Current going to load is controlled by Simple Current Limiting Circuit using ICs LM317/LM338.
These chips also work as current limiters if set properly.
Power comes in as Vin.
LM317/LM338 have 3 pins: input, output and adjust.
Vin goes to input pin.
Output pin (Vout) goes to load and gives limited current.
Circuit Working:
Parts List:
| Category | Item | Quantity | Notes |
|---|---|---|---|
| Resistor | Resistor Rx | 1 | |
| Semiconductors | IC LM317/LM338 | 1 | Choose either LM317 or LM338 based on your voltage and current requirements |
Power source give input voltage which is called Vin.
LM317 or LM338 chip has 3 pins: input, output and adjust.
Vin goes to input pin.
Output pin (Vout) goes to load which get limited current.
One resistor called Rx connects between output and adjust pin.
Rx decide how much current can go to load.
Current limit use this formula:
ILIMIT = VREF / Rx
where,
- ILIMIT is max current goes to load and unit is in amp.
- VREF is the fixed voltage inside LM317/LM338 which usually is 1.25V.
- Rx is the resistor between output and adjust pin its unit is in ohms and this resistor set the current limit.
VREF is inside chip which is usually 1.25V.
When load take more current then voltage on Rx goes up.
Chip see this from adjust pin and lower Vout.
This way chip keep voltage on Rx same so current stay same too.
Chip act like smart resistor and control current with feedback.
Important points:
Rx must be strong enough and should not get hot.
Power used by Rx: P = I² × R
LM317 can give max 1.5A and IC LM338 can give 5A.
Chip limit current only up to its max limit.
Rx value range is 0.8Ω to 1200Ω which is too low or too high can make chip not work right.
Advantages:
Protect load from too much current.
Give steady current, even if input voltage changes a little.
How to Build:
To build a Simple Current Limiting Circuit using IC LM317/LM338 follow the below mentioned steps:
- Gather all parts like shown in circuit diagram.
- Connect adjust pin of LM317 to other side of Rx resistor and this is also current output.
- Connect Vout pin to load through Rx resistor.
- Connect Vin pin to positive of DC power.
- Connect loads negative wire to ground.
Safety Tips:
- Always check LM317 or LM338 datasheet we are using.
- Datasheet tells limits for voltage, current, temperature and safety rules.
- Use heat sink, capacitors, resistors with correct ratings.
- Do not give part more voltage or current than it can handle.
- LM317/338 can get hot if voltage or current is high.
- Read datasheet to know if heat sink is needed.
- Heat sink helps cool down the IC.
Conclusion:
Simple Current Limiting Circuit using IC LM317/LM338 is good to protect our devices from too much current.
Build it carefully, choose good parts, follow safety steps and know how it works.
Always check datasheet and stay safe with electronics.