Fan control help needed

Other Hardware General Electronics
3 
int LDR = A0;                 
int LDRValue = 0;                    
int light_sensitivity = 750;  
 
void setup()
  {
    Serial.begin(9600);           
    pinMode(13, OUTPUT);    
  }
 
void loop()
  {
    LDRValue = analogRead(LDR);        
    Serial.println(LDRValue);                  
    delay(50);                                              
 
    if (LDRValue < light_sensitivity) 
      {
         analogWrite(13, 100);
        }
        else
        {
          analogWrite(13, 200);
          }
          }

This is the code I got running. Basicly it's hooked up to a lightresistor so when the lights dim, the fan turns on. Like I mentioned earlier it works on another CPU fan I got, the problem might be the PWM frequency.