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

[Bruce Wrenn]

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

Multiwire branch circuits use a double pole breaker. Each hot leg is connected to a recpt. along with a neutral. This gives you two 120 circuits. Then I feed through the recpts. into another box with a 240V, 20 amp socket Only one of the hots passes through sensor. Either the 240, or the 120 leg that passes through sensor will operate DC. I have a total of six machine circuits (220) in my shop. On each set of circuits, I have multiple recpt. sets. I chose to make the left recpt.at each location the one that controls DC for 120 machines. No specific reason, just did. When I bought sensors back in 2000, they were about $40 and delay timer was about $20. They relay I used has a 120V coil, so no extra transformer is needed. It was a stock Grainger item. You only need to break one leg of the 240 to DC. By using 3/4 EMT., I didn't have to pull a ground wire. With the price of #12 copper THHN being what it is, that is both a savings in time and money. You still have to bond recpts. to the metal boxes though. PM me an address and I will do you a diagram, along with specific parts #'s

[Rob Will]

I think this discussion is leaning toward the complicated side.

All you need is a magnetic starter on the DC equipped with a time delay auxillary contact (off the shelf stuff at the local electric motor shop).

How you handle the control is up to you. With most current sensing relays, remote control switches and push-buttons, you will simply be "switching" the coil voltage to the much heavier magnetic starter.

If you can't source a magnetic starter with built-in delay off, I would get a time delay relay (and the cooresponding mounting socket) from a local electronics shop or Grainger.

Rob

[Daryl Upole]

Hi Doug & All;

Doug, I purchased my SSAC sensor from Grainger a few years ago. But I could not find the instruction sheet recently so I Googled to find it; which I did. In the process, I did see it somewhere for under $50. So, you might want to Google the exact part number and you may find better pricing.

I went with Tom's suggestion & ordered the delay relay from the link he provided. ( they may even have the SSAC) It will go between the sensor and the 220Volt relay to the D/C. I should have it in a few days.

TOM - on that - I think I've got the resistor stuff figured out. I'm barely knowledgeable in this area. I found the package of 1/4 watt Radio Shack resistors I had. With the scale on 2000k - I get the reading of approx 1,500. And, the color banding looks like 1 and 5 @ 100k multiple. So, this would be the 1.5MEG? Which is 1,500,000 ohms? From the charts, that should give me a little over two minutes.

I ended up taking an old breaker panel and setting up all the relays, etc. in there just underneath the power panel. That gave me a lot of room to work.

On the split circuits - in my shop I use GFCI for all 110 circuits. Without special receptacles, breakers, etc. you need a single circuit to work properly - I won't work right on a split circuit. I have alot of respect for electricity and with my concrete floor and the possibility of using tools outside the shop, I feel better doing so. I'm pretty sure local code would require that for me as well. Also, outlets need to be 24" off the floor - I think that has to do with minimizing problems from vapors settling near the floor. I've never had a problem with false tripping of the GFCI.

I'm glad to know that one leg of the 220 runs through the sensor - that's the way I've got it set up awaiting the rest of the parts and my new saw.

Doug - yes, you can adjust it from 2 amps up. From the spec sheet, I think you can wrap the wire around the sensor if you need it to be a lower start up. For example, I leave mine at the minimum. A 100 watt bulb (less than 1 amp) will not trigger it - but just my 3/8" electric drill will. There is also a delay on setting you can use.

[Larry James]

... 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.

Tom, I think you are correct about switching only one wire being acceptable. The double pole/single throw disconnect, as shown in your drawing, is required. Nice control circuit Tom.

In a 120 volt circuit the neutral (usually color coded white) is the "grounded conductor" and connected to ground at the service entrance. Therefore, the neutral is not "hot" with respect to ground at the equipment. However in a 220 volt circuit you may control the load by switching only one leg (wire), but unlike the 120 volt circuit, the second un-switched wire remains "hot" at the equipment.

Safety: it is very important to understand that if only one wire is switched in a 220 circuit, you must unplug, or turn the circuit breaker off to remove voltage to the equipment before working on the equipment.

Larry

[Matt Meiser]

Safety: it is very important to understand that if only one wire is switched in a 220 circuit, you must unplug, or turn the circuit breaker off to remove voltage to the equipment before working on the equipment.

That's safe working practice regardless. There was a recent thread on magnetic switches that started on their own after a physical shock. A disconnect switch by your DC might even be a good idea.

[Tom Veatch]

...I think I've got the resistor stuff figured out. I'm barely knowledgeable in this area. I found the package of 1/4 watt Radio Shack resistors I had. With the scale on 2000k - I get the reading of approx 1,500. And, the color banding looks like 1 and 5 @ 100k multiple. So, this would be the 1.5MEG? Which is 1,500,000 ohms? From the charts, that should give me a little over two minutes. ...

It's been a long time since I tried to read resistor color codes, but if I remember the "Bad Boys Ravish..." mnemonic correctly, a 1.5Meg resistor would be Brown, Green, Green and yield about 150 seconds or 2 1/2 minute delay. Two resistors in series would double that to 3Meg and 5 min delay, etc.

[Daryl Upole]

Hi Tom,

I got the delay unit in and installed today. Works great. I put in 470k ohm resistor and that gives me about 70 seconds which seems about right.

What would I use if I wanted this to be variable? Can you point me to the right part?

Daryl

[Bruce Wrenn]

Daryl, did you think about the fact that you have to have GFCI on the 110's, but not the 220's. Here , we only have to have GFCI on outlets closest to the door. You can do the "multiwire branch circuits with GFCI recpt. on the 110 legs. You will have to feed a couple of hots for the 220.

[Tom Veatch]

Hi Tom,

I got the delay unit in and installed today. Works great. I put in 470k ohm resistor and that gives me about 70 seconds which seems about right.

What would I use if I wanted this to be variable? Can you point me to the right part?

Daryl

Assuming that you got the same module I did, the adjustment range runs from 2 sec @ 0 ohms to 1000 sec at 10Meg ohms. You can use a potentiometer in place of the fixed resistor and dial in whatever resistance/delay time you want. A 0-10meg pot will give you the full adjustment range.

I just went by a local "mom and pop" electronics store and picked up a single-turn linear pot similar to this. They didn't have a 0-10Meg in stock so I got a 0-5Meg plus a 5Meg carbon resistor and a small SPST switch. I got a little fancy with it (for no good reason other than just playing around) and wired them such that the switch and resistor are in parallel and that combination is in series with the pot. I believe the first schematic I posted shows that arrangement. Doing it that way gives me a High (500-1000sec) and Low (2-500sec) range on the potentiometer/delay setting. With the switch closed, the resistor is shorted out and the total resistance in the timing circuit is the potentiometer's 0-5meg (2 to 500 sec). With the switch open, the 5meg resistor is in the circuit and the total resistance is 5-10meg (500 to 1000 sec). All times and values are approximate.

What parts? If you are pretty sure about what maximum delay you want, just get a potentiometer with a maximum resistance at or above that required for your maximum delay time. You should be able to find one at any local electronics shop for a couple of bucks. You don't need a "precision", "high wattage", "low-noise", "audio", etc. type. Cheap is good. I would say Radio Shack except the stores around here seem to have become cell phone stores and are very short stocked on any electronics components.