How to Turn a Basic Converter into a Current Regulator

I’ve mentioned before a circuit that indicates when the load current exceeds a certain threshold. Today I’ll show how to use this circuit to modify a basic voltage converter and turn it into a current regulator.


Many radio hobbyists probably have little voltage converter boards like these lying around. They cost next to nothing, and are often sold by weight in batches of ten.


The board is tiny but very useful. However, it can only operate in voltage regulation mode, with the output voltage set by a trimmer potentiometer.


Sometimes you need to put together a current regulator using whatever you have on hand—for example, to power LEDs, charge batteries, and so on.
A simple current consumption indicator can help with this. I covered it in detail in a separate video.


It uses a very simple circuit.
When current flows through the circuit, a voltage develops across resistor R1, and its value depends on the current.
The voltage across R1 turns on the transistor once it reaches a sufficient level. Typically, the transistor turns on when the voltage across R1 is around 0.6–0.7 V.
Once on, the transistor supplies current to the LED circuit, lighting it up. Changing the value of R1 changes the current at which the LED lights up. For example, with a 1 Ω resistor, this current is about 0.6–0.7 A. If you use a resistor with half the resistance, the current will be 1.2–1.4 A, so it changes in inverse proportion to the resistance.
The transistor used in this circuit is a BC557B, though there are plenty of alternatives, such as the common KT361—or, if you build the circuit with reversed polarity, a KT315.


As an example, I’ll try making a current regulator to power this LED array. Its LEDs are connected in series-parallel, so the total voltage drop is about 7 V at a current of 700 mA.


Of course, I could have made a current regulator using the familiar LM317, but it’s a linear regulator, so it would get noticeably hot.
Instead, we’ll take a different approach.

On the left, highlighted in blue, is a simplified diagram of the step-down voltage regulator I showed at the beginning. The IC monitors the output voltage through the FB (Feedback) pin.
The board shown above is highlighted in red.


To connect everything correctly, we need to find the IC’s feedback input, also labeled FB or Feedback in schematics.
My board uses an LM2596. We can look up its datasheet and see that this is pin 4.

Solder a wire directly to the IC pin. The pins are usually tinned, so they’re very easy to solder.


Connect this wire to the collector of the transistor on the current-sensing board. At the same time, connect the converter board’s output to the input of the current-sensing board.
Apply the input voltage to the converter. In my case, I used about 17 V. Set the output voltage higher than the LED array requires—for example, 10–12 V—and connect the array to the output of the current-sensing board.


Great, the current in the circuit is 650 mA, and everything works perfectly.


In some cases, you may need to add a diode between our board’s output and the converter. This prevents our circuit from affecting the converter’s output voltage setting (depending on the PWM controller used).
If you also want the LED to light up when current limiting is active, it’s a good idea to add a resistor as shown in the diagram (R6), with a value of around 56–470 Ω.


I mentioned batteries earlier.
If you move the upper resistor in the divider from the converter output to the current-sensing board’s output, as shown in the diagram, the board can also charge batteries. You can charge them without this resistor too, but the voltage drop across R1 will affect the charge termination voltage to some extent.


I also made a video to go along with this; it may be useful.


That’s all for now. As always, I’ll be happy to answer your questions. By the way, there’s a similar modification you can make to a power supply instead of a converter.