D/C Current Sensor Turn off Delay - How to?

[Daryl Upole]

I've read a lot of the posts on parts of this question. I am using a Grainger current sensor to a relay to turn on my D/C system when I turn on my machine power on that circuit. It has worked great for a few years now. As I read these forums, I'd like it to stay on for a little while when the power to the machine goes off. Can someone fill in the blanks on how I could do this? I've read some of you have done this - but could not quite tell how. I know a fair amount about electrical wiring - but not on how to accomplish this delay. The sensor has a delay feature to delay the turn on - but that's not what I want. My D/C is on a 220volt 20 amp circuit that the relay controls. I also have a manual override switch to power the relay.

[Tom Veatch]

I installed one of these in the circuit that powers the relay coil. There are many similar products that could be used.

In a nutshell, this device is inserted into the circuit between the switch and the relay coil. A variable resistor is used to set the time delay - from 2 sec to 1000 sec in this particular case. When the switch is turned on, power is fed to the load - the relay coil in this case. When the switch is turned off, the device maintains output power to the load until the delay time expires.

Here is the spec sheet for the device. It shows how this specific device is wired into the circuit. Other devices might be different.

[Tom Veatch]

Here's the wiring diagram for the installation in my system. The control system for my DC has two modes, Manual and Automatic. With the main switch on, setting the mode switch (DPDT center off) to "Manual" turns on the DC. Setting it to "Auto" puts the timer and tool sensor switch in the circuit such that when a tool is turned on, the "tool sensor" switch closes and turns on the DC. When the tool is turned off, the "tool sensor" switch opens and the delay time interval begins. The DC is turned off at the end of the delay interval.

The delay interval is controlled by the 5meg potentiometer and the 5Meg resistor in parallel with the SPST High/Low range switch. With the range switch closed, the 0-5meg potentiometer selects a delay interval of 2sec to 500sec. With the range switch open, the total resistance in the delay circuit is 5meg to 10meg and the delay interval is 500sec to 1000sec.

Hope this helps.

[Daryl Upole]

Thanks, Tom. That is exactly what I was looking for. Do I just put a resistor of different values to get the time I want between the two terminals? Are they just the cheap low-watt resistors you get in a bundle at Radio Shack?

Thanks again for the schematic.

[Tom Veatch]

Thanks, Tom. That is exactly what I was looking for. Do I just put a resistor of different values to get the time I want between the two terminals? Are they just the cheap low-watt resistors you get in a bundle at Radio Shack?

Thanks again for the schematic.

It will vary depending on the specific timer you use. Some have the variable resistor (trimmer pot.?) built into the device. This particular one uses an external resistor (each additional Meg is another 100 seconds of delay up to 10Meg/1000sec). The spec sheet in the link in the previous post shows the Seconds/Ohms relationship. According to the specs for this device, the worst case for the timing delay power dissipation is 3 milliwats, so a cheap 1/4 watt resistor is more than adequate.

Note, this is specific to this particular timing module. For other modules, the specs may be quite different.

[Matt Meiser]

Tom, that's great! For a while I was using a surplus PLC as the heart of my DC and heating/cooling control, but the comm module went bad and I didn't want to spend the $$$ to buy a new one so I ripped it out. Back to regular control for the heating/cooling and just blast gates for the DC. But having microswitches on the blast gates really lends itself to an off delay to give you time to move from one tool to the other without cycling the blower. I had mine on about a 15s delay before which worked great for me.

[Daryl Upole]

Thanks for the info. I have a package of these from when I did not want to pay to have a new resistor key for my Caddy (the old one started to work kind of funky in the ignition switch) - so I just wired them right into the theft control feature to bypass that and had regular keys cut.

[Tom Veatch]

Thanks for the info. ...

You're more than welcome.

My current shop upgrade project is to automate the blast gates so they actuate with the tool - like the Grizzly/Ecogate gates. But, the commercial 6" gates are outside my financial window. Right now, damper type gates with push-pull solenoid actuation is the front runner being considered.

[Bruce Wrenn]

For my DC system, I use the SSAC current sensor that was used in article in FWW, Aug 2000. I added a delay on off timer, from Grainger, to it. DC runs about ten seconds after machine shut down. All parts, including the relay are mounted in bottom of the power panel. One leg of each 240 circuit, except welder and air comp. pass through sensor. I use multiwire branch circuit wiring, so half of my 110 outlets also control DC. At each location, I have two 110 duplex recpt. and a 220 recpt. The duplex on the left controls DC.

[Doug Hobkirk]

...the more I write in this post, the less I realize I know...

Daryl, Tom, Matt -

First, you guys open my eyes and make me start to think... I had planned on using a $20 Sears current sensor in-line plug but I do not know if it can handle triggering a 240V device. Now I am considering other issues.

Turn-on DC Sensor

1. Could the Sears sensor rated at 15A control my DC by controlling just one leg of the 240 circuit? (The induction motor on my 2HP Grizzly DC lists it as 12A at 220V)
2. This current sensor relay (Grainger item 4GY65) cost $60 has a 12 amp "resistive" amp rating ("inductive" rating would be about half that?) - is this sufficient?
3. Does an adjustable trigger (e.g., 2A to 20A) mean if I set it at 2A anything over 2A will trigger the relay? (My tools that I would like to trigger the DC range from 15A down)
4. Does the "maximum inrush current" (when listed) apply to the triggered tool, acting like a time-delay fuse to accommodate the motor "over-drawing" on startup?

Delayed DC Shutoff

I thought that delayed DC shutoff couldn't really be done unless it was built in, but now I wonder if something like
this for $60 would be sufficient? (Sorry Tom, but I just don't know enough to fully understand your schematic.)

Bruce -

Then, I read "...I use the SSAC current sensor... I added a delay on off timer... DC runs about ten seconds after machine shut down... One leg of each 240 circuit, except welder and air comp. pass through sensor. I use multiwire branch circuit wiring, so half of my 110 outlets also control DC. At each location, I have two 110 duplex recpt. and a 220 recpt. The duplex on the left controls DC."

You made my knees buckle! And you make me rethink my soon-to-be-started garage sub-panel wiring job.

• Does "multiwire branch circuit wiring" mean you have two 120V outlets, each to one 120V circuit breaker, and a 3rd 240V outlet?
• Are all of your left outlets (your tool outlets) (excluding welder and compressor) on one circuit?
• Would draw on a 240V outlet also trigger the DC?
• Continuing my point 1 above, how much does a current sensing relay that can handle a home shop cost? (My only 240V tools is my DC, although I had planned on having at least two extra 240V outlets)
• Do you have a diagram?

Thank you all for your stimulating discussion!

[Matt Meiser]

Delayed DC Shutoff

I thought that delayed DC shutoff couldn't really be done unless it was built in, but now I wonder if something like
this for $60 would be sufficient? (Sorry Tom, but I just don't know enough to fully understand your schematic.)

Here's a similar but much less expensive 120v off delay relay.

Something like this can be used to detect current flow, but if it was me, I'd use it to control a contactor that actually switched the motor.

[Ken Garlock]

Hi Gents.

I also did a semi-automatic dust collector system. I had bought some 50 Amp/250V solid-state relays several years ago to use as the motor relay on the cyclone. To drive those two relays(one on each 240v hot line) I built a small 12V DC supply to provide the needed switching voltage for the relays. At five of my stationary tools, I placed a solid-state current switch, and then ran thermostat wire back to the relay panel from each switch. When the tool is turned on, the current switch activates a NPN transistor internally, that in turn completes the 12V circuit that closes the big solid state relays and starts the motor. I placed about a 12,000 MFD capacitor with current limiting resistor across the switched side of the 12V line that discharges slowly after the tool is turned off, and keeps the cyclone running for an additional 10 seconds.

I too would like to build an automatic blast gate, but that is down the list of things to do.

[Matt Meiser]

Tom, have you thought about using pneumatic cylinders for automation? Seems like they would be relatively simple to deal with and surplus ones aren't too expensive.

[Tom Veatch]

Could the Sears sensor rated at 15A control my DC by controlling just one leg of the 240 circuit? (The induction motor on my 2HP Grizzly DC lists it as 12A at 220V)

I'm going to decline comment on the specific devices since I'm not familar with them. However, controling the motor by switching only one leg of the circuit will work. In reality, that's what you do when you switch only the "hot" wire in a 120v circuit. Someone please correct me if I'm wrong, but I believe switching only one leg of a single phase circuit is acceptable to code as long as the voltage is less than 400 volts.

I built my current sensors from the plans in "Automated Dust Collector", Woodwork Magazine, Issue 91, Feb '05.

I thought that delayed DC shutoff couldn't really be done unless it was built in, but now I wonder if something like
this for $60 would be sufficient? (Sorry Tom, but I just don't know enough to fully understand your schematic.)

Something like that could work. That particular one couldn't be used to directly control the DC motor because of the low HP rating of the relay. It could be used to control the motor contactor coil, but you'd need a 24v power supply. I think you'd be better off with something like the solid state timer I linked in a prior post. That particular one was about half the cost of the Grainger link and can be used with 120v power. There are a lot of other brands/models of solid state timers available.

Most of the complexity of that schematic is due to the frills - dual mode and variable timing. I've simplied the schematic for a fixed time automated system without the frills. See the attached image below. That schematic is pin-specific to the one that I used and might not be pin-compatible with a different brand/model.

[Tom Veatch]

Tom, have you thought about using pneumatic cylinders for automation? Seems like they would be relatively simple to deal with and surplus ones aren't too expensive.

Yes, indeed, I have. In fact, post #5 in this thread shows a pneumaticly operated gate built by Todd Crow that I'm very interested in. I haven't looked into surplus cylinder prices, but Grainger prices for all the components would run about $100/gate. Much better than the Grizzly/Ecogate 6" gate prices, but I'm thinking I can get there for somewhere in the $30 - $50 range using the design of Post #9 (Jeffrey Makiel) in that same thread. I'll almost certainly build a prototype based on the Makiel gate for testing and design verification.