Build an Experimenter’s Power Supply

Many laser projects require a steady supply of low-voltage dc, typically between 5 and 12 volts. You may use one or more batteries to supply the juice, but if you plan on doing lots of laser experiments, you’ll find that batteries are both inconvenient and anti- productive. Just when you get a circuit perfected, the battery goes dead and must be recharged.

A stand-alone power supply that operates on your 117 VAC house current can sup ply your laser system designs with regulated dc power without the need to install, re place, or recharge batteries. You can buy a ready-made power supply (they are common in the surplus market) or make your own.

Several power supply designs follow that you can use to provide operating juice to your laser circuits. The designs show you how to construct a:

* 5-volt dc regulated power supply

* 12-volt dc regulated power supply

* Quad ±5- and ± 12-volt regulated power supply

* Adjustable (3 to 20 volts dc) regulated power supply.

Note that the power supplies presented within this section are similar with the exception of different values for capacitors, diode bridges, and other components. You may use the schematics to create power supplies of different voltage levels. The multi voltage supply is designed to provide the four voltages common in laser support systems: + 5 volts, + 12 volts, -5 volts, and -12 volts. These voltages are used by motors, solenoids, and ICs.

SINGLE POWER SUPPLY

Refer to FIGS. 12-1 and 12-2 for schematics of the single-voltage power supplies. Ill. 12-1 shows the circuit for a + 5-volt supply; ill. 12-2 shows the circuit for a + 12-volt supply. There are few differences between them, so the following discussion applies to both. For the sake of simplicity, we’ll refer just to the + 5-volt circuit. Parts lists for the two supplies are provided in TABLES 12-1 and 12-2.

For safety, the power supply must be enclosed in a plastic or metal chassis (plastic is better as there is less chance of a short circuit). Use a perforated board to secure the components and solder them together using 18- or 16-gauge insulated wire. Don't use point-to-point wiring where the components are not secured to a board.

Alternatively, you can make your own circuit board using an etching kit. Before constructing the board, collect all the parts and design the board to fit the specific parts you have. There is little size standardization when it comes to power supply components and large value electrolytic capacitors, so pre-sizing is a must.

ill. 12-1. Schematic diagram for the 5 Vdc regulated power supply.

ill. 12-2. Schematic diagram for the 12 Vdc regulated power supply.

To explain the circuit in ill.12-1, note the incoming ac routed to the primary terminals on a 12.6-volt transformer. The “hot” side of the ac is connected through a fuse and a single-pole single-throw (SPST) toggle switch. With the switch in the OFF (open) position, the transformer receives no power so the supply is off.

The 117 VAC is stepped down to about 12.6 volts. The transformer specified here is rated at 2 amps, sufficient for the task at hand. Remember that the power supply is limited to delivering the capacity of the transformer (and later, the voltage regulator). A bridge rectifier, BR1, converts the ac to dc (shown schematically in the dotted box). You can also construct the rectifier using discrete diodes (connect them as shown within the box).

When using the bridge rectifier, be sure to connect the leads to the proper terminals. The two terminals marked with a “—“ connect to the transformer. The “+“ and “—“ terminals are the output and must connect as shown in the schematic. A 5-volt, 1-amp regulator, a 7805, is used to maintain the voltage output at a steady 5 volts.

Note that the transformer supplies a great deal more voltage than is necessary. This is for two reasons. First, lower-voltage 6.3- or 9-volt transformers are available, but most don't deliver more than 0.5 amp. It is far easier to find 12- or 15-volt transformers that deliver sufficient power. Second, the regulator requires a few extra volts as “overhead” to operate properly. The 12.6-volt transformer specified here delivers the minimum voltage requirement, and then some.

Capacitors C1 and C2 filter the ripple inherent in the rectified dc at the outputs of the bridge rectifier. With the capacitors installed as shown (note the polarity), the ripple at the output of the power supply is negligible. LED1 and R1 form a simple indicator. The LED glows when the power supply is on. Remember the 270-ohm resistor; the LED will burn up without it.

The output terminals are insulated binding posts. Don’t leave the output wires bare, or they could accidentally touch one another and short the supply. Solder the output wires to the lug on the binding posts, and attach the posts to the front of the power supply chassis. The posts accept bare wires, alligator clips, or even banana plugs.

Differences in the 12-Volt Version

The 5- and 12-volt versions of the power supply are basically the same, but with a few important changes. Refer again to ill. 12-2. First, the transformer is rated for 18 volts at 2 amps. The 18-volt output is more than enough for the overhead required by the 12-volt regulator and is commonly available. You may use a transformer rated at between 15 and 25 volts.

The regulator, a 7812, is the same as the 7805 except that it puts out a regulated + 12 volts instead of + 5 volts. Use the T series regulator (TO-220 case) for low-current applications and the K series (TO-3) for higher capacity applications. Lastly, R1 is increased to 330 ohms.

MULTIPLE-VOLTAGE POWER SUPPLY

The multi-voltage power supply is like four power supplies in one. Rather than using four bulky transformers, however, this circuit uses just one, tapping the voltage at the proper locations to operate the +5, +12, -5, and -12 regulators.

The circuit, as shown in ill. 12-3, is composed of two halves. One half of the sup ply provides +12 and —12 volts; the other half provides + 5 and —5 volts. Each side is connected to a common transformer, fuse, switch, and wall plug. See TABLE 12-3 for a parts list.

The basic difference between the multi-voltage supply and the single-voltage supplies described earlier in this section is the addition of negative power regulators. Circuit ground is the center tap of the transformer. Make two boards, one for each section. That is, one board will be the ± 5-volt regulators and the other board will contain the ± 12-volt regulators. The supply provides approximately 1 amp for each of the outputs.

Use nylon binding posts for the five outputs (ground, +5, +12, -5, -12). Clearly label each post so you don’t mix them up when using the supply. Check for proper operation with your volt-ohmmeter.

ill. 12-3. Schematic diagram for the quad power supply (± 5 and 12 volts).

ADJUSTABLE-VOLTAGE POWER SUPPLY

The adjustable power supply uses an LM317 adjustable voltage regulator. With the addition of a few components, you can select any voltage between 1.5 to 37 volts. By using a potentiometer, you can select the voltage you want by turning a knob.

The circuit shown in ill. 12-4 is a no-frills application of the LM317, but it has everything you need to build a well-regulated, continuously adjustable, positive-voltage power supply. See TABLE 12-4 for the parts list. The regulator is rated at over 3 amps so you must mount it on a heavy-duty heatsink. Although you don’t need to forcibly cool the regulator and heatsink, it’s a good idea to mount them on the outside of the power supply cabinet, for example on the top or back.

ill. 12-4. Adjustable power supply.

Remember that the case of the regulator is the output, so be sure to provide electrical insulation from the heatsink, or a short circuit could result. Use a TO-3 transistor mounting and insulator kit. It has all the hardware and insulating washers you need. Apply silicone grease to the bottom of the regulator to aid in heat transfer.

INSPECTION AND TESTING

All of the dc power supplies should be inspected and tested before use. Be particularly wary of wires or components that could short out. Visually check your wiring and check for problems with a volt meter. When all looks satisfactory, apply power and watch for signs of problems. If any arcing or burning occurs, immediately unplug the supply and check everything again. When all appears to be operating smoothly, check the output of the power supply to ensure that it's providing the proper voltage.

BATTERY PACK REGULATORS

Voltage regulators can also be used with battery packs for portable equipment. A 5-volt regulator can be used with a single 6-volt battery to provide a steady supply of 5 volts. The schematic in ill. 12-5 shows how to connect the parts. Refer to TABLE 12-5 for a parts list. Alternatively, use a 12-volt regulator. The battery should put out a nominal 13 volts to accommodate for the 1- to 1.2-volt drop across the regulator. Most lead-acid and gelled electrolyte batteries put out 13.8 volts when fully charged. See TABLE 12-6 for a chart of voltage values for various types of batteries.

BATTERY RECHARGERS

With a rechargeable battery, you can use it once, zap new life into it, use it again, and repeat the process several hundred—even thousands—of times before wearing it out. The higher initial cost of rechargeable batteries more than pays for itself after the third or fourth recharging.

ill. 12.5. Batt. pack regulator.

Rechargeable batteries can’t be revived simply by connecting them to a dc power supply. The dc supply delivers too much current and tries to charge the battery too quickly. If you are recharging gelled electrolyte or lead-acid batteries, you might be able to get away with using an ac power adapter, the kind designed for video games, portable tape recorders, and other battery-operated equipment (the output must be dc). By design, these adapters limit their maximum current to between 250 to 600 mA. A 300 mA recharger can be effectively used on batteries with capacities of 2.5 AH to 5 AH. A 400 mA or 500 mA ac adapter can be used on batteries with capacities of 3.5 AH to 6.5 AH.

However, one problem is that you must be careful the battery doesn’t stay on charge much longer than 12 to 16 hours. Leaving it on for a day or two can ruin the battery. This is especially true of lead-acid batteries. The circuit shown in ill. 12-6 minimizes the danger of overcharging.

*Gelled electrolyte and lead-acid battery; single cell, 6volt cells in series), 12-volts (six cells in series).

ill. 12-6. Circuit diagram for the battery charger. See page 180 for values of R and pg 182 for settings for R4 and R5.

Build the Universal Battery Recharger

The universal battery recharger shown in ill. 12-6 is built around the LM317 adjustable voltage regulator IC. As indicated in TABLE 12-7, this IC comes in a TO-3 transistor case and should be used with a heatsink to provide for cool operation. The heatsink is absolutely necessary when recharging batteries at 500 mA or higher.

The circuit works by monitoring the voltage level at the battery. During recharging, the circuit supplies a constant-current output; the voltage level gradually rises as the battery charges. When the battery nears full charge, the circuit removes the constant- current source and maintains a regulated voltage to complete or maintain charging. By switching to constant-voltage output, the battery can be left on charge for periods longer than recommended by the manufacturer.

Table 12-8. Common Currents and Resistor Values

Before you build the circuit, you should consider the kind of batteries you want recharged. You’ll have to consider whether you will be recharging 6-volt or 12-volt batteries (or both) and the maximum current output that can be safely delivered to the battery (use the 10 percent rule or follow the manufacturer’s recommendations).

Resistor R1 determines the current flow to the battery. Its value can be found by using this formula:

R1 = 1.25/Icc

where Icc is the desired charging current in mA. E.g., to recharge a battery at 500 mA (0.5 amp), the calculation for R1 is 1.25/0.5 or 2.5 ohms. TABLE 12-8 lists common currents for recharging and the calculated values of R1. For currents under 400 mA, you can use a 1-watt resistor. With currents between 400 mA and 1 amp, use a 2-watt resistor.

If the resistor you need isn’t a standard value, choose the closest one to it as long as the value is within 10 percent. If not, use two standard-value resistors, in parallel or in series, to equal R1. If you’d like to make the charger selectable, wire a handful of resistors to a one-pole multi-position rotary switch, as shown in ill. 12-7. Dial in the current setting you want.

ill. 12.7. Rotary switch for selectable change currents.

The output terminals can be banana jacks, alligator clips, or any other hardware you desire. You might w to use banana jacks and construct cables that can stretch between the jacks and the batteries or systems you want to recharge. E.g., you can connect the charger to a 12-volt He-Ne laser battery pack. The pack is outfitted with a common ¼-inch phone plug for easy connection to the laser. To recharge the battery, you simply remove the cable attaching it to the laser and replace it with the one from the recharger.

Building the Circuit. For best results, build the circuit on a printed circuit board. Alternatively, you can wire the circuit on perforated board. Wiring isn't critical, but you should exercise the usual care, especially in the incoming ac section. Be sure that you provide a fuse for your recharger.

Calibrating the Circuit. After the circuit's built, it must be calibrated before use. First set R4, the voltage adjust. This potentiometer sets the end-of-charge voltage. Then set the trip point, which is adjusted by R5. Follow these steps.

1. Before attaching a battery to the terminals and turning the circuit on, set variable resistors R4 and R5 to their mid ranges. With the recharger off, use a volt-ohmmeter to calibrate R4, referring to TABLE 12-9. Adjust R4 until the ohmmeter displays the proper resistance for the current setting you’ve chosen for the charger.

2. Connect a 4.7k, 5-watt resistor across the output terminals of the charger (this approximates a battery load). Apply power to the circuit. Measure the output across the resistor. For 12-volt operation with gelled electrolyte cells and lead-acid batteries, the output should be approximately 13.8 volts; for 6-volt operation, the output should be approximately 6.9 volts. If you don’t get a reading or if it's low, adjust R5. If you still don’t get a reading or if it's considerably off the described mark, turn R4 a couple of times in either direction.

3. Connect the volt-ohmmeter between ground and the wiper of R5, the trip-point potentiometer. Turn R5 until the meter reads zero. Turn the charger off.

4. Remove the 4.7k resistor, and in its place connect a partially discharged battery to the output terminals (be sure to use a discharged battery), observing the correct polarity. Turn the charger on and watch the LED. It shouldn't light.

5. Connect the volt-ohmmeter across the battery terminals and measure the output voltage. Monitor the voltage until the desired output is reached (see step 2, above).

6. When you reach the desired output, adjust R5 so that the LED glows. At this point, the constant-current source is removed from the output, and the battery float charges at the set voltage.

Application Notes. If you have both 6- and 12-volt batteries to charge, you might find yourself readjusting the potentiometers each time. A better way is to construct two battery rechargers (the components are inexpensive) and use one at 6 volts and the other at 12 volts .Alternatively, you can wire up a selector switch that chooses between two sets of voltage adjustment and trip-point pots.

At least one manufacturer of the LM317, National Semiconductor, provides extensive application notes on this and other voltage regulators. Refer to the National Linear Databook Volume 1 if you need to recharge batteries with unusual supply voltages and currents.

Table 12-9. Values for R4

Depending on your battery and the tolerances of the components you use, you might need to experiment with the values of two other resistors. If the output voltage cannot be adjusted to the point you want (either high or low), increase or decrease the value of R2. If the LED never glows, or glows constantly, adjust the value of R6. Be careful not to go under about 200 ohms for R6, or the SCR could be damaged.

When recharging a battery, you know it has reached full charge when the LED goes on. To be on the safe side, turn the charger off and wait five to 10 seconds for the SCR to unlatch. Reapply power. If the LED remains lit, the battery is charged. If the LED goes out again, keep the battery on charge a little longer.

BATTERY MONITORS

A battery monitor simply provides an aural or visual indicator that a battery is either delivering too much or too little voltage. Ill. 12-8 shows a schematic for a simple “window comparator” battery monitor (see TABLE 12-10 for a parts list). It is designed to be used with 12-volt batteries, but you can substitute one or more of the zener diodes for use with other voltages.

ill. 12-8. A simple battery condition indicator. Choose the zener diodes to provide a “window” for over/under voltage indication.

In normal operation, LED1 glows when the voltage from the battery is at least 10 volts. It is also desirable to know if the battery is delivering too much voltage, so a second zener diode is used. If LED2 is on, the circuit's receiving too much power, and it could be damaged. More likely, however, the battery level will drop, and LED1 will grow dim or flicker off completely. If LED1 isn't lit or is dim, the battery needs to be recharged.