Circuit Design Help

I haven't designed a circuit in more years then I care to mention so I know I need help. What I want to do is .. control a table saw and a shop vac. When I turn the circuit ON...table saw and shop/vac on immediately When I turn the circuit OFF... table saw off immediately and shop/vac to run for approx. 5 seconds longer and then turn off. If your able and/or willing to help I'll give you the specs for both pieces of equipment.

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all you need is the main switch, a capacitor, and a relay.

The main switch will automatically turn on the saw as it would be directly connected. The vac will be connected through the relay that will be controlled by a capacitor.

essentially the capacitor will charge up(you'll need a diode for this and a few resistors and stuff) and when it charges to the correct voltage it will switch the relay on which allows the vac to run. In the same sense it will discharge and turn of the vac. The problem here is that the vac will not start for the same time that it will be delayed. (Which is probably not an issue but if it is you can use a little more circuitry to solve the problem).

The switch that your controlling will need to to be a DPDT. One so that if controls the mains on the saw and the other will be used to charge the capacitor. The relay will always be connected to the vac.

In essenence->

Mains ---- Switch ----------- Saw | |----------Diode-Resistor-Cap->Relay Control |------Relay In --- Relay Out---> Vac

Hopefully that makes some sense.

While I think it works in theory and its just off the top of my head I'm sure some of the "experts" around here will point out all the flaws. I'm sure you probably could find something that is essentially a delayed time switch that will do what you want. You'll basically want a dual delayed time switch and set one of the timers to 0 and the other to 5 seconds. This is essentially what the above does and you would adjust the resistor and capacitor values to get the correct time constant and voltage needed to run the relay.

There are also SCR circuits that do this too but I would imagine you would want something with a true bypass.

Anyways, Just thought I'd through that idea out there as its pretty basic and probably will get the job done just fine. There might be a few issues there but I'm sure you can figure them out and if not you can ask.


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Jon Slaughter

Save yourself some trouble and buy a ready made "delay on off" time delay relay?

Plan B would be a solid state relay with a large (~1,000 uf- 5,000ur) cap holding up the "coil" voltage for 5 seconds. But that would require a simple basic low voltage power supply to energize the relay.

Plan C use a low voltage control relay, power supply, and mosfet transistor to switch the relay off when a cap on the gate discharges below its threshold value (cap can be ~10 uf). Cap charges through a diode and turns on relay immediately turning on the dust collector. Cap discharges through a suitably large resistor across it after power is removed (1-4.7 megohm). Downside is the LV supply has to stay on

24/7. The LV supply could be off and dust collector started manually with a push button, or switched with the power to the saw if that's convenient/desirable. '-- '

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You need a power cord and a switch capable of handling the power requirements of both motors, a metal case for mounting the swicth, a relay, a relay socket, and appropriate AC outlets.

The delay relay and socket are available here:

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Everything else is available at your local home supply store.

A suitable case might be a sub-circuit breaker box with just one 30 amp breaker to use as the master power switch.

If you need a wiring diagram, post another message or send email.


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You need to get a Time off delay relay. they already come made.

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P.S. you'll need an octal socket base for it also.

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A delay off relay has been suggested, and would be a simple, but expensive, solution. Here's a "poor-man's" solution that will cost < $20.00

TABLE SAW --------------------------------------------- | Switch | | / | | AC Hot ---o o---+---------------------+ | | | | | | | ------- | | | | | | | | | +--| | | |---+ | | | | o | | | | | | | | | | | ------- | [MOTOR]| | | | | | | AC Gnd ------------------+ | | | | | | | | AC Neutral ----------------------+------+ | | | ---------------------------------------------

Wire up a junction box with a receptacle and install on the table saw per the diagram above.

Plug a regulated 12V wall-wart supply into the receptacle. Use the circuit below to operate a relay which will control the vacuum.

a +12 ---+----[D1]-----+----------+ | | | [R1] [D3] | | a| | | a | | +---[D2]------+ [Rly] | | | | + | | [R2] [C1] | | | | | | | Gnd ---+-------------+----------+

C1 = 1 Farad super cap CAT # CBC-17 Rly = Automotive 12V relay CAT # RLY-351 R1,R2 = 50 ohm, 25 W CAT # 50-25 Diodes = 1N4004 CAT # 1N4004 Wall Wart 12V regulated CAT # PS-1251

All parts from Allellectronics

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Some notes: R1,2 were picked based on availability at Allelectronics. 5 watts would have been more than adequate if they had that wattage.

The delay time is based on approximation. The exact voltage that will allow the relay to drop out is unknown, but should be about 2 volts. To shorten the delay time, increase the value of R1 by adding power resistors in series with R1. You might want to include CAT # 16-5 in your order from Allelectronics. That will give you three 16 ohm resistors for $1.00, which you can add one at a time, as needed.

Your 12 volt supply *must* be regulated, and produce no more than 12 volts. If it is not regulated, or produces over 12 volts, add a second super cap in series with C1. That will also shorten the delay time.


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