delabs Circuits

Showing posts with label LED-Lighting. Show all posts
Showing posts with label LED-Lighting. Show all posts

Monday, September 21, 2015

FET Current Source and MOSFET

This is a constant current source using a FET. This is the most simple replacement to series resistor to limit current. The N-Channel FET BF256C can give 15mA current.


Simple Methods

Before you get to use chips, experiment with some methods, which will help you learn about the LEDs better. The first is just One Resistor in series. This is to Limit the max current in a Series LED Chain. If you have a Regulated Supply with a Fixed Voltage, then you can use this method.

Let us take a 12V SMPS, Each HB White LED has a drop of around 3.2 (please see datasheet). If you put 3 LEDs in series it is a drop of 3 X 3.2 = 9.6 V.

12V - 9.6V = 2.4 V. This is the drop across the Resistor, let us keep the current at 20mA for a Long life for LED. Some LEDs will get damaged at 30mA some take more that that. We now have LED Modules which can take even 1 or 2 A.

V/I = R as per OHM. 2.4V/20mA = 120 E or Ohms.

How Hot? W = VI Power in Watts. 20mA x 2.4V = 48mW. This is where you lose the Money. Keep it low, else the Green Goblin will frown. Unless you want LED Lighting to double up as a Room Heater, Nice idea if you are in the Artic.

Now you have a chain of 3 LED with one R, make many such chains and put it in parallel to around 70% of SMPS capacity. If you have 20 Strips of 3 LEDs each, 20 X 20mA = 400mA. You will need a 12V 600mA SMPS .

MOSFET Drive for LED Constant Current

Let us assume, you have a supply that is varying and not stable. Then use a SMPS and Resistor as shown above. Closely matching the LED Chain to the SMPS voltage to keep the heat loss minimum. At Low voltages the above idea may not work. So you can try a MOSFET circuit shown.

You can use Transistors too but The Heat is more, as the Drop is more. When you use batteries, you cannot afford to lose even 0.5V. So the MOSFET is the answer.

This circuit is a nice design idea about LED drive with low voltage and watts burden. In combination with a Joule Thief - and PWM you can make many White LED utilities like Lanterns and Flashlights. PWM is to modulate brightness and also Extend LED Life.

The Essence is The LED has to have a long life, constant current is the answer. The Efficiency has to be High, Switching MOSFET is the answer. For just a LED or Two, you do not need to bother about Efficiency, but Constant Current, No Compromise. Why? Because it is in the Absolute Maximum Rating.

"You do not cross the road, when the light is RED. You do not Drive Faster, than the Speed Limit. You do not Eat, more than you can Digest.You do not Stress the Absolute Max in the Ratings."

The Elektrik Jedi
Read More

Monday, June 08, 2015

Perpetual Candle - White LED Lamp on Ni-Cd

This is a easy to build LED lamp circuit for Learning and building skills. This is the first draft schematic V 1.0. It will need improvements for Higher Power Lighting.

Perpetual Candle Project


I will give a short summary, The LM317 here configured for around 6.4V DC. The Q3 BC547 limits the current, you can select R3 to suit, make it 1/2W. The Ni-Cd battery pack 1.2 * 4 will not get Over-Current or Over-Voltage due to this circuit.


The IRF540 Mosfet or any other equivalent you have around, along with Q2 BC547 forms a current source for the parallel 12 LED array. Ultra-bright White LED at 20mA each or use a 1W ready LED Chip. R4/R6 can be selected for the Max LED current. The voltage of LED is around 3.1 and 20mA * 12 = 240mA is the max current. You can Tweak the design for even 5A or more but then you will need a DC/DC High frequency converter in place of LM317. The Current source also needs to switch to improve efficiency. A PWM on the mosfet gives brightness control. The entire solution (switching) can be found in many chips with semiconductor vendors today.


One Single High Current LED may work well. 12 Matched LEDs also can be used. In LED Array some are dim, put min. resistor (3.9 ohm) for all 12 for current sharing. The resistor addition will impar the ability of Candle to work at lower battery voltages. Also resistor is less green, It wastes power, so use PWM and Single Die high current LED. Ni-Cd system may last over 5 Years if Candle is allways on Mains. Sealed Lead Acid system may go upto 2-3 years life but will have more punch. A SuperCap system may last more than 12 Years, i feel.

Source in Cadsoft Eagle format - candle.zip

Circuits by Application

Analog Circuits

  1. Battery Level Indicator
  2. Simple Sample and Hold
  3. Sample and Hold Standby
  4. Voltmeter Attenuator
  5. Precision Current Source
  6. Opamp Supply Virtual Ground

SCR and Triac

  1. Solid State Relay
  2. Normally Closed AC SSR
  3. AC-AC-SSR
  4. DC-DC SSR
  5. 2N2646 based Pulser
  6. Drive SCR thyristor

Mains Power

  1. Flashing Neon Lamp
  2. Dimmer power control
  3. Edison Bulb Life Extend
  4. Mains Current LED
  5. Mains Voltage LED

Digital Circuits

  1. Simple Digital Counter
  2. Running Lights
  3. Frequency Divider
  4. Crystal Oscillator
  5. Simple High speed switch
  6. Differential TTL converter

Measureall DMM

  1. Ohmmeter Measure Resistance
  2. Precision Digital Attenuator
  3. Precision Amplifier

Mixed Circuits

  1. Monostable Multivibrator
  2. Digital to Analog
  3. LM311 Oscillator
  4. PLL using 4046
  5. VCO with LM331
  6. BCD Thumbwheel to Analog
  7. V to F Converter ICL8038
555 Circuits
  1. OR gate with two 555
  2. fixed frequency duty cycle
  3. Pulse width modulation
  4. Astable Multivibrator
  5. uC Reset Generator
  6. LM555 Voltage Doubler
  7. 555 Power Oscillator
Discrete Circuits
  1. Isolated dual supply
  2. Sound to light converter
  3. Water operated relay
  4. Telephone Indicator
  5. Passive volume control
  6. RS232 Opto-Isolation
  7. Voltage Level Indicator
  8. Relay Driver
  9. Constant Current LED
  10. Voltage Doubler
  11. FET Current Source
Opamp Circuits
  1. Three Opamp Differential
  2. Two Opamp Differential
  3. Buffer Opamps
  4. Differential Op-Amp
  5. Inverting Opamp
  6. Non Inverting Opamp
  7. Digital gain control
  8. Square Triangle Oscillator
  9. Dual Polarity Output Amps
  10. Ammeter Precision Rectifier
  11. Voltage / Current 4-20 mA
  12. Current Source for RTD
Power Electronics
  1. Dual Power Supply
  2. Single Power Supply
  3. Battery Backup Supply
  4. 5V 1A Supply LM2575
  5. 5V Power Supply L296
  6. Dual Power Supply
  7. Tubelight Electronic Choke
  8. Voltage Doublers Multipliers
  9. White LED Lamp on Ni-Cd

uC and uP

  1. PC RS232 with MAX232A
  2. Battery Backup SRAM
  3. watchdog uC uP systems
Instrumentation Circuits
  1. Mains monitor LM3914
  2. Simple Mains monitor
  3. single digit voltmeter
  4. High Resistance Meter
  5. Diode Thermometer
  6. Function Generator
  7. Diode Leakage Tester
  8. Analog LED Ohm Meter
  9. Millivolt Source Current Loop
Process Control
  1. AD590 - temperature
  2. Thermocouple Amplifier
  3. Linearizing Thermocouple
  4. Thermocouple Amplifier
  5. 0-1V to 4-20 mA
  6. 1-5V to 4-20 mA
  7. InfraRed - Optical Switch
  8. InfraRed Detector