An Add-On Current Limiter For Your Power Supply

Description: This circuit allows for setting a limit on the maximum output current from a power supply unit (PSU). It is particularly useful for initial project power-ups or during soak tests. By establishing an upper current limit, it protects both the power supply and any connected devices. This circuit serves as a simple and cost-effective alternative to a current-limiting power supply. The basic circuit is illustrated in the first schematic. Two diodes stabilize the voltage on the base of the power transistor at approximately 1.4V, resulting in a fixed voltage across resistor R2 of about 0.7V. If R2 is set to 10 ohms, the maximum emitter current is calculated to be about 70mA (0.7V divided by 10 ohms). Consequently, the collector current will also be limited to around 70mA at the output terminals. Exceeding this current will cause the output voltage to drop. A BD131 transistor is used, although any NPN power transistor with similar or superior specifications should be suitable. The choice of a 70mA limit is arbitrary; altering the value of R2 will change the current limit, as indicated in the diagram. It is important to note that increasing the current will also increase the power dissipation. The second schematic introduces additional features; an LED can replace a voltmeter on the output, dimming or extinguishing if the output voltage decreases, indicating excessive load. For unattended operation, such as during a soak test, a buzzer can be included to sound an alarm if the output voltage drops by 2 volts or more. The layout for the second schematic is flexible; components like the LED and R3 can be omitted if not required, as well as the alarm components (D3, D4, D5, R4, R5, and Q2). The heatsink is constructed from a folded aluminum strip approximately 2mm thick, 6cm long, and 3cm tall. If the maximum current from the limiter circuit is significantly increased, a larger heatsink may be necessary. Support material for this circuit includes a detailed construction guide, a parts list, and a complete circuit description.

This circuit functions as a current limiter that safeguards both the power supply and connected devices during testing phases. The design includes a basic configuration, as depicted in the initial schematic, where two diodes are employed to regulate the base voltage of the power transistor. This regulation ensures that the emitter current remains within safe limits, preventing damage due to excessive current draw.

The choice of components, particularly the BD131 transistor, is critical for the circuit's performance. While the BD131 is suitable, alternatives with comparable or enhanced specifications can be utilized, allowing for flexibility in component sourcing. The resistor R2 plays a pivotal role in determining the current limit; its value can be adjusted to set different maximum currents as needed. The provided formula in the diagram facilitates this adjustment, ensuring that users can tailor the circuit to their specific requirements.

The second schematic enhances the circuit's functionality by incorporating an LED indicator and a buzzer. The LED serves as a visual cue for output voltage conditions, providing immediate feedback without the need for additional measuring equipment. This feature is particularly beneficial in scenarios where the circuit is left unattended, as it simplifies monitoring.

The buzzer adds an auditory alert for significant voltage drops, ensuring that users are notified of potential issues during prolonged operation. The design's flexibility allows for the omission of these features if desired, catering to varying user preferences and application needs.

Thermal management is addressed through the use of a heatsink, which is essential for maintaining safe operating temperatures, especially when higher currents are involved. The specifications for the heatsink should be carefully considered, particularly if the circuit is modified for greater current limits.

Overall, this current limiting circuit provides a robust and adaptable solution for protecting power supplies and connected devices, with features that enhance usability and safety during testing. The accompanying support materials further aid in the successful implementation of the circuit.This circuit allows you to set a limit on the maximum output current available from your PSU. It`s very useful when you power-up a project for the first time - or carry out a soak-test. By setting an upper limit on the current available from your PSU - you can protect both your power supply - and any device connected to it. It offers a simple and cheap alternative to the Current Limiting Power Supply. The basic circuit is shown in the first schematic. The two diodes fix the voltage on the base of the Power Transistor at about 1v4. This means that the voltage across R2 is fixed at about 0v7. If R2 is 10 ohms, then the maximum emitter current is (0v7 G ½10) about 70mA. Since the collector current is always more or less equal to the emitter current - you cannot draw more than 70mA from the output terminals. If you try to do so - the output voltage will fall. I used a BD131 because that was what I had available. However, any NPN Power Transistor with a similar - or better - spec should work fine. I had no special reason for chosing a 70mA maximum. If you want to set a different current limit - change the value of R2. The formula is in the diagram. Always remember that if you increase the current - you`ll also increase the watts. The second schematic has a couple of added features. If you don`t want to use a voltmeter on the output - use an LED instead. If the output voltage falls - the LED will dim or extinguish completely. This is enough to let you know that the load on the output is excessive. Where the Current Limiter is to be left unattended for any length of time - say during a soak-test - the Buzzer is useful.

Should a problem develope - and the output voltage fall by 2-volts or more - the circuit will sound the alarm. The layout provided is for the second schematic - but it`s flexible. If you don`t want the LED feature - just leave out the LED and R3. If you don`t want the alarm feature - leave out D3, D4, D5, R4, R5 & Q2. The heatsink is a folded strip of aluminium about 2mm thick, 6cm long and 3cm tall. If you increase significantly the maximum current available from the limiter circuit - you`ll probably need to increase the size of the heatsink as well.

The Support Material for this circuit includes a detailed guide to the construction of the circuit-board, a parts list, a complete circuit description and more.