Modern HeNe laser power supplies are of the high frequency switching type, operating between 20 KHz and 110 KHz. They are made to accept almost any input from 5 VDC to 240 VAC, and to operate any HeNe from 1/2 mW up to 30 mW and larger. Physical sizes range from 1.3 cu. in. to about the size of a brick. They are generally potted in solid thermally conductive potting material to contain the high voltages, up to 11 KV, to conduct the heat throughout the package, and for mechanical stability. Certain applications may require slightly different packaging.
If AC power is available, it is preferable because it is pretty much inexhaustible. Suitable AC voltages are 100 (85-120), 120 (108-132), and 220 or 240 (215-265). If no AC is available, battery or DC voltages of 5 V, 6 - 9 V such as a couple of 9 V alkalines in parallel, 10 - 14 V such as a vehicle battery, 12 - 18 V such as two 9 V batteries in series, or 22 to 30 V such as is found in some copiers, etc.
A very good strategy might be to use a DC input supply in conjunction with a wall plug converter, to avoid the cost and problems of complying with UL, CSA, VDE, etc. if your application would otherwise require it. Most applications, however, do not.
Beware of feeding the power supply with a switching converter. Unless the output is very clean, the switching action may interfere with the laser power supply, causing instability and malfunction.
Whatever input you use, though, there must be sufficient current available to provide the inrush current required be the power supply. This will typically be from several amps. for a low voltage DC input running a large laser to perhaps one amp. for an AC input. Most batteries handle this easily; fusing, however, must take this into consideration.
C.1 Output Voltage:
C.2 Start Voltage:
Depending on the model of power supply, it will produce more than eight thousand or more than ten thousand volts for starting the laser. Generally, this voltage starts them virtually instantly. Occasionally, though, we encounter a hard starting laser which may take as long as several minutes to start. A laser such as this should just be returned to the maker.
C.3 Output Current:
Output current should always be set, give or take a half milliamp, at whatever the laser manufacturer recommends. Settings much lower than recommended may get below the laser's dropout current, with the result being intermittent, or oscillating, operation. As noted previously, this condition is not good for either the laser or power supply and should be avoided. Settings much higher than recommended will likely result in laser-induced noise and, at higher levels, reduced laser power.
C.4 Noise and Ripple:
If your application is one of most, noise & ripple are of little concern. Where you need a coherent, highly focusable light, it generally matters not if the intensity of the light varies slightly, invisibly. Remember that most lasers attenuate ripple effects which come from the power supply by three to ten times. If your application is one which is sensitive to variations in beam intensity, specify a level of current ripple which is near the level you can tolerate from your laser.
C.5 Current Pot - Yes or No?
If you intend to operate more than one laser requiring different currents from one power supply model, specify a current pot. Be prepared to measure the current, in the cathode lead,with everything connected securely. High voltage can be uncomfortable, to say the least.
Otherwise, specify a current setting which is right for the laser you are using. One or two tenths of a milliamp one way or the other are not normally a concern.
C.6 How to Measure Current and Dropout Current
To measure laser current, use an analog meter of 1 to 10 mA inserted in series with the laser in the cathode lead. A digital meter can also be used, but it should be shunted by two back-to-back fast diodes to protect the meter in case of a fault. 1N4148's are fine for this job. When measuring dropout current, start the laser with a normal setting, then back the current down and note the reading at the point where dropout occurs.
D.1 Laser Tube & Ballast:
By far the largest share of the heat developed in these systems is by the laser and ballast resistor. HeNe's turn virtually all of their input power into heat. In the case of a five mW laser, for example, we might feed it 2500 V at 6.5 mA, or 16 + watts, and only about 5/1000 watt of laser light is produced. The rest is given off as heat.
D.2 Power Supply Heat: Is Heat Sinking Required?
This type of switching power supply is very efficient, under most conditions better than 75%. The power supply, then, might draw as much as 22 watts and deliver 75% of that to the laser and ballast. That means the power supply is turning about 5.5 watts or less into heat, which the thermally conductive potting material spreads fairly evenly throughout the package. What this means is that, as long as the power supply is not surrounded by some thermally insulating material or exposed to excessive ambient temperatures, no heat sinking is ever required.
caution : The user must be careful that ambient temperature, especially inside the equipment which contains the laser and power supply, does not exceed that specified by the laser or power supply manufacturer.
D.3 Positioning of the Components
The power supply does contain several components which may have their service life reduced by excess heat. Therefore, it is suggested that the user keep the power supply away from the laser housing so that it will keep as cool as it can.
E.1 Effect of Current Variation on Laser Output:
With larger lasers, small variations in current cause much smaller changes in laser output. Thus, current variations of a couple of tenths of a milliamp one way or the other are cause for little concern in most applications. However, most of these power supplies are designed to meet even the most demanding applications, thus they have very tight regulation, under all specified conditions of line and load.
E.2 Dropout Considerations:
When designing a HeNe system, it must be remembered that placing a metal housing around a plasma tube will raise its dropout current by as much as a milliamp. It does this by increasing the capacitance close to the anode. Also, if a tube is in a nonconductive housing, for example Lexan, placing the hand around the housing will have nearly the same effect. Therefore, dropout current must be determined under conditions similar to the above and the power supply current set or specified accordingly. A laser should always be provided with about one milliamp more than its measured dropout current to avoid encountering dropout. Also, remember that dropout current increases by perhaps half a milliamp as the laser warms up.
F.1 Relation of Input Voltage to Current:
Since these power supplies are true power converters, i.e. watts to watts, it will be seen that input current varies inversely with input voltage, so that with a constant load (laser and ballast), input current increases with reduced voltage and vice versa.
F.2 Battery Considerations:
For battery operation, four voltages are recommended: 9 V, 12 V, 18 V and 24 V. For small, hand-held laser systems, 9 V or 18 V systems each can make use of two 9- volt alkaline batteries, which will give about 30 to 45 minutes of laser operation, depending on the laser selected. For larger systems, such as a construction laser, a vehicle battery of 12 or 24 V can operate a two milliwatt laser for several hours and still start the vehicle.
The 5 volt input power supplies are optimized for use with a computer system power supply.
F.3 Low Input Voltage Effects:
When the battery's voltage falls below the power supply's minimum input value, the supply will go out of regulation or "squat" and the laser will start dropping out. The power supply will then go into start mode and re-start the laser, repeating this cycle until it is shut down. This shows up as a type of oscillation at around 10- 20 Hz. The laser will make a series of spots if scanned or swept across a surface such as a wall, and the power supply will make an audible buzzing. This condition is not good for either the laser or power supply and should be stopped when it becomes apparent. For a situation where the laser is needed, such as when using a laser pointer while speaking, an effort should be made to minimize use of the laser and replace the batteries as soon as possible. Spare batteries are recommended for these situations.
G.1 Mounting Required:
Most of these power supply modules are provided with at least two mounting holes centered 2.3 inches apart. These holes are located close to the side of the module and will accommodate size 6-32 screws. Modules should be kept at least 1/2 inch from the laser tube or housing to allow them to stay cool. As noted previously, it is not necessary in most cases to mount the module to any heat sink, although if one exists in the form of a metal case, for example, mounting it to this can only help extend service life.
G.2 Output Wires:
The high voltage positive wire is, in every case, a high voltage stranded wire with at least 20 KV insulation. The return wire generally is, too, although some configurations use a thinner wire with perhaps a teflon insulation. Where the return lead must pass close to the laser anode, though, the heavier insulation is preferred.
Many of the larger power supplies are made with a high voltage connector molded onto the leads. This style of connector was popularized by the Alden company, so is often called an "Alden" connector, although they are now made by other companies.
G.3 Special Configurations Available:
If an application requires a special form or fit, special configurations can often be supplied with short notice and at little or no extra cost. Special output voltages and currents can also be easily and inexpensively arranged, in most cases.
G.4 Fusing Requirements:
These devices should be fused to protect the line or batteries providing power. Fusing must be able to provide enough current for the turn-on inrush. For a 12-V input supply operating a 5 mW laser, a three to five amp fast blow fuse is recommended. For the same size system operating from the 120 V line, a one amp fuse is appropriate. A one-half to one milliwatt laser operated by two 9-volt batteries should not require fusing. For further guidance on this, contact the power supply manufacturer.
H.1 Adjust the potentiometer:
1). Connect the power supply to the input power, laser, and the current measuring equipment
2). Turn on the power
3). Look at the output current
4). Turn the potentiometer counter clockwise to lower the output current, turn the potentiometer clock wise to increase the current
5). Stop adjusting the potentiometer at the required output current