what I want to achieve: delay start a dust collector when table saw is turned on. I would like a short delay turning the unit on, and a longer delay [7-10 secs] when turning off to pull remaining sawdust.
I have a heavy duty contactor with 24v input and want to make a simple circuit that would close a relay supplying the 24v. My skill level is entry level plus.
From what I can decern so far, I need to create a latching switch circuit with 555's or possibly resistor/capacitor in parallel before a relay. I've read that the simpler resist/cap setup could lower the voltage too much, so it seems like a 555 based circuit might make more sense. Again, I have only basic skills, but my hope is that this circuit would receive voltage when table saw is energized, and that would power the trigger and close the relay after a set delay. The relay would remain closed indefinitely, and then when the saw goes off and the trigger voltage drops, the relay would open after another set delay.

I would appreciate any thoughts. I seems like it might be a common scenario, but much googling has not helped

 

Why not just switch on at start and a delay at off.

 

Why not just switch on at start and a delay at off.

The reason is to stagger the startup of the two motors, it only needs to be about a second. Both draw a lot for the first split second.
I can purchase a 120v commercial product that does what I’d describing for about $50 and up. But my saw is wired for 240, and devices for 240 are much more expensive. The saw motor can be wired 120v, so I could go that route.

 

But my saw is wired for 240,

Are you planning on tapping into the 240 on the saw switch as a trigger for the delay circuit?
If so here's a simple circuit to provide both delays.
R2 sets the delay ON time and R1 the Delay OFF. The values listed should be in the ball park.

 

The above circuit will work with a constant 24-volt supply that is present before you throw the saw switch.

Do you have such a supply?

If not, you will need RL1 to switch on an AC to 24-volt DC converter that in turn powers the contactor.

 

Thanks, this looks great!
Just to make sure :
A. A 5 volt usb charger that takes 240v input that is switched with the saw motor.
B. When the saw is switched off, the capacitor will power the relay for a few more seconds.

thanks again!

 

That's correct but I made a big boo boo in the circuit. See edited schematic below and corrected in post #4
Probably can eliminate the relay. Do you have a link to the 24 volt contactor?

 

See edited schematic below

The 24Vdc will need to be there after the saw power is removed.

The 5V power can be from any common wall-wort supply.

 

Also, 24V- and 5V- will need to be connected together.

 

The 24Vdc will need to be there after the saw power is removed.

Yes, that's a given.
There is away to activate the 24 volt supply via the relay contacts and back feed the 24 volts to the relay coil if required.

crutschow said:
The 5V power can be from any common wall-wort supply.

I find the USB chargers less expensive and smaller.

Also, 24V- and 5V- will need to be connected together.

Correct as shown below in the latest edit.

 

Thanks so much for all the input!

My other concern is that currently [pun not intended], the dust collector in on another circuit and I'm assuming it is not good to have have this wiring bridging the two. The dust collector is wired for 120, but it can be rewired to 240, and the combined 13.5A load would be plenty safe on one 20a 240v circuit.

The other thought I had was to use a current sensing switch in place of direct connection.

 

Don't see a problem with the wiring since the 120 for the dust collector is isolated by the contactor although the collector would operate more efficiently at 240.
Do you have a particular current switch in mind?

 

I did find a couple options on Amazon I’d have to pull up the specs. But I do agree that all on 240 is the way to go.

Why is 240 more efficient? When asking woodworking forums most people mention the lower amp draw, and subsequent smaller gauge wiring. But most people had anecdotal reasons as opposed to factual reasons why the motor would operate better at 240v

 

Yeah I saw a couple of units that would work but connecting them might be problematic depending on your setup.

 

It makes more sense to start the dust collector before the saw, so that even the very first dust will be collected. But the saw should never be started automaticly for safety reasons.

 

It makes more sense to start the dust collector before the saw

But it makes sense also to wait until the collector is up and running before making the first cut.