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The electrical circuit of a heat-lamp is not different from that of an incandescent lamp for lighting purposes: An AC- or DC voltage source connected to the two leads of the lamp directly or through an on/off switch and/or a safety fuse. In rare occasions an
incandescent lamp with two separate filaments in the same bulb was used. Using a 3-pole socket, the filaments of for instance 100- and 150 Watt than could be switched individually or in parallel, resulting in a total wattage of 250 Watt in that case. The electrical circuit of a sun-lamp in its basic form consists of two carbon rods or a gas
discharge tube in series with a resistor. The objective of the resistor is to stabilise the current since the conductance of a voltaic arc or a gas discharge increases with the current through the circuit. When connected to a supply with constant voltage directly this would lead to a vicious circle since an increase in conductance (= lower resistance) consequently would result in a further increase of the current through the circuit. Without any measures to limit the current, the process would end in the destruction of the ultraviolet source, the voltage source or both. To limit the current I, a load with a resistance R is switched in series with the voltage source U. The value of R has to be constant or, even better, has to increase with increasing current. According Ohm's law (see infrared emission) the current through the circuit is limited to its short-circuit current I = U/R. The power to be dissipated in the resistor equals U x I, the product of the supply voltage and the current. At a supply voltage of 230 volts and a desired working voltage over the discharge tube of for instance 55 volts at a working current of 1,4 Ampere, a serial resistor of (230-55) V/ 1,4 A = 125 Ohm would be needed. Its maximum power capability than would be (230-55) V x 1,4 A = 245 Watt. The discharge tube under this circumstances would dissipate 55 V x 1,4 A = 77 W so with this kind of limitation about three quarters of the applied power is dissipated in a resistor whose only purpose is to limit the current through the discharge tube. Most of this power is dissipated in the form of infrared radiation and the idea to apply this radiation as part of the ultraviolet therapy is not surprising
therefore. There are a few basic implementations for the serial resistor, depending on whether the infrared radiation is considered to have a positive contribution to the therapy or not. When the infrared contribution is not desirable, the resistor is placed more or less out of sight. Furthermore in that case it is dimensioned in such
a way that glowing of the material is avoided. When the infrared radiation is desired however, the resistor will be placed in sight
and its dimensioning is focussed on an intense glowing of the material. The resistive element is often made of Kanthal, an alloy containing iron, chrome and aluminium. The most important property of Kanthal is that it can be made glowing under atmospheric conditions without any vaporisation of the material. In early configurations the
Kanthal wire was wound around a relatively large and open ceramic bearer. More recent is the so-called quartz element, consisting of
an open coil of Kanthal wire. This coil is fitted into an open tube made of quartz glass which glass can withstand the high temperatures of the glowing Kanthal. Commonly the quartz glass is semitransparent with a milky white colour, caused by a fabrication process that leaves a great number of tiny air bubbles in the glass. Most sun-lamps equipped
with quartz elements are given the possibility to switch off the discharge tube and use the quartz elements only. In the most
basic configuration the discharge tube is simply by-passed, making the quartz element to operate at the full supply voltage. As a result the quartz element will glow up and the wavelength of the emitted infrared radiation will become shorter and, for therapeutic use, of a better quality. With this construction the wavelength during combined operation of
the infrared- and the ultraviolet radiator is consequently longer and of a lower therapeutic quality. A little more sophisticated therefore
is the application of separate sections of quartz elements. When used as an infrared device only, both sections are switched in series and connected to the voltage source directly. When used in combination with the ultraviolet device, one of the infrared sections is replaced by the gas discharge tube. The total amount of infrared radiation than decreases but
the wavelength of the emitted radiation roughly stays the same. Due to dimensioning constraints or aesthetic considerations the quartz tube sections as drawn in the circuit diagrams are often composed out of two separate quartz tubes in series. From electrical point of few this does not change the configuration. In
Since the physical- and electrical operation of a gas discharge tube is a polarity sensitive process, operation in an AC environment requires additional measures. One of the options is to use a configuration with two anodes, one active in the positive half of the alternating current and one in the negative half. Another solution is to use specially designed electrodes that can operate both as a cathode and as an anode, depending on the momentary polarity of the voltage. Most sun-lamps with high-pressure mercury vapour discharge tubes operate according this concept as well as tube-lights like applied in face-tanners. In carbon rods like applied in carbon-arc sun-lamps the binding forces that hold together the carbon atoms in their grid is relatively weak. Due to the high temperatures that come with an arc discharge the rods will gradually get ionised and vaporised. The greatest loss of material takes place at the tip of the anode rod where the temperature is at his highest level due to the never lasting bombardment of electrons coming from the cathode. A part of the mostly ionised carbon atoms will sublimate on the relatively cold cathode. The
the 70's and 80's of the 20th century a number of models have been sold under the type classification "impulse sun-lamp". The main feature of these lamps was the strongly shortened treatment time. This was realised by decreasing the serial ballast for a short period of time, thus causing the ultraviolet radiation to intensify strongly. The discharge tube
triggered automatic pulse switches. The therapeutic value of combined ultraviolet- and infrared treatment is not undisputed. The effectiveness of ultraviolet therapy is not proven to become any better when combined with a simultaneous infrared therapy. For a proper treatment of skin diseases the additional infrared radiation even may be a disadvantage due to the heat that prohibits a short radiation distance. For other medical indications the additional warmth may add some comfort to the treatment at best since the wavelength of the infrared radiation that comes with Kanthal- or quartz elements is longer than desired for therapeutic infrared applications. The maximum working temperature of Kanthal for instance is about 1400K, giving a corresponding maximum for the emission spectrum of about 2100 nanometers. This
was able to withstand this overburdening for only a few or at most ten seconds and the control was therefore done by manually
makes a Kanthal element less effective for therapeutic applications than the specially designed infrared incandescent lamps that operate at temperatures as high as 2900K and a corresponding wavelength maximum of about 1000 nm. As stated in most user manuals, application of these quartz tubes is limited to cosmetic applications. Instead of a Kanthal element an incandescent lamp
may be used to limit the current through a discharge tube. The filament of an incandescent lamp can withstand much higher temperatures than a Kanthal element, resulting in infrared radiation with wavelengths that are more suitable for therapeutic applications. A special variation of such a configuration is the so-called blended lamp in which a gas
discharge tube is mounted directly in series with the filament of
the incandescent lamp while both are mounted in one and the same glass bulb. It is uncommon for a blended lamp to provide independent use of its infrared- and ultraviolet capabilities but the Philips Biosol 11911/28 was an exception to this rule. With this sun-lamp the gas discharge tube and the filament were indeed mounted in the same glass bulb but their leads were connected to a three-pole socket. This allowed the discharge tube to be ballasted either by the filament in the bulb or by a separate ballast, mounted in the base of the lamp.
small crater in the anode and the complementary tip in the cathode caused by this process are typical for a DC-operated arc-lamp whereby the speed at which the anode burns away is almost twice that of the cathode. In order to maintain a quiet and stable arc discharge the crater in the tip of the anode must be kept more or less in the center of the carbon rod. This can be achieved by using an anode rod that is either hollow or filled with a softer type of carbon. A softer core can also easily be enriched with small parts of metal in
order to alter the spectrum of the radiation. Adding some iron to the anode core for example increases the radiation in the ultraviolet region. The cathode does not need to be hollow but it has to be fabricated from a type of carbon that easily can supply a large quantity of electrons without getting too hot. When operated with an AC-supply the arc-lamp has to be equipped with two identical carbon rods from the same type as the discussed anode rod. In case of an AC operated sun-lamp, a coil or a transformer may be used as a ballast instead of a true resistor. Unfortunately coils and transformers that need to operate at the relatively low line frequencies of 50 (Europe) or 60 Hertz (USA) are heavy and expensive but this disadvantage is often more than compensated by the gain in power efficiency due to the reduced power dissipation. Another appreciated advantage of a heavy transformer is its ability to stabilise the armature mechanically when placed in the base of the armature. For TL-illumination it has been proven to be profitable toincrease the frequency above the standardised line frequencies of 50 or 60 Hz. The ballast coil or transformer can than be constructed much lighter and cheaper without loosing its current limiting properties. For this reason lighting applications operating at a frequency of about 400 Hz can be found in ships, aeroplanes and in industrial plants. While application of coils or transformers is limited to AC sources, incandescent lamps and resistors (among which the quartz tubes) can be used in AC- as well as in DC configurations. To allow for comparison between AC- and DC sources the voltage of an AC source is characterised by its
effective value instead of its amplitude. The effective value of an AC-voltage with amplitude x is the value y for which a DC-voltage with this value causes the same energy dissipation as the AC-voltage. Our well-known 230 volts AC line supply in fact delivers a sinusoidal voltage with an amplitude of 325 volts, giving the same dissipation as a 230 volts DC supply
high frequency operation
Ballasting coils and -transformers can be kept small by the application of frequency converters. In a frequency converter the low frequency alternating voltage is first rectified and than electronically transferred to an alternating voltage with a much higher frequency. By operating the gas discharge lamp at this higher frequency, the desired current limiting effect can be reached with a much smaller and cheaper coils or transformers. Frequency converters however, are too expensive for most sun-lamps and face-tanners for domestic use who therefore always are operated at the standard lower line frequencies. A special form of high
frequency operation had been applied in sun-lamps from Sun-Kraft. This company specialised themselves in contactless discharge tubes. In these tubes a glow discharge was enforced by placing the tubes in a high frequency electromagnetic field, generated by an electronic oscillator. Due to the lack of electrodes the lifetime of these contactless tubes was virtually unlimited as long as they were not mechanically damaged.
In order to maintain an arc discharge, the cathode of a discharge tube has to emit large quantities of electrons. This requires a thermal emission from a hot cathode. To obtain this thermal emission you can simply connect the discharge tube to a serial resistor and a voltage source and than just wait until a spontaneous dark discharge develops into a glow discharge and finally into an arc discharge (see ultraviolet emission). This process however is quite unpredictable and can take a long time if it happens at all. To overcome this uncertainty, the arc discharge has to be forced to start more quickly and over time many methods to ignite the arc discharge have been tested and applied.
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would do. When using an AC-supply, the addition of a diode can overcome the earlier mentioned problem that the quartz tubes will glow up more brightly when the ultraviolet source is switched off.
Next to here the electric circuit of a Philips HP3111 is shown, an AC-operated sun-lamp with one quartz tube section consisting of two serialised quartz tubes. In the infrared mode the current will flow to the quartz tubes during the positive half of the alternating voltage only, due to the diode that is now switched in series with the tubes. As a result of this configuration the dissipated energy in the quartz tubes is almost the same during the combined- and the infrared mode and prevents the application of an additional quartz tube section. The resistor left to the discharge tube is responsible for the ignition of the lamp as will be explained in one of the following paragraphs.
The most basic method to start the arc discharge was to temporarily increase the temperature of the electrodes to a point were thermal emission started easily. With carbon-arc discharge lamps this was indeed the only way to start the arc discharge at all. The carbon rods were shifted or tilted into such a position that
their tips just touched each other. Due to the short-circuit current the rods started to glow at the point were the electrical resistance was maximal, the point were the rods touched each other. When the rods were separated carefully, an arc discharge developed at the tips of the rods which than could be moved apart gently until their desired operational position was reached. Burning and evaporation of the carbon material widened the gap between the rods slowly and the position of the rods required a re-adjustment from time to time to prevent the
arc from extinguishing. With carbon-arc lamps for lighting purposes this re-adjusting process often was automated by some electro-mechanical construction but due to their relatively short treatment- and burning periods this was rare with carbon-arc sun-lamps.
A similar method from the early days of the discharge tube sun-lamps was the so-called tilt ignition. After connecting the voltage source, a discharge tube containing a cathode in the form of a
large pool of mercury was manually tilted until the pool of mercury reached the anode. The mercury now carried the full short-circuit current, only limited by the serial ballast. When the tube was tilted back slowly, the mercury would eventually disconnect from the anode, causing a small arc discharge (a spark) at the place were the last contact occurred. This discharge would locally cause some mercury to evaporate and the gas pressure and the temperature within the tube would rise slightly. This caused an environment in which an arc discharge that was a little larger than the initial one could be maintained. After a short while and, if necessary, some repeated tilting forth and back, the discharge
would become continuous even when the tube was tilted back completely and gas pressure and temperature would adjust themselves to the steady state working conditions of the circuit.
Apart from making contact between the electrodes, the initial discharge could also be obtained by a temporarily increase of the
field intensity around the cathode. For this purpose a coil was mounted around the outside of the tube at the level of the mercury pool. Through a switch and a capacitor the coil was now temporarily connected to the voltage source. Disconnecting the coil again caused a short voltage peak and thus a local increase in the field intensity. This
triggered a glow discharge that, instantly or after some repeated switching, developed into an arc discharge. In later designs a push button (S in the diagram next to here) started a relay-circuit that automatically opened and closed the switch until the start button was released. A little transformer T in series with the cathode induced a voltage pulse,
a method that was also suitable for high-pressure discharge tubes that lacked a pool of mercury.
An ignition method that uses a combination of inductive ignition and pre-heating of electrodes can still be found in Tube-light (TL) armatures and in TL based sun-lamps. Tube-lights are commonly started with a specially designed automatic starter consisting of a
small neon gas discharge tube S and a capacitor C. When switched on in a cold position the full supply voltage U of (in Europe) 230 volts AC is applied to the discharge tube S of the starter through the coil L and the two filaments G. This causes a glow discharge in the starter for which the coil and the two filaments form the current limiting ballast. Due to the resistance of the coil and the filaments the voltage over the starter drops to about 200 volts. Since the current is still relatively small, the filaments are hardly heated and most of the time there is no gas discharge yet in the Tube-light itself. This situation would not alter if not one of the electrodes of the starter was constructed as a bi-metal. A bi-metal
consists of two strips of metal with different expansion coefficients, firmly pressed against each other. This causes a straight strip of bi-metal to bend when it is heated and to straighten again when it cools down. Although there is only a little heat dissipation in the discharge tube of the starter, this dissipation is sufficient to make the bi-metal bend. The bi-metal electrode is now positioned in such a way that it comes in touch with the other electrode when it
bends over, causing a complete short-circuiting of the starter. The short-circuit current that starts flowing now is only limited by the coil and the two filaments and is large enough to make the filaments glow. Since the short-circuiting of the starter also terminates the glow discharge within the starter, the bi-metal electrode will cool down and straighten again until it comes loose from the other electrode. The serial circuit of the coil L and the capacitor C now causes a voltage pulse to occur and it is this pulse that temporarily increases the field intensity between the two filaments G of the Tube-light. Combined with the heated state of the filaments this pulse starts an arc discharge between the two filaments of the Tube-light. Since a much larger electrical current now starts flowing through the Tube-light and the coil, the voltage over the starter drops to about 160V. The discharge tube S is then dimensioned in such a way that this voltage is not sufficient to start a new glow discharge that can bend the bi-metal again and a steady state is reached as long as the arc discharge within the Tube-light is maintained. In case the arc discharge in the Tube-light is not ignited in the first cycle, the process in the starter repeats until it does. This causes the well known blinking and clicking of a defective Tube-light which will only end when the supply voltage is switched off or the starter or one of the filaments breaks down. Basicly the same automatic ignition circuit was used
to start medium pressure discharge sun-lamps like the Philips Biosol 11856. Since these discharge lamps lacked the filaments of the Tube-light, an additional resistor R provided the ballast and current limiter for the starter. Also a
second and larger capacitor C2 was added which, in combination with a heavier coil, induced a sufficiently large field to ignite the lamp when the starter contacts opened.
Another method that is still used quite frequently is that of the glow ignition in which case a second anode is mounted near the cathode. The serial resistor R2 limits the current to such a level that only a glow discharge between the cathode and the additional start-anode can develop. This glow discharge causes
the gas pressure and the temperature to increase until a level were an arc discharge between the cathode
and the main anode can be established. The diagram next to here is that of a PMC Capri Super, an AC-operated sun-lamp with one quartz tube section consisting of two quartz tubes, a double glow ignition and a fall-over protection. The fall-over protection consists of a small glass tube, partly filled with mercury. The mercury connects the two electrodes as long as the tube stays in a horizontal position but it disconnects as soon as the tube (and the sun-lamp it is mounted in) tilts over.