The lifetime of LED bulbs (now available in the REUK Shop) is strongly influenced by the amount of current passing through them. Though typically rated for 100,000 hours of use, if the current used is double that recommended the LED bulb lifetime can fall to 100 hours or less.
It is therefore essential to regulate the current flowing through an LED circuit. This can be achieved using resistors however it is safer to use an LM317T chip as an adjustable current regulator.
Typically LED circuit current regulation is achieved by putting a resistor in series with the LED bulbs. (Read our article on making your own LED spotlights to find out more about selecting resistors for LED circuits.) Resistors are very cheaply available in an enormous range of sizes. However using resistors to regulate current in a circuit is not a very safe solution – particularly when the power supply used is not stable. A resistor in series with an LED may result in the exact desired current going through the LED at 12V, but at 15V that current could be much greater.
Problems Using Resistors to Regulate Current
- The circuit Diagram of the high power LED driver is shown in the image below. As you can see the driver has nothing more than a LM317 IC and a resistor. The above circuit is designed to drive a LED with 0.2A. This current rating is fixed by the Resistor R1 in the circuit.
- In this project, we are going to design a simple Laser diode driver circuit using an LM317T voltage regulator IC. LM317 is a three-terminal voltage regulator IC, with a high output current value of 1.5A.
I built a minimal LED driver circuit using a LM317LZ and a 12 Ohm resistor between OUT and ADJ (identical to e.g. Understanding this LM317 LED Driver circuit). According to the datasheet this shoul.
If you wire three LEDs in series – each with a nominal current of 25ma (0.025 Amps) and a voltage drop of 3.6V – there is a total of 10.8 volts being dropped across the LEDs. If the power source is a 12 Volt battery then there is an additional 12-10.8=1.2V to drop to keep the LEDs happy.
Resistance is equal to voltage divided by current and so 1.2/0.025 = 48 Ohms is the size of resistor required. If a suitable resisitor (50 Ohms since that is the next manufactured resistor over 48 Ohms) is put in series with the LEDs then everything will work correctly – 25mA of current will flow through the LEDs in the circuit as desired and 0.03 Watts (1.2 volts * 25ma) will be dissipated as heat by the resistor.
However, in most renewable energy systems the voltage of the battery changes dramatically depending on the charging current. When the battery is not being charged it may give out only 12 Volts, but when it is being charged it can easily reach in excess of 14.5 Volts – particularly if a small battery is being charged by a large PV solar panel or wind turbine generator.
If the circuit described above has 14.5 Volts going through it rather than the 12 Volts for which it was designed then the extra voltage to be dropped is 14.5-10.8 = 3.7 Volts. This means a current of 3.7 Volts divided by 50 Ohms = 74ma would be passing through the LEDs (since all components connected in series receive the same current), enough to burn them out in a few hours.
Current Regulation with an LM317T
Getting a fixed current from power source using the LM317T is actually very easy. All that is required to fix the current is a resistor. The resistor is placed in series between the LM317T and the LED circuit, with the ADJ input of the LM317T connected to the circuit between the resistor and the LED circuit.
The LM317T always regulates the voltage on the ADJ input to 1.25 Volts – and therefore the current through the resisistor and therefore through the LED circuit will be constant. All we need to do is pick a resistor across which the nominal current of the LEDs will flow at 1.25 Volts using Ohm's Law:
Resistor Required (Ohms) = 1.25 (Volts) / nominal LED current (Amps) |
For example, with the white LEDs in the example given above had nominal currents of 0.025 Amps (25ma). Therefore we need a resisitor of 1.25/0.025 = 50 Ohms in our current regulation circuit. (The fact the resisistor needed in this circuit was the same value as the resistor in the example above was a coincidence!)
The power dissipated by the resistor is given by the regulated 1.25 Volts multiplied by the 25ma current = 0.03 Watts, so a standard 1/4 Watt resistor is more than sufficient to cope.
Click here to go to ourLM317 Current Calculator (pictured above). Simply enter your desired fixed output current, and the value and power rating of the required resistor will be displayed.
Don't Forget the Volts Dropped in the LM317T
A couple of volts will be dropped in the LM317T and therefore the power supply required for the example above would have to be at least equal to the voltage drop across the LEDs (10.8V) plus 2 Volts = 12.8 Volts. Below this value the LEDs would not light – therefore the circuit would be better if it were modified to have two LEDs connected in series rather than three if a 12 Volt supply was to be used.
Limitations of the LM317T
The LM317T accepts an input voltage from 3 Volts to 40 Volts, and it will cope with an average current of 1.5 Amps. The higher the voltage and the higher the current, the hotter will get the LM317T – therefore a suitable TO-220 heatsink can become essential or the chip will automatically shut down and/or be damaged. Click here for our article on LM317 Heatsinking.
However these regulators are cheap and easily available if you make a mistake – and a burnt out LM317T is a lot less expensive than a string of of burnt out LEDs.
If you need more than 1.5 Amps of continuous current, consider the LM338T – a product which works in exactly the same way as the LM317T but with a continuous current rating of 5 Amps with suitable heatsinking.
Lm317 Application Circuits
Current Limited Solar Battery Charger
The current limiting circuit used here to safely light LEDs can be easily modified for use in a simple Solar Battery Charger. The resistor just needs to be changed so that the output current is around 10% of the capacity of the batteries to prevent overcharging.
Do you want a constant current source for LED? To builds a power supply for a battery charger circuit.
Why should we use these circuits?
Imagine your load needs fixed current like LED. We cannot power it over 20mA. It may damage the LED.
LED needs to have a constant current and voltage. As usual, we provide the current limiting resistor to it.
But in some cases, we can not use it. Because the input voltage changes all the time. We should make a constant current through the LED.
Other events when you charge a battery. Normally it requires a fixed current only. You need these circuits too.
If you do not understand. Fallout 4 vault 113.
Let's get started to learn in 7 circuits below.
1# FET Constant current drivers for LED display using BF256
This is a FET Constant current drivers circuit for drive LED display which can use FET-BF256 instead resistor so well.
Normally when you use LED display in any circuits often use a resistor for limit LED current. Because easy and cheap.
But it is not best, this way is ideal for the stable voltage source only. When we change voltage source, the current that flow through the LED will also changes, causes LED not stable brightness. It may be damaged, it must be a constant current flowing through it.
Such as in digital logic probe circuit, which we need to test to TTL type that use 5-volts only, and a CMOS type that wide voltage of 3-volts to 16-volts. When we need to have the LED that stable brightness all voltage source.
I have a good way. A ' FET' is requirement because when we connect Gate and Source together, then put it instead the resistor. They are similar as Figure below.
I use number : BF256 normally it is used as N-channel RF amplifier (in VHF/UHF frequency) , it is small size FET to-92 type. Use in voltage under 30-volts. And see a position lead (Gate,Source and Drain lead) or BF256 Pinout in Figure.
And I test it on a breadboard as video below. I use the power supply is … Adjustable dc voltage regulator circuit using ic-7805. which have voltage output of 5V to 22V as we need. (TTL and CMOS voltage)
Firstly I adjust voltage at 5V (see on meter above) At the same time, I measure current that flow through LED have 4.22mA only.(see on meter right) But LED is normal brightness. Normally LED has current required of 15mA.
Then, I adjust voltage up, while the current is stable about 5mA only and the LED also stable brightness as we need…happy circuits.
2# Constant current circuit using LED
This is Constant current circuit using LED. Normally the voltage drop across LED while forward bias will be about 1.2 to 1.4 volts depend on type of LED by has the temperature coefficient is -1.5 mV per degree Celsius. Which similar to the temperature coefficient of junction. between a base and emitter of silicon transistors.
From this relationship can the constant current circuit that no temperature coefficient as show in Figure 1.
Constant current circuit using LED
From in the Figure current I flowing through the value.
(U LED – U BE) / R
And since the temperature coefficient of transistor and LED fully offset.
Thus, Temperature occurs It does not affect the current flowing yet.
3# 7805 current constant circuit
We use also a 7805 regulator to build a constant current circuit. It is a simple charger circuit.
Recommended: 7805 datasheet and sample circuits
Or the current regulator using IC-7805.
Basic current constant or current regulator using 7805
In the datasheet, if using a resistor-R1 pass current from a pin output of IC to load.
Then, it gets the current output to pin ground, too.
The circuit inside 7805 can keep the output current is solid status.
Even we change any input voltage. But do not forget it run well over 5V input.
Read more: about how to find R1 in any case.
4# Precision LED Regulator using LM337T
Here is alternatively use LED with power supply many the level Voltage.
Look at the circuit.
The LED1 will get a stable current. Some called Precision LED Regulator Circuit using LM337T.
The Pros of this circuit is using a few parts.
And you should use input voltage from -5V to -37V. Because this IC is a negative voltage regulator.
Firstly I adjust voltage at 5V (see on meter above) At the same time, I measure current that flow through LED have 4.22mA only.(see on meter right) But LED is normal brightness. Normally LED has current required of 15mA.
Then, I adjust voltage up, while the current is stable about 5mA only and the LED also stable brightness as we need…happy circuits.
2# Constant current circuit using LED
This is Constant current circuit using LED. Normally the voltage drop across LED while forward bias will be about 1.2 to 1.4 volts depend on type of LED by has the temperature coefficient is -1.5 mV per degree Celsius. Which similar to the temperature coefficient of junction. between a base and emitter of silicon transistors.
From this relationship can the constant current circuit that no temperature coefficient as show in Figure 1.
Constant current circuit using LED
From in the Figure current I flowing through the value.
(U LED – U BE) / R
And since the temperature coefficient of transistor and LED fully offset.
Thus, Temperature occurs It does not affect the current flowing yet.
3# 7805 current constant circuit
We use also a 7805 regulator to build a constant current circuit. It is a simple charger circuit.
Recommended: 7805 datasheet and sample circuits
Or the current regulator using IC-7805.
Basic current constant or current regulator using 7805
In the datasheet, if using a resistor-R1 pass current from a pin output of IC to load.
Then, it gets the current output to pin ground, too.
The circuit inside 7805 can keep the output current is solid status.
Even we change any input voltage. But do not forget it run well over 5V input.
Read more: about how to find R1 in any case.
4# Precision LED Regulator using LM337T
Here is alternatively use LED with power supply many the level Voltage.
Look at the circuit.
The LED1 will get a stable current. Some called Precision LED Regulator Circuit using LM337T.
The Pros of this circuit is using a few parts.
And you should use input voltage from -5V to -37V. Because this IC is a negative voltage regulator.
Change R2 to control the trend (Adjustable (+/-)15%).
For R1: if get from I LED1 = 1.5V / R1, R2 such as ILED1 = 15mA , R1 = 100 ohms.
5# Stabilised Current Battery Charger using LM723
Normally battery Charger Circuit, will use the way gives Stabised Current or stable current. For this circuit also the integrated circuit LM723 and electronic parts a few with follow the circuit appraise R1 = 11ohm for fix current at 60mA.
We can seek the value R get from R = 700/I and The transistor 2N3055 add keep for enlarge current the paramour at LM723 , durable get , make have the wastage of power to electricity work about 1.6Watt only. For voltage output be valuable about 7.5V then choose use battery voltage low get only. The detail is other, Friends see in the circuit has please yes.
6# The Safe constant current source
Look at the circuit below. It is a Safe constant-current source circuit, how it works?
A CMOS op-amp (number ICL 7611) controls the input current through a P-channel Hexfet power transistor (No. IRF 9520), then to keep up a constant voltage across the R1.
As they are connected in a serial form, so use the together current by I = VREF / R1, while the Vref to be defined by the IC2 is 1.25V.
The advantage of this outline are:
1. The load current is limited by R1 when the load is too heavy.
2. The op-amp and Hexfet there is the overhead voltage very low.
7# Precision current sink circuit
This is a current sink circuit that uses a transistor, Jfet and LM101 IC op-amp, so there is high precision.
The 2N5457 Jfet and PN2222 bipolar have normally high output impedance.
The R1 is used as a current sensing resistor, to provide feedback to the LM101 op-amp that it supplies a large amount of loop gain for negative feedback, to enhance the real current sink nature.
The value of Iout is Vin/ R1, by Vin more than 0V.
For low current values, the 10K resistor and PN2222 may be clear out, if the source of the Jfet is connected to R1.
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Lm317 Voltage Regulator
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