Electronics Activity

Part 1: The Constant Current Regulator

Purpose: To construct a regulator to limit the current through a high intensity LED.

Resources

bullet LM217T adjustable voltage regulator TO220 package
bullet R1 - 3.6Ω 1 watt resistor
bullet Aluminum heat sink
bullet Pre-etched circuit board
bullet Heat sink compound
bulletStranded hook-up wire
bulletMounting hardware

Background

The LM217 positive voltage regulator can be configured to supply a constant current to a load even if the input voltage changes. Any type of light emitting diode (LED) needs a constant current to operate. In a simple application, a resistor can be used to control the current but fixed resistors can only be used if the supply voltage is fixed, otherwise the current will change as the voltage flucuates (Ohm's law). In any battery operated circuit, the voltage will change as the energy from the battery is used.

When using high power LEDs it is best to use the new generation of efficient drivers but these can be more expensive than the LED itself. The LM217 can be used to provide a constant current and is considerably cheaper (but is less efficient because of heating) and uses only one extra component, a resistor whose value determines the size of the current. The resistor is connected between he adjust terminal (leftmost) and the voltage out terminal (middle). The LM217 is configured to maintain 1.25 volts between these 2 terminals regardless of changes in the input voltage. In the circuit shown below, a 3.6Ω resistor is used between these 2 terminals. Using ohm's law, the current through the resistor will be I = V/R = 1.25v / 3.6Ω = .347a or roughly 350milliamperes. Any current up to 1.5a can be passed as long as a heat sink is used and the power rating of the resistor is within limits.

Circuit Configuration

The LM217 is a three terminal device. Please see the datasheet for terminal identification.

The basic circuit configuration is shown next. The components will be mounted on a pre-etched circuit board.

Circuit Fabrication

The components for the circuit are shown next. The pc board is a section from a larger board.

Step1

Start by soldering the heat sink to the circuit board. Two holes have been enlarged with a 5/64" dill bit to accommodate the heat sink legs. Make sure it is seated as far as possible.

Step2

Mount the LM217 voltage regulator. DO NOT SOLDER YET! First put a dab of heat sink compound on the back tab and bolt it to the heat sink with a short machine screw and nut. Now solder the three terminals to the circuit board as shown. Trim the leads.

Step3

Mount the fixed resistor vertically by bending one of the leads, Make sure the resistor is flush with the board.

Step 5

Finally, solder 2 wires to the board, a red wire for the positive voltage input and a yellow for the positive voltage output. The wires should be attached to the board with a cable tie or hot glue for strain relief.

Part 2: The High Intensity LED

Purpose: To construct a high intensity light using a white LED and the current limiting circuit from part 1.

Resources

bullet Luxeon III LED
bullet Lens holder
bullet Lens
bullet Aluminum heat sink
bullet Thermal adhesive tape
bulletStranded hook-up wire
bulletMounting hardware
bulletCurrent regulator (part 1)

Background

The Luxeon III LED is a high intensity white light emitter that is soldered to a substrate that acts as a circuit board and heat sink contact. The circuit board has multiple contacts for the positive and negative leads. This device can use up to 1 ampere of current but in this application, you will limit the current to ~ 350 milliamperes. The Luxeon Star III can be attached to a heat sink mechanically using nuts and bolts but in this application you will use a special double sided tape that acts as both adhesive and thermal paste.

Circuit Configuration

The Luxeon III LED assembly will be wired to the constant current circuit fabricated in Part 1. The circuit diagram follows.

 

Circuit Fabrication

Step 1

The Luxeon III assembly has 3 negative and 3 positive solder points. Since we are using a lens assembly in tthe final stage, solder a length of wire (black - 30cm) to the lower negative terminal as shown. If you tin the wire first, it is easier to solder.

Step 2

Next solder the yellow (voltage output) lead from the constant current circuit (part 1) to the lower positive lead.

 

Step 3

You need to attach the Luxeon III to a heat sink. You can use a double sided thermal tape for this purpose. Peel the clear cover from the tape and press in place on the heat sink. It would help to clean the heat sink first with suitable de-greaser.

Step 4

Peel the white cover from the thermal tape and carefully position the LED. When ready press and hold for a few seconds.

Step 5

Position the lens holder as shown. Make sure it is flush with the LED assembly. Secure it with a small dab of hot glue. (At higher currents, silicon glue could be used). When the glue sets, snap the lens in place.

Testing

Connect the red (voltage in) lead to the positive terminal of a  12v DC source. Connect the black lead (from the Luxeon III) to the negative lead on the same same source. The LED should light. Don't look at the light directly!

Part 3 - Controlling the high intensity LED

Purpose: To interface the LED to a Phidget Interfacekit888 using a solid state relay.

Resources

bullet Phidget Solid State Relay
bullet Phidget InterFacekit888
bulletLED system from part 1 and 2
bulletHookup wire

Background

The digital outputs on the IFKit can only source 20 milliamperes of currrent. The LED system needs 350 ma to operate. Clearly, the LED cannot be connected directly. The solution is to use a relay which can be operated by the IFKit. A relay is usually an electro magnet which is used to open and close a switch. Most relays require more than 20 ma to operate. However, a solid state relay (no mechcanical parts) can operate on very low currents. In the final part of this activity you will use a Phidget SSR (solid state relay) to control the LED. This device can control up to 2 amperes and operates from a low current input.

Circuit Configuration

The Solid State Relay(SSR) will act as a simple on/off switch for the LED. The SSR is in turn controlled by InterFaceKit through one of its 8 digital outputs. The InterfaceKit is not shown in the next illustration. Note: It doesn't matter which way the wires connect to the green terminal.

Connect the + INPUT and - Ground wires to a 12v DC power source. Connect the black wire from the white digital plug to the InterFaceKit ground on the IFKit's digital output terminal strip. Connect the red wire from the white digital plug to one of the digital outputs (0-7). Make sure the IFKit is connected to the computer via a USB cable.

Testing

Open the Phidget control panel by double clicking the Ph icon on the task bar (hidden icons). The IFKit should be in the general tab. Double click to open the IFKit window. In the Digital Output section, check the box that corresponds to the digital output that the SSR is connected to. The LED should light. If it doesn't, trouble shoot the circuit!