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    carbon arc modalities

    Typically carbon arcs are made with either DC or AC but in no cases does one find them being generated using a constant current method. This is because constant current arcs are extremely power inefficient.

    Would a 1amp 200Vdc constant current source driving a carbon arc reduce heat while still permitting it to have considerable distance between electrodes?

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    Re: carbon arc modalities

    I have no experience of this but I would guess it would be difficult to establish an arc with a power limited source. If my thinking is right, it takes more power to take the arc temperature to plasma state than to sustain it afterwards, with 200W power limitation you may not have enough to start it.

    Brian.
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    Re: carbon arc modalities

    The arcs are started by separating them slowly heating and ionizing the area in between. Should be fine if done slower to prevent gap from being prematurely quenched. But the CC source might prevent rods from wearing out rapidly.

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    Re: carbon arc modalities

    This is because constant current arcs are extremely power inefficient.
    Why do you think so?

    In basic operation, carbon arcs are like discharge lamps. Say a fluorescent lamp or a neon bulb. All discharge lamps love constant current operation mode.

    Carbon arcs work in open air and you must maintain a constant distance between the electrodes. It also does not like AC (particularly low frequency AC like 50Hz!!)

    You should use a ballast (inductor for AC or a resistor for DC) in series with carbon arcs (like all discharge lamps). 200V DC and 1A you may have a very short arc (1-2 mm)

    You need to keep a const gap between the electrodes (that ensured a const current)- can be done with a servo that looks at the current through the arc.

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    Re: carbon arc modalities

    Howcome the 200v 1A method would only yield a 1-2mm arc?

    Carbon electrodes put out copious UV and heat which readily ionizes gasses.

    The CC method is inefficient since its speced with a CC source using opamps driving mosfets.

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    Re: carbon arc modalities

    The original carbon arcs used AC and a ballast choke to give an approx constant RMS current - the arc is a plasma in fact - and there were numerous mech systems to wind the carbon rods slowly together, as they were consumed, to give a constant arc width - yes plenty of UV came off as well as a blinding blue white light ...

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    Re: carbon arc modalities

    Arc I/V characteristic has negative slope (negative dynamic impedance), you won't achieve a stable arc with CV supply.

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    Re: carbon arc modalities

    The negative I/V characteristic means voltage increases as current decreases?

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    Re: carbon arc modalities

    Correct, and current goes up as volts go down ( very counter intuitive ). Due to IR drop in the carbon rods - there will be a small element of V going up as I goes up - but swamped by the dynamic neg res of the arc.

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    Re: carbon arc modalities

    Carbon electrodes put out copious UV and heat which readily ionizes gasses....
    But this is typically a blackbody radiation. The temp can be computed using the power input and the thermal mass.

    The ionization is caused by the electrons- and they are not having the same thermal temp as the gas.

    The CC method is inefficient since its speced with a CC source using opamps driving mosfets.
    Just like most discharge lamps; you cannot run a common neon indicator lamp with a const voltage source!

    Const current drivers can be as efficient as const voltage drivers.

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    Re: carbon arc modalities

    If it is turning electricity into light - then it is not just, or all, black body radiation - there will be some black body radiation from the hot carbon rods and from the plasma itself which is hot ( very hot ) but there is an intrinsic process of electrons falling in energy levels - giving of all types off light, before being re-energised by the electricity...

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    Re: carbon arc modalities

    there is an intrinsic process of electrons falling in energy levels - giving of all types off light, before being re-energised by the electricity...
    You are right, this is the process that keeps the plasma alive. The ionised gas is also being heated by the current and that is what makes the plasma hot. Just like a filament lamp.

    But you will see the light coming from the electrons falling into lower energy levels only at lower pressure. At lower pressure, the current will reduce and the voltage will increase and the light output will decrease (most of it will be in UV).

    The carbon arc works at one atmosphere when the mean free path is small and the number of ionized molecules is small (as a fraction of the total).

    It is possible to figure out, just by looking at the spectral distribution of the light output, whether the source is a black body or electronic transitions in an ionized gas.

    For C, O and N, electronic transitions will produce mostly UV and little visible light.

    Fluorescent lamps use chemical phosphors to convert the UV to visible. They produce little visible light (and that applies to most discharge lamps). Sodium vapour lamps are an exception (as are neon, argon discharge lamps).

    The hot plasma produces lots and lots of black body radiation and that is independent of the light produced by electron transitions.

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    Re: carbon arc modalities

    Does this mean a carbon arc can be wider with a 5v 200w supply instead of the 200v 1amp?

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    Re: carbon arc modalities

    Does this mean a carbon arc can be wider with a 5v 200w supply instead of the 200v 1amp?
    I do not think it will strike at 5V; you will need some minimum voltage (I do not know the exact value) like 50-70V.

    Usually the arc strikes at a point; it gets wide at the centre (because of repulsion of the current paths). The arc length depends on the voltage applied.

    Pointed electrodes are better to get the arc started: on DC power, one electrode will be consumed faster than the other. On 110-220V AC, you will need a ballast to regulate the current.

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    Re: carbon arc modalities

    You are right, this is the process that keeps the plasma alive. The ionised gas is also being heated by the current and that is what makes the plasma hot.
    the above statement tends to contradict the other statement you have made:

    The carbon arc works at one atmosphere when the mean free path is small and the number of ionized molecules is small (as a fraction of the total).

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    Re: carbon arc modalities

    Ballasting with AC can be done with a series film capacitor to electrodes, it would have to be rather large value to permit the low frequency but its lighter and more compact than inductors.

    Do carbon arcs make the type of UV which makes ozone?

    If there is any ozone it would likely be reduced per due the immense heat but then the UV can ionize gasses further away from the arc which aren't in the heated vicinity.
    Last edited by Zak28; 23rd August 2019 at 09:37.

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    Re: carbon arc modalities

    Quote Originally Posted by Easy peasy View Post
    the above statement tends to contradict the other statement you have made:
    In a typical gas discharge curve, the glow region has low current. The number of active carriers are very small.

    During the arc region, the current increases and the voltage decreases; the number of active carriers is increases at this point. This is the region having a dynamic negative resistance.

    In the last stage, the current increases very rapidly, the gas is highly ionised (dynamic resistance is very small but positive) but this is not a stable discharge because a large fraction of the gas is ionised.

    The arc discharge region is characterised by avalanche multiplication (that is the major cause for the negative dynamic resistance)- that cannot take place if the carriers are already large in number (if most of the gas is ionized, recombination is more important than further ionization).

    I hope I am clearer now.

    - - - Updated - - -

    Do carbon arcs make the type of UV which makes ozone?
    I am not so sure but the major chemical product of an open arc is nitric oxide (NO) which further reacts with oxygen to form NO2. Production of ozone by UV (far UV; not near UV) is efficient only at low temperatures.

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    Re: carbon arc modalities

    Is the nitric acid byproduct a major contributor to electrode wear?

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    Re: carbon arc modalities

    Is the nitric acid byproduct a major contributor to electrode wear?
    Nitric acid is NOT a byproduct; as I mentioned, nitric oxide is the primary byproduct and it is converted into NO2 by atmospheric oxidation. Nitric acid is formed later from reaction with moisture.

    But the electrode wear is caused by (i) combustion and (ii) volatilization. Also a part of the electrode simply disintegrates and falls off.

    Nitric acid has no role in the electrode wear in a carbon arc.

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    Re: carbon arc modalities

    Are there off the shelf constant DC current drivers which can power the arc without its protection features blocking the arc from operating?

    Any decent LED driver which has protection features built in wouldn't power an arc since its not akin to a typical LED load.
    Last edited by Zak28; 24th August 2019 at 01:40.

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