Laser Power Supplies

Imagine a world without electricity. Without the motive force of electricity and more importantly a way to harness it, we would be without 90 percent of our creature comforts. Everything from the family car to the kitchen food processor operates on electrical power, and without juice, these things would come to a grinding halt.

The same is true of lasers and their support systems. Without power, your laser is useless and no more worthwhile than a rock paperweight. In this section, you’ll learn how to construct universal power supplies, including:

* High-voltage power supplies for operating a helium-neon laser; both 117 volt ac and 12 volt dc versions.

* Regulated power supplies for diode lasers.

Low-voltage power supplies for operating electronic equipment are in the next section. There you’ll find designs for single- and dual-voltage regulated supplies, including an all-purpose adjustable version and battery pack regulators.

ABOUT HELIUM-NEON POWER SUPPLIES

A helium-neon laser tube must be connected to a high-voltage power supply or it won’t work. You have two options to provide the required juice: buy a ready-made laser power supply or build your own. Commercially made power supplies for helium-neon lasers are available from a variety of sources, and if you are just starting out, this is the best route to go. As detailed in Section 5, you need to be sure that the supply is rated for the tube you are using. Some tubes require more operating current than others and might not work properly with a power supply that can’t deliver the milliamps.

He-Ne laser power supplies you build yourself are not overly complicated and they don’t need lots of parts. But the parts they do require can be hard to find. Specifically, the laser power supply must use high-voltage diodes and capacitors—the higher the rating, the better. The 1N4007 diode is rated at 1 kV, the minimum you can use. Such diodes are bound to burn out when running a laser that consumes more than 5 milliamps, so 3- to 10-ky diodes are preferred. High-voltage capacitors of the typical values used in laser power supplies — 0.1 to 0.001 F, are even harder to find. Most high-voltage capacitors have very low values, usually in the tens of picofarads.

Perhaps the most troublesome component is the transformer. The ideal laser power supply transformer is specially made to conform to the specifications required by the job, but a number of ready-made step-up switching type transformers can effectively be used. The hard part is finding them. The typical transformer for use in a dc-operated helium-neon laser steps up 12 volts to between 300 and 1,000 volts. High-voltage transformers designed for use with photocopiers can also be used. These transform 117 Vac to 1,000 to 4,000 Vac. Most laser tubes require between 1,200 and 3,000 volts.

A local surplus or electronics outlet can carry suitable high-voltage diodes, capacitors, and transformers, but you might have better luck trying surplus mail-order outlets. See Section A for a list of mail-order surplus dealers. Ask for their latest catalog, and if you don’t see the items you want, write or call. Some outlets carry stock that isn't included in the general distribution catalog.

Section A also lists several sources for laser components. These include Meredith Instruments, MJ Neal Co. Information Unlimited, and General Science and Engineering. These mail-order companies are prime sources of laser power supply components, and you should obtain their catalogs before beginning any serious laser project.

Many also offer power supply kits, with all the parts conveniently pre-packaged for you. In fact, one of the dc power supplies discussed below is available (at the time of this writing) in kit form from General Science and Engineering. In the event that the kit's no longer available, you can still construct the power supply using the schematic and parts list, provided in this section.

Before building any of the laser power supplies described in this section, read the following very carefully:

* Any laser power supply delivers high voltages that, under certain circumstances, can injure or kill you. Use extreme caution when building, testing, and using these power supplies.

* Don't attempt to build your own power supply unless you have at least some knowledge of electronics and electronic construction.

* Although the power supply projects are not difficult to construct, they should be considered suitable only for intermediate to advanced hobbyists.

* Power supplies and laser tubes retain current even after electricity has been removed. Be sure to short out the output of the power supply before touching the la ser or high-voltage leads.

ill. 11-1. An easy approach to building a high-voltage He-Ne power supply. Be wary of the high voltages present at the secondary of T1.

• C2-C5 2KV
• D2-D5-2KV
• R2 3watt minimum
• Q1 on heat sink
• 200 ma input minimum
• Try IRF511 or TRF61O for Q1

The schematic in ill. 11-1 shows a basic, no-frills power supply suitable for use with helium-neon tubes rated at 0.5 to 1 milli-watt. TABLE 11-1 contains the parts list. The circuit's shown more as a lesson in high-voltage power supply design than a full-fledged project. You will probably want to supplement the supply with additional features, such as a 10-kV trigger transformer or current feedback circuit. A number of books and web sites provide details on advanced high-voltage power supplies; see our References for a selected list.

Table 11-1. Basic Dc He-Ne Power Supply Parts List

• R1 270 ohm resistor
• R2 180 k-ohm resistor, 3 to 5 watt
• C1 22 uF electrolytic capacitor
• C2 uF electrolytic capacitor
• C3-C6 0.02 uF capacitor, 1 kV or more
• D1 1N4001 diode
• D2-D5 High-voltage diode (3 kV or more)
• Q1 TIP 140 power transistor
• T1 High-voltage dc-to-dc converter transformer; see text for specifications.

All resistors are 5 to 10 percent tolerance, ¼ watt, unless otherwise indicated. All capacitors are 10 to 20 percent tolerance, rated 35 volts or more, unless otherwise indicated.

At the heart of the power supply is a dc switching transformer, T1. This oscillation transformer is designed for use as a dc-to-dc converter and is available from Meredith Instruments. It has the following characteristics:

* Input voltage (primary): 6 volts

* Output voltage (secondary): 330 volts

* Ferrite core size: EE19

* Maximum power output: 7 watts

* Oscillation frequency: 15 kHz

* Winding ratio (Ns/Np): 57.4

Operation of the Power Supply

Here’s how the power supply works: Q1, R1, and C1 form a Hartley-type astable multivibrator (a free-running oscillator) that switches the incoming 12 volts dc between the two secondary windings of the transformer. T1 is a high-turns-ratio transformer that steps up the incoming voltage to about 660 volts. Capacitors C2 through C5 as well as diodes D2 through D5 form a voltage multiplier that increases the voltage to about 2,500 volts at approximately 3 to 4 mA.

Resistor R2 is an important component. All laser tubes are current-sensitive and try to consume as much current as the power supply will deliver. The resistor limits the current to a safe level; without it, the tube might burn out. Resistor R2 is chosen for a typical 1 mW laser tube. If your laser sputters or doesn’t fire, the resist or value might be too high or too low, causing the tube to be unstable. Later you will see how to test the power supply to discover how much current the tube is drawing and to adjust R2 to deliver just the minimum to keep the tube lasing.

Note that the battery power requirement is rather steep. The power supply consumes about 350 mA of current, so you should use only heavy-duty batteries. Although the supply will work on “C” alkaline cells, you’ll have better luck with “D” cells. The best results are obtained when using high-output lead-acid or gelled electrolyte batteries. A pair of 6-volt, 4 AH batteries will power the laser for several hours before needing a recharge.

ill. 11-2. Use a 7812 voltage regulator with a 14- to 16-volt supply to regulate the voltage to the high voltage power supply presented in ill. 11-1.

The power supply works best when the input voltage is as close to 12 volts as possible. Because most batteries deliver a range of voltages during their discharge period, you might want to add the regulator circuit provided in ill. 11-2 (parts list in TABLE 11-2). The schematic uses a positive 12-volt regulator that requires about 1 volt as “overhead.”

====

Table 11-2. 12 Vdc Battery Regulator Parts List

IC1 7812 + 12 Vdc voltage regulator

C1 2200 uF electrolytic capacitor

C2 1 uF electrolytic capacitor

All capacitors are 10 to 20 percent tolerance, rated 35 volts or more.

===============

When fed by the typical lead-acid or gelled electrolyte battery—which have an average output of about 13.8 volts—approximately 12 volts reaches the power supply.

Components R1 and C1 determine the frequency rate of the circuit. By adjusting R1, you change the frequency and therefore the output voltage of T1. If your supply is having trouble igniting and running your laser tube, try a slightly higher or lower value for R1.

Building the Circuit

The basic power supply should be constructed on a printed circuit board. Component placement isn't crucial, but you should allow as much room as possible for the high- voltage components. Keep the anode lead as short as possible (2 to 4 inches) and place the ballast resistor close to the anode terminal on the laser tube.

ill. 11-3. Connect a volt-ohmmeter (set to read millivolts) as shown to determine the amount of current consumed by the laser tube.

Testing the Current Output

Resistor R2, the ballast resistor, determines the amount of current delivered to the tube. Although you can calculate the exact value of the resistor using design formulas, you need to know the parameters of the particular tube you are using. Dial the meter to read dc milliamps. Turn on the power supply and watch the meter. The current shouldn't exceed 6 or 7 mA (it probably won’t with the basic power supply described earlier, anyway). If the current is too high, you should immediately remove the power. Short the leads of the power supply to remove any remaining current, and replace R2 with a higher value resistor. Be sure to use a resistor rated to at least 3 to 5 watts. Re-apply power and take a new reading.

Most likely, the laser will sputter or not turn on at all. The usual cause is a ballast resistor that's either too high or too low; either way, the sputtering is caused by unstable operation and can usually be corrected by selecting another ballast resistor. The tube won't ignite or lase if the current is less than about 3.5 mA. If the tube stays on with out sputtering and the current output is between about 3.5 and 6 mA, you have selected the proper ballast resistor.

If the ballast resistor is too low, excessive current will flow through the tube, damaging it or at the least severely shortening its life. Besides doing harm to the tube, the power supply consumes excessive current, prematurely draining battery power. You will realize the longest battery life by careful selection of the ballast resistor.

Note that some sputtering is caused by arcing of the anode and cathode leads. Be sure the leads are securely attached to the power supply and the laser. You can often see the result of arcing by turning off the lights and looking carefully for a tell-tale blue glow around the anode and cathode terminals. The glow is a corona caused by the ionization of air by high-voltage discharge.

PULSE-MODULATED DC-OPERATED HE-NE SUPPLY

The basic helium-neon laser power supply is good for low-output tubes, but it doesn’t deliver sufficient current for higher power and hard-to-start tubes. The advanced power supply shown in ill. 11-4 can be used with 1- to 5-mW tubes, depending on the transformer you use. The parts list for this supply is included in TABLE 11-3.

About the Circuit

The advanced laser power supply uses an LM555 timer IC as a pulse width modulator (PWM). Two potentiometers, R12 and R13, adjust the width of the output pulses from the 555, and therefore change the currents used to trigger and operate the tube.

Capacitor CS and resistors R8, R9, R12, and R13 determine the pulse width of the 555. Initially, the R12 and R13 are dialed to their center positions and relay R1 is de energized, effectively removing R8 and R13 from the circuit. When 12 volts is applied to the circuit, the 555 pulses and triggers Q1, C1, and R2. This in turn drives transformer T1. This transformer steps up the 12 volts to approximately 1,000 volts. Capacitors C7 through C10 and diodes D4 through D19 form a four-stage cascaded voltage multiplier that increases the output to about 3,500 volts.

If the tube doesn’t fire, R12 adjust to increase the duty cycle of the 555 pulses. When the tube ignites, sensing resistors R3 through R6 trigger Q2, which closes relay R1. That brings R8 and R13 into the circuit. Adjusting R13 controls the duty cycle of the 555 while the tube is operating. Shortening the duty cycle of the pulses decreases the current delivered to the tube; lengthening the duty cycle increases the current.

ill. 11-4. The circuit schematic for the pulse-width-modulated power supply.

Building the Circuit

The PWM power supply can be built on a perforated board or printed circuit board. When using a perforated board, be sure that lead lengths are kept to a minimum and that the high-voltage capacitors and diodes are not placed too close together. To prevent arcing, place the diodes at 45-degree angles.

The leads for the anode and cathode should be 6 inches or shorter. Reduce the chance of arcing by wrapping high-voltage dielectric tape around the leads. Or, slip a length of neoprene aquarium tubing over the wires.

Construct clips for the laser terminals as detailed in Section 6, “Build a He-Ne La ser Experimenter’s System.” You can also form heavy-duty steel or copper wire and bend it in a clip shape. Make the clip slightly smaller than the diameter of the laser tube terminals. When made properly, the wire should clip securely around the tube. Wrap a length of high-voltage dielectric tape around the clip and terminal to hold them in place. Be sure that you don’t cover the mirrors on either end of the laser.

Using the Power Supply

Operating the power supply is straightforward. Once the tube is secured, rotate potentiometers R12 and R13 to their center positions. Apply power and watch the tube. Slowly rotate R12 until the tube triggers. You will hear the relay click in. If it chatters and the tube sputters, keep turning R12. If the tube still won’t ignite, rotate R13 slightly.

Once the tube lights and stays on, rotate R13 so that the tube begins to sputter and the relay clatters. This marks the threshold of the tube. Advance R13 just a little until the tube turns back on and remains steady. Every tube, even those of the same size and with the same output, have slightly different current requirements, so you will need to readjust R12 and R13 for every tube you own.

Resistor R11 is the ballast, limiting current to the tube. The schematic shows a 80-k-ohm resistor, but you can experiment with other values to find one that works best with your tube. If the laser doesn’t trigger or run after adjusting R12 and R13, try reducing the value of the ballast resistor. Use a voltmeter, as explained in the previous section, to monitor the output current to ensure against passing excessive current through the tube.

ill. 11-5. Minimum configuration for an ac-operated high-voltage He-Ne power supply.

AC-OPERATED HE-NE POWER SUPPLY

The schematic in ill. 11-5 shows a basic ac-operated helium-neon power supply (parts list in TABLE 114). A high-voltage transformer converts the 117 Vac line current to 1,000 volts or more. The voltage multiplier increases the working voltage while rectifying the ac. The circuit shows a transformer with a 1,000 volt secondary and the voltage multiplier section used in the basic dc-operated power supply presented earlier in this section. The output voltage is about 4 to 5 kV (rectified and unloaded). Some tubes require extra voltage to start and might need a transformer with 2,000- to 2,500-volt secondary. Note that the circuit's basic and lacks current sensing or high-voltage start capabilities.

ENCLOSING THE HE-NE POWER SUPPLIES

Laser power supplies should never be used without placing them in protective, insulating enclosures. After you have built and tested your power supply, tuck it safely in a plastic enclosure. If you plan on using the supply to power a variety of tubes, mount heavy-duty (25-amp) banana jacks to provide easy access to the anode and cathode leads. Keep the jacks separated by at least 1 inch and apply high-voltage putty around all terminals to prevent arcing.

ill. 11-6. Hookup diagram for building an all-in-one battery-operated helium-neon laser power supply. The battery can be recharged in the enclosure without removing it.

A functional schematic for a completely self-contained, rechargeable battery pack/power supply is shown in ill. 11-6. The parts list for the battery pack/power sup ply is provided in TABLE 11-5. Note the addition of the key switch, power-on indicator, and battery-charging terminal. The key switch prevents unauthorized used of the power supply and acts as the main ON/OFF switch. For maximum security, you should get the kind of switch where the key can be removed only when it's in the OFF position (CDRH requirements call for a key that cannot be removed in the ON position).

The power-on indicator is simply an LED with a current-dropping resistor. The battery-charging terminal provides a means to recharge the batteries without removing them from the enclosure. Note that you can operate the laser while the battery is connected, but in most cases, the power supply will consume too much current and the batteries won't be recharged.

A fuse is added to provide protection against an accidental short circuit in the battery compartment. Lead-acid and gelled electrolyte batteries can easily burn plastic and even metal when their terminals are shorted. The fuse helps prevent accidental damage and fire.

ABOUT LASER DIODE POWER SUPPLIES

As discussed in Section 10, laser diodes come in two basic forms: single- and double heterostructure. The single-heterostructure (or sh) diodes are regarded as the “older” variety and can only be operated in pulsed mode (unless you cool them with a cryogenic fluid, such as liquid nitrogen). Sh laser diodes are capable of multi-watt operation, but only when the pulses are 200 nanoseconds or shorter. Therefore, sh diode supplies may be built around some type of astable multivibrator.

Double-heterostructure (dh) laser diodes can be operated in pulsed or CW modes. Like any diode, excessive currents can destroy the laser, so you must take precautions to operate the unit within its design parameters. Dh lasers are capable of multi-watt operation when used in pulsed mode, but most are designed for CW operation and emit 1 to 10 mW of light energy. Remember that although you can often see a red glow from a diode laser, this light represents only a fraction of the total radiation from the diode. The bulk of the radiation is in the near-infrared spectrum and is largely invisible to your eyes.

PULSED SINGLE-HETEROSTRUCTURE INJECTION DIODE SUPPLY

A common method for powering an sh injection diode is shown in ill. 11-7 (see the parts list in TABLE 11-6). The power supply provides pulses of about 10 to 20 amps at a short duration of around 50 ns. The supply provides sufficient drive current to exceed the threshold of the laser (typically about 7 or 8 amps), with some room to spare. The laser might still glow at currents less than threshold, but the light won’t be stimulated emission. In other words, the device won't emit laser light but behave like an expensive LED.

ill. 11-7. (A) High-current drive circuit for a single heterostructure laser diode. (B) Power leads for the typical sh laser diode, showing single lead for the anode. Typical Injection laser diode (pulsed type).

The sh laser diode circuit uses a common npn transistor operated in avalanche mode. The batteries are 67.5-volt type (NEDA 217, Eveready number 416) used in older tube- type equipment. You’ll have better luck finding the required batteries at an electronic store specializing in communications or ham gear. The price can be steep—up to $10 each depending on the source—so make sure they are fresh before you sign the check.

Quality control in low-cost plastic npn transistors isn't great, so not all transistors will work in the circuit. The schematic calls for a 2N2222, but you might need to experiment with several until you find one that oscillates in the circuit. Construct the circuit using component leads that are as short as possible and test the transistor by substituting the laser with a short piece of copper wire (magnet wire works well). Use an oscilloscope across current-monitor resistor R4 (1 ohm, carbon composition) and watch for the pulses from the transistor. Avoid the use of a logic probe, as most are not de signed for circuits exceeding 18 volts.

After you have determined that the transistor is oscillating (adjust R2 as needed), substitute the laser, being careful to observe polarity. Most sh lasers use the case as the cathode and the single lead as the anode. Yours might be different, so be sure to check the specifications or information sheet that came with the unit. The diode operates at a wavelength of about 904 nm, which is beyond that of normal human vision, so don’t expect the same bright red beam that’s emitted by a helium-neon laser. You can test the operation of the laser by using one of the infrared sensors described in the previous sections.

PULSED DOUBLE-HETEROSTRUCTURE INJECTION DIODE SUPPLY

The popularity of compact audio discs, as well as many forms of laser bar-code scanning, have made double-heterostructure laser diodes plentiful in the surplus market. A number of sources (many of which are listed in References) offer dh laser diodes for prices ranging from $5 to $15. Depending on the power output of the laser, new units are even affordable. A typical 5 mW laser diode lists for about $25 to in low quantities. Sharp is a major manufacturer of dh laser diodes; write them for literature and a price list.

As discussed in the previous section, one of the most attractive features of dh laser diodes is that they work with low voltage power supplies. A dh laser can easily be run off a single 9-volt transistor battery. However, dh laser diodes are sensitive to temperature. They become more efficient at lower temperatures, and their power output increases. Unless the temperature is very low (such as when the diode is immersed in liquid nitrogen, as described in Section 22), the increase in power output can damage the laser. That’s why most dl lasers are equipped with a monitor photodiode. The current output of the monitor photodiode is used in a closed-loop feedback circuit to keep the power output of the laser constant.

Although dh lasers are designed for CW operation, they can also be used in pulse mode. An astable multivibrator, such as a 555 timer, can be used to pulse the laser. A circuit's shown in ill. 11-8, with a parts list in TABLE 11-7. Because the laser is pulsed, the forward current can exceed the maximum allowed for CW operation (generally 60 to 80 mA). However, care must be taken to keep the pulses short. Pulses longer than about a 50 percent duty cycle (half on, half off) can cause damage to the laser. Duty cycle isn't a critical consideration when the current is maintained under 80 mA. The circuit shown in the figure lets you alter the frequency of the astable multivibrator (and therefore the duty cycle).

A closed-loop feedback system constantly watches over the output of the monitor photodiode and maintains the proper current to the laser diode. One such circuit's shown in ill. 11-9. This circuit's designed around the 1R3C02 chip, which is a special-purpose IC manufactured by Sharp. See TABLE fl-8 for a list of required parts. This IC is made for use with their extensive line of dh lasers, and while hard to find, it's relatively inexpensive (obtain the chip through Sharp’s parts service or from a distributor dealing with Sharp components).

ill. 11-8. A double heterostructure laser can be connected to a 555 timer IC for pulse operation. With the components shown, pulse rate is about 300 Hz; pulse repetition is about 3 milliseconds.

Another method using discrete components is shown in ill. 11-1 (parts list in TABLE 11-10). Here, an op amp, acting as high-gain comparator, checks the current from the monitor photodiode. As the current increases, the output of the op amp decreases, and output of the laser drops. The gain of the circuit—the ratio between the incoming and outgoing current—is determined by the settings of R1, R4, and R5.

The circuit in the schematic was adapted from an application note for a General Electric C86002E laser diode and uses a CA3130 CMOS op amp. You can readily modify the circuit if you use another op amp or laser diode. Both output transistors are available through most larger electronics outlets, but if you have trouble locating them, you might have luck substituting them with a single TIP12O Darlington power transistor.

ill. 11-9. Basic schematic for the Sharp 1R3C02 laser diode driver IC.