# How to Detect if Motor is not Working?

sorry for my english, I am from Indonesia.

I try to make a (car) relay active - switch on, if the motor/fan is working normal, so when the fan is out, not working or broken, automaticly the relay is off. But it is not working.

Edited Transistor NPN 2N3055

Motor 12 Vdc / 3 A

Car Relay Hela 12V

• Please explain how you believe your current circuit should work. – Ignacio Vazquez-Abrams Jan 7 '16 at 9:23
• What fault conditions on the motor are you trying to detect? – Icy Jan 7 '16 at 9:25
• As long as the motor works fine, the relay should still on. – Herbrata Moeljo Jan 7 '16 at 9:29
• 'not working' could be seized - high current; open circuit - zero current; disconnected load - low current + possibly lots of other conditions. – Icy Jan 7 '16 at 9:33
• so you require base current of 3.75mA (150/40) - and you have sized 270R resistor to give 10mA at 3A motor current. If this is rated current typical operating current will be much lower perhaps less than 1A - try using 1 or 2 Ohm resistor instead of 270R - the BD139 can take 1.5A of base current - and this will also give a reduced voltage drop on the motor at full torque. – Icy Jan 7 '16 at 10:01 Start by assuming the motor is drawing 3 A, and the relay coil is drawing 150 mA. Then the transistor collector current is also drawing 150 mA. Because the transistor should be acting as a switch, its base current should be in the range of 1/10 to 1/20 the collector current, or about 10 mA. Then the voltage across RSENSE should be $$V_{RSENSE} = 0.7 + I_b \times RBASE$$ For a 47 ohm RBASE, this works out to $$V_{RSENSE} = 0.7 + .01 \times 47 = 0.7 + .47 = 1.17\text{ volts}$$ and the power dissipated in RSENSE $$P = i VSENSE= 3\times1.17 = 3.5\text{ watts}$$ so you'll need RSENSE to be at least 3.5 watts, and a 5-watt resistor would be a reasonable choice.