the relationship between average and peak current over a range of duty cycles - frequencies between 25 and 60Hz.

Remember that this valve is only suitable for intermittent operation - bursts averaging 12.5 seconds - and that in any 0.6 sec period the maximum ratio of average to peak current must not exceed 0.166.

You'll notice that it can handle a surge current 12.5 times higher than the peak current for 0.15 sec maximum periods - so you could get it to switch 50kA in very short pulses! A splendid device!

saturatEd

Mon Nov 17 2008, 10:59PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

Yes that ratio rating is what made me ask the question in the first place. It seems to be a limiting factor which doesn't allow anything nearly as smooth as DC current.

I think the safest bet for square pulses is to take a half-wave at 50-60 cycles per second, and then fit my square somewhere within the surface of the half wave, that was I can be 100% sure it is within rated parameters.

Thanks for the help

saturatEd

Mon Nov 24 2008, 09:26PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
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Posts: 14

Hi

one more question

In the datasheet there are 3 example ratings. for intermittant rectifier operation. In one rating it says max peak current of 8000A, at max peak anode voltage of 600, second rating is 6000A at 1200V and third 4800A at 1500V.

I'm not sure I understand why is there a reduction of max anode voltage for higher current ratings?
Shouldn't peak voltage be simply limited by maximum hold-off voltage for the tube?

I mean, the voltage drop on the tube will be the same if the current is the same, whether you have 1500V on anode (before conduction) or 100V. (depending on the load outside)
So why did they derate the current for higher anode voltages?

Proud Mary

Mon Nov 24 2008, 11:16PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The device has to be maintained within the safe operating area of its dissipation rating - perhaps even more so than a vacuum valve, because of the thermodynamics of the mercury evaporation-condensation cycle.

Increase the input watts beyond a critical limit, such that the device becomes so hot that the mercury can no longer condense back into the reservoir, and something will fail.

saturatEd

Sat Nov 29 2008, 01:20PM

Registered Member #1810 Joined: Thu Nov 13 2008, 02:46PM
Location:
Posts: 14

But what does network voltage have to do with the ignitrons dissipation, if its voltage drop is always say 30V at say 1000A.
Whether you have 100V input voltage or 1500V, you will still have 30V voltage on the ignitron if the current is 1000A, which in both cases gives you about 30 kW dissipation.

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