Inrush-resistor-dat
🔍 Purpose of 2W 300Ω Resistor Before LM317 Input
🔧 Common Use Cases
1. 🛡️ Inrush Current Limiting
- Function: When power is first applied, capacitors (especially large filter caps) can draw a big inrush current.
- The 300Ω resistor slows the charging rate of the input capacitor.
- Helps protect:
- Transformer
- Diodes (in rectifier)
- LM317 itself
2. 🔥 Power Dissipation / Pre-Regulation
- Drops excess voltage before it hits the LM317
- Reduces the heat load on the LM317, especially if there's a large Vin − Vout difference.
For example:
Vin = 24V, Vout = 12V, Load = 50mA
→ Voltage drop across LM317 = 12V
→ Power = 12V × 0.05A = 0.6W (hot!)
Insert a 300Ω resistor:
V = I × R = 0.05A × 300Ω = 15V drop before LM317
New Vin = 24V − 15V = 9V → not enough
So you may adjust value to drop just a few volts- You must ensure enough voltage remains after the resistor so that LM317 operates normally (requires at least Vin ≈ Vout + 2V).