March 31, 2024
March 31, 2024
THIS PROJECT HAS RISK OF SHOCK AND FIRE!
The sodium-vapor lamp is a kind of discharge lamp that utilizes the light emission of excited sodium-vapor in the arc. There are two types of sodium-vapor lamps, high-pressure sodium (HPS) lamp and low-pressure sodium (LPS) lamp. Both of the soidum-vapor lamps are widely used for the lightings in the streets, parking lots, facilities and anywhere outdoor. The sodium-vapor lamp produces a characteristic night view filled by pale orange light or amber light as shown in right image.
The LPS lamp is a type of vacuum discharge lamp, such as fluorescent lamp. It has some features shown below:
The sodium D-line emission in vacuum discharge becomes nearly a single line (589 nm). This monochromatic amber light enhances the eyesight in smoky air, and the high energy efficiency benefits all-day lightings in economy. These are the reason why LPS lamps have been widely used for the road lightings in tunnels, underpasses and foggy regions. However LPS lamps, as well as HID lamps, on the time to maintenance are being replaced with energy efficient LED lamps, because the diesel exhaust has become clean by automobile NOx/PM reduction law. The production of LPS lamps has been ended in 2019 worldwide, and LPS lamps will be disappear from the road ligtings in near future.
The LPS lamp has some unique features, especially its monochromatic light emission, that any other light source does not have, so that every light bulb maniac will not stop to play with LPS lamps. In this project, I made LPS lamps light with home-built control gears and luminaires.
| Size (L/D) | SOX | SOX-E |
|---|---|---|
| T50 (216/52) | - | 18 W |
| T50 (311/52) | 35 W | 26 W |
| T50 (425/52) | 55 W | 36 W |
| T65 (528/66) | 90 W | 66 W |
| T65 (775/66) | 135 W | 91 W |
| T65 (1120/66) | 180 W | 131 W |
In comparison with HPS lamps that in many and various styles, there are only a few types of LPS lamps in the market that listed in Table 1. The SOX lamps were sold in Japan as product code of NX, and SOX-E lamps were not sold in the end.
The optimum temperature of discharge tube for maximum light output is approx. 260 ℃ and the luminous efficacy of the lamp improves as the power consumption to maintain this temperature lowers. However the discharge tube of LPS lamp is large in size compared to the HID lamps, and the thermal insulation was not sufficient in early days. The standards of LPS lamp have continually upgraded as the improvement of thermal insulation as SO → SOI → SOX → SOX-E. SO lamps consist of a U-bend discharge tube with a detachable dewar jacket. SOI lamps are in integrated and evacuated outer tube for thermal insulation like HID lamps. SOX lamps have IR-reflective coating inside the outer tube to reduce the thermal radiation loss. SOX-E lamps are in improved corting, optimized lamp current and higher lamp voltage. The SOX and SOX-E lamps ware used until the end of age after the '80s. There was linear stlye LPS lamp (SLI) like linear fluorescent lamp, but it was superseded by SOX lamps and gone from the market.
Figure 2 shows the control circuit in electromagnetic ballst. The input voltage is applied to the lamp via a simple choke coil or a leakage transformer. The igniter, in mechanical or electrical, switches the lamp terminals and generates inductive kickback to start to discharge. There is another type of ballast, lead-peak type ballast, which eliminates the igniter by its high peak factor output waveform even with limitations of open circuit voltage.
The LPS lamps are mainly used outdoors, in the tunnels and somewhere in moistly environment, so that the electronic ballast was not used for LPS lamps as much as one for fluorescent lamps that mainly used for indoor lightings. Of course, LPS lamps are the same vacuum discharge lamp as fluorescent lamps, and the LPS lamps can also be driven in efficient high frequency inverter. As shown in Figure 3, the electronic ballast for LPS lamps is quite simple construction compared to the electronic ballast for HID lamps.
| Item | AC Powered | DC Powered |
|---|---|---|
| Lamp Type | SOX-E 36W | SOX-E 18W |
| Input | AC 100-120V | DC 24V |
I designed two different type of electronic ballasts shown in Table 2 and built the luminaires for LPS lamp. The one is powered in AC mains and the another is powered in DC supply. These electronic ballasts are almost identical in basic design, so I describe AC type ballast in details, but only in differences about DC type ballast.
Figure 3 shows the schematic of the built electronic ballast. The markers indicated in alphabet in the schematic are the probing points for waveforms reffered in this document, voltage A as Ch-1, current B as Ch-4 and volatage C as Ch-2.
The half-bridge inverter consist of Q2 and Q3 and outputs sqare wave of 65 to 75 volts depends on input voltage. The inverter is driven in PWM output of microcontroller. C14 and R14 at the inverter output are an RC snubber. The absorbed energy is fed to the auxiliary supply.
The output of inverter is DC-blocked with C9 and tied to the LC matching network consist of L1 and C10. The L1 (*1) is an inductor in a couple of ferrite cores B66317G0000X187(g=0.4mm), 58 turns of litz wire in 0.1 mm by 40 strands as main winding and a turn of wrapping wire as auxiliary winding. The saturation characteristics is shown here. The C10 is a polypropylene capacitor for high current resonant circuit rated 1.6 kV / 10 nF. The LPS lamp is connected at C10 in pallarel.
The inverter is driven at a frequency around the resonant frequency of LC network, f = 1 / (2π√(LC)), so that a large current by series resonance will flow throuth the LC network. The resonant current produces a high voltage several times to ten times of inverter output across L1, C10 and also the lamp. When the lamp breaks down and starts to discharge, it means a load resistance is inserted into the LC network. As the result, the Q factor is lowered and the resonance is quenched. The drive frequency and the value of LC are the factor needs to be chosen to set the lamp current to optimal condition. The drive frequency may need to be adjusted in the control sequence if needed.
The state of discharge needs to be detected and controled for safe operation. In this project, the inductor current is monitored by a microcontroller to detect the state of discharge. The induced voltage on the auxiliary winding of L1 is rectified by a diode, and the output voltage, voltage C in schematic, is input to the microcontroller. When the inductor current is assumed a sinusoidal wave, the voltage at point C as V becomes:
V = (2π・f・L・IL・N - VF)・R6 / (R6 + R7) -- Equation 1
(where IL is inductor peak current, N is turn ratio (1/58), VF is forward volatage of D6.
When the bus voltage comes to a certain level to operate, the control changes to the BLANKING state. After a blanking time, 500 ms, elapsed, it changes to the IGNITION state and enables the inverter at 80 kHz. There is no PRE-HEAT state like start-up sequence of fluorescent lamps, so LPS lamp starts to discharge from IGNITION state. This scheme accelerates the wear of electrodes on ignition but it can be negligible for road lightings and any use with low frequency of on-off cycles. Figure 4 shows the wave forms at the start of ignition. The lamp not in discharge is actually an insulator, so that the Q-value of LC network is quite high and a large resonant current will flow through the LC network. From the wave forms, the inductor current increases quickly and the peak voltage across the lamp reaches 1 kV at some cycles, about 50 μs, after start of ignition and the lamp breaks down.
The discharge always starts from glow discharge. If the lamp current is ehough to heat electrodes (take-over current), the lamp will immediataly come into an arc discharge. Figure 5 shows the process of a take-over. The reason why wave forms change in two steps is that there is a delay in time of take-over on each electrode. In this figure, the inductor side of electrode takes-over at 50 ms and the another one takes-over at 150 ms. The difference in electrical characteristics between glow discharge and arc discharge is its discharge impedance. Because the cathode fall in glow discharge is much larger than arc discharge, the discharge impedance of glow discharge is high. The damp factor of resonance depends on load impedance, so that break-down and take-over of lamp can be detected by monitoreing the resonant current. The inductor current lowers as the lamp impedance lowers. Ch2 in the figure is at current monitor signal to the microcontroller, probing point C, and the detection gain seems to be the nearly same as Equation 1.
If the lamp does not discharge due to end of life or lamp removal, a large resonant current continues to flow and it can result failure of some component. To avoid this problem, IGNITION state needs to be interrupted if a discharge could not be detected at a certain time. Figure 6 shows the wave forms on an ignition failure. The resonant current is estimated to be several ten amperes by calculations in SPICE, but the measured current is only several amperes in peak value due to a dampning by core saturation of inductor. If any discharge could not be detected at 10 ms after start of ignition, it is considered ignition failure and control returns to the BLANKING state. If arc discharge could not be detected at 300 ms after start of ignition, it is considered take-over failure and the control returns to the BLANKING state. If arc discharge could be detected at 300 ms after start of ignition, it changes to the RUN state. In RUN state, drive frequency is changed to set lamp current to proper level if needed. However, any frequency with the current phase leading causes significant heat of transistors due to a reverse-recovery loss of body diode, so that the frequency must be higher than resonant frequency. In this project, drive frequency in RUN state is 85 kHz. If an extinguish or short-circuit is detected in RUN state, the control returns to the BLANKING state.
Figure 7 shows the state diagram of the operation described above. If the ignition failure repeated certain times, it will be considered "cannot work" and stay blanking state until power off.
The LPS lamps are large in size compared with HID lamps, I built the luminaires that designed specially for each LPS lamp. Figure 8 shows the built luminaire and its rough sketch. It consists of aluminium plates and transparent acrylic boards in 2 mm of thickness each and is shaped like a lantern. The acrylic board is bended with a certain radius to make it in rounded shape as a design, and also it can minimize the glueing and maintain the transparency. However bending the acrylic board in arbitrary radius is not an easy work and it needs some jig. I used a furniture with a rounded corner for a bending die and succeeded. The base cap of LPS lamp is BY22d, a variant of B22d withstands high-ignition voltage, but the lamp holders for LPS lamps are not in the market any longer, so that I modified a E27 to B22d socket converter to use it for this project.
There is an essential characteristic of LPS lamps that needs some consideration to use it, because not a few metallic sodium is sealed in the discharge tube. The sodium is in liquid phase while the lamp is in work and about 15 minutes after power off. Within this time, any shock or vibration should not be applied to the lamp, or the sodium can flow in the discharge tube and spread on the inner surface or lead-in wires. This can cause loss of lumen or seal failure of discharge tube. In general, the burning position of LPS lamsp is in horizontal ±20 degrees, to avoid flow of sodium and to maintain even thermal and sodium vapor distribution of discharge tube. As for small LPS lamps in T50, they can burn in base up ±110 degrees.
The luminaire can be put anywhere on the table or floor, but the position of luminaire needs to be under restriction of burning posision of lamp. When set the luminaire in vertical, the lamp holder needs to be upward. I made two keyholes on the back plate to hang the luminaire on the wall or ceiling easy.
Figure 9 shows the built DC powered luminaire for 18 W SOX-E lamp. The only difference in the construction is the length of body and any other dimensions is same as 36 W AC powered one.
The configurations of electronic ballast and the control program except configuration parameters are almost the same. There is a significant difference in supply voltage. The supply voltage is much lower than the lamp voltage, and it lowers the efficacy of the matching network at too high matching ratio, so that 24 V, the maximum voltage in generic wall adapter, of input voltage is chosen and a full-bridge inverter is used to make the output voltage of inverter high as possible.
