Constant current circuit using transistors

Here is a constant current circuit using transistors. Because the Ni-HM battery should only be charged with a constant current. Our friends need it. Also, I and my daughter are interested in learning/trying out this circuit.

Constant current circuit using transistors

Normally the current will change always according to the voltage level. Imagine an LED display, when higher voltage, it gets a higher current, too. Of course, it can get damaged.

Learn first: transistor circuit

For the battery, when it runs out of power. Its internal voltage and resistance are very low. While charging, the current is very high. It causes by very high heat, which is not a good result at all.

Would it be better if charged with the constant current? The current flowing through it is low and constant. It doesn’t get hot, so it can be recharged many times.

Basic Constant current circuit Learning

There are many types of constant current generator circuits. This time we choose to study a transistor circuit. It’s cheap and easy. Sure, we should start in a simple way. Look at below.

Basic constant current generator circuit using transistor

We believe that problems make the best learning. From; How to use easily LEDs. But the LED emits light unsteadily when used with a higher voltage source.

The LED needs constant current. For the voltage, it will be pulled up by itself, as required. When we keep the current constant. It will pull the voltage is almost constant. Of course, the light is more stable, too. Is this nature of LED true?

Let’s found out.

Experiment basic circuit

We try to set the condition to use the 3mm red LED again and a voltage source of 5V to 12V. From the basic circuit above, let’s choose the parts list.

  1. Q1: When we use the 3 mm LED, the voltage level is just like that. We can use any small NPN transistor, e.g. BC547, BC337, 2SC1815, 9013, 2N2222, etc. We use 2N3904. Note: Be careful with the different leg positions.
  2. R2: Bias current (IB) resistor to a transistor. We use 4.7K because of the low lC.
  3. D1, D2: We use 1N4148, this circuit requires low current.
  4. R1: It’s the most important Because it defines constant current to the load (LED).

We can easily calculate its value:
R = 0.7V ÷ I
We know I, is constant current, is ILED. The current lowest of LED is about 3mA(0.003A).

So, R1 = 0.7V ÷ 0.003A = 233Ω. But we use a 220Ω resistor.

Or see Shopping List:

0.25W Resistors, tolerance: 5%

  • R2: 4.7K, Quantity: 1
  • R1: 220Ω, Quantity: 1


  • Q1: 2N3904, 45V 0.1A, NPN TO-92 Transistor; Quantity: 1
  • LED1: 3mm red LED; Quantity: 1
  • D1: 1N4148, 75V 150mA Diodes; Quantity: 2

Then, look at the circuit diagram below. Even, fewer pieces of equipment. But it might seem confusing for beginners.

Basic constant current circuit using transistor

Assemble this circuit on a breadboard, and compare the results when we change input voltage, between 5V to 12V. Notice the LED, it’s steadily bright. Great!

If you still don’t understand clearly, try reading this: Electronics for beginners

Measure the current of the LED. It is constant at 2mA in all voltage ranges. And measuring the LED voltage is 1.9V constant as well. See the illustration below.

Measuring constant current of LED

Note: The meter gets less precise than the calculated formula a bit.

Why current is constant?

Now coming to the difficult, I have a big question: how to explain the operation of the circuit in a simple way? until my daughter understands. This content may not be suitable for people who like complex principles.

Here are some facts that you should understand for yourself.

When the voltage and resistance are constant, the current is surely constant. This can be confusing. Look at the circuit again.

See flowing current

various currents that flowing on basic circuit
The current flows through the points of the basic circuit.
  • No 1: a little current flows through R2, D1, and D2 to GND.
  • No2: while some currents flow through R2 to B pin and E pin of the transistor, and R1 to GND, too. It is a bias current to the transistor. We call it that IB. So, Q1 conducts. And,
  • No 3: Making a large current flows through LED to C pin and E of the transistor, the R1 to GND, as well. Thus, LED glows.

We will see that: IC of transistor = ILED, while IE is IR1. From principle: IE = IC x IB, but because the IB is very small. We may ignore it as IE = IC (almost the same). In conclusion, ILED = IR1.

We’re going to link it to find out.

The voltage at different points

What’s more. Meare a voltage at various points.

various points of voltage on basic circuit.

1. VD—D1, D2

There is a constant voltage drop across both diodes of approximately 1.4V. Each diode has a voltage drop of about 0.7V. While is Vin 5V to 12V range. Why?

The principle is: that when a little forward current flows through the diodes. It will conduct when the forward voltage reaches a certain threshold point. For Silicon diode is 0.6-0.7V.
It will maintain the voltage level while it is still conducting.

2. VBE and VR1

Look at the above illustration circuit. Both diodes D1-D2 are across B-E of transistor and R1. Sure, they are also forced to keep the voltage constant,1.4V. Then,

  • The internal structure of the transistor between the B-E pin is like a diode. So, the voltage across B-E is 0.7V.
  • Thus, the voltage at R1 is 0.7V (constant as well) So we can easily link to calculate the formula above.

    R1 = 0.7V ÷ ILED

In conclusion, when VR1 is fixed at 0.7V. So, the ILED (load) is stable too, meeting our goal.

Make Ni-MH battery charger circuit

We have learned enough about the constant current circuit using transistors. Let’s make a Ni-MH battery charger circuit. Currently, the Ni-MH battery has more capacity more than 1500mA or 1900mA, high efficiency.

AA 1900mA NiMH Battery

Next, let’s see how to choose the right devices.

  • We should charging current—ICharging about 0.1C or capacity battery x 0.1 = 1900mA x 0.1 = 190mA. We set it about 170mA.
  • Sure, a small transistor cannot get it. We need to use a larger one.
  • There be should LED display battery are charging.
  • Easy same above circuit.

Look at the circuit diagram below.

Ni-HM battery charger circuit using transistors, constant current
  • Change D1, and D2 into a LED. Also, It can keep a stable voltage. Besides, It indicates battery connecting.
  • Put another power transistor—Q2-2N3055, to deliver 170mA of output current. Also, we put Q1 and Q2 in Darlington pair form, causing have high gain to drive the load.
    Note: You may use TIP41, MJE3055, and more. They are smaller than one, on TO-220. But I love 2N3055, it is more durable and has high performance.
  • Calculate the value of R1 = 0.8V/ICharging = 0.8V/0.17A = 4.7Ω. And, Power = V x I = 0.8V x 0.17A = 0.13W, or use 0.25W resistor.
  • Use LED1 as a 5mm Green LED. It will keep a voltage at about 2.1V and be stable, too.

Then, assemble this circuit on a breadboard. Measure the voltage at various points. Look at the circuit diagram below.

Measuring voltage constant-current Ni-MH battery charger circuit

There are 3 points we need to measure voltages.

  1. Measure voltage across LED1 should read about 2.1V. It will keep the circuit running and stable.
  2. Point 2 is about 1.4V, results from Q1 and Q2 conducts.
  3. The voltage of R1 needs to be about 0.8V, Output current will be constant to the battery (load).

We tried charging one AA Ni-MH battery. See the Figure below.

Testing Ni-HM-battery charger circuit using transistors

Then, measure the current is about 0.170A. Meanwhile while charging, we measure the voltage drop across The battery as 1.4V. No more than the specified value is 1.5V. Then, we tried charging it for 16 hours. The battery is normal, not more than 40 degrees Celsius.

Note: As the circuit operates, Q2 rises in temperature. Therefore should be installed on the heatsink. and if charging only one battery we may try to reduce Vin to 5V. The Q2 has a lower temperature. Sure, Vin should be regulated power supply.

Thanks, Harald Brötell our friend who sparked trying this circuit. We had a lot of fun.

I and my kids love to learn electronics. I also found a Simple Constant Current Generator using Transistor with helping of TL431, Even though it uses IC. But it is good learning more.

In conclusion, We hope this circuit will be helpful to you. If you have tried What results did you get? Please don’t delay, share with us

Here are three circuits similar to the circuit above:

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5 thoughts on “Constant current circuit using transistors”

  1. Took courses long ago, starting to get back into it! would you have a manual with basic projects in it to start.

    • It’s a good sound. Electronic learning experiments can be fun. We spend a lot of time worthwhile. Time passed really quickly. I’m not sure you mean the academic manual, right?

      Hello Robert Elliott,

      I don’t have any academic manuals. When I was in school, I was rather lazy to study academically. I prefer to experiment with creating projects to solve problems.

      Of course, we use academic principles in every circuit, it’s challenging to understand or calculate its value.

      In short, I don’t have any high academic manuals, I only have manuals on projects or related calculations at a basic level.



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