I have just finished build my table saw and now planning for integrated vacuum system for dust collection.
I've designed the circuit to detect if saw motor is switched on and automatically switch on the vacuum pump. Current sensing resistor Rs is on neutral line of the saw. One side of this resistor is floated on DC common as shown in the schematic below.
The voltage drop at the other end of Rs is used to turn on NPN transistor which drives relay. The contacts of the relay will act as the switch to turn on the vacuum pump.
Care have been taken to clamp Vbe to safe values using 1N4004 diodes.
Before I build this circuit and test it, I need some expert advice for Rs. My saw motor is 1500 watts universal motor. It has no soft start feature, meaning it may consume very high starting current which Rs will have to bear.
As such, what should be the right value for Rs in terms of resistance and power rating?
PS; Based on comments on this question, i would like to clarify the reason for this current sensing auto switch is to accommodate few more power tools so that when any of those dust making tool is switched on will start the vacuum unit.