Need help with switch... - Page 1
Forum section: electronics / beginners

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 01, 2011, 11:45:54 pm
content: Using (magnetic) switchone, I turn on a motor running on household current. I press a spring loaded button (switchtwo). As I hold the switch in the motor continues to run. When I release the switch the motor shuts off. Is there such a switch? Thanks for any help.

author: Jimmy
author_rank: Regular Contributor
timestamp: March 02, 2011, 02:58:06 am
content: If you want to only run the motor while you have your finger on the button a normally open momentary push button is what you require

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 02, 2011, 04:06:10 pm
content: The motor is already running because of switch one (has to be this way). Then I press switch two. The motor continues to run unless I let go. Sort of like holding a grenade with the pin pulled but lever held in. I know it sounds crazy but this is what I need for an invention.

author: xygor
author_rank: Regular Contributor
timestamp: March 02, 2011, 11:54:40 pm
content: Yes. There are push button switches that toggle their state upon release, not when pressed.

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 03, 2011, 02:16:47 am
content: Thanks. Do you know how I might Google it? Does it have a name that would make it easier?

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 03, 2011, 02:33:41 am
content: A few other things to consider. You will have to get a switch rated for the load of the motor you intend to use at the voltage it will operate. If you have a standard small motor like for a fan or so (less than 1/2 HP), there should be a large choice of switches available. You didn't mention how this switch combination is supposed to reset. If you buy a standard pushbutton switch for switch 2, you will have to manually reset it each time you restart the motor as it will be wired in series with switch 1 since it will lock in both on and off positions. There is also a momentary contact type switch, but that wouldn't work unless you have a controller of some type. You also didn't mention what type of magnetic switch you're using - that might affect things as well. Are these switches directly controlling the motor or are they controlling a programmable relay? Can you give some more details about what you're trying to accomplish?

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 05:34:42 am
content: switch one activated by magnetic (relay?), but when switch two is pressed, does it deactivate switch one? and switch two will be the main supply to the motor? so when release the switch two, both switch will be off and off the motor? is it like that?

author: xygor
author_rank: Regular Contributor
timestamp: March 03, 2011, 11:36:02 pm
content: Quote from: easilyconfused on March 03, 2011, 02:16:47 amWhat I had in mind is called "push-on push-off" or "push-push". Something like this:http://push-button-switches.carlingtech.com/heavy-duty-pushbutton-switch__64.aspI am not saying that the above switch does what you want. I just can't figure it out whether it does or not.I don't know what the action (switch on release) is called. I looked at several manufacturers and cannot find any terminology or indication of where in the cycle the switch actually changes state. You'll need to ask your rep or obtain samples.You could also work out something with momentary switches and relay logic.

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 04, 2011, 01:27:07 pm
content: Thanks for all the help. I'm just going to go ahead and spill my guts. Probably won't work anyhow. Table saws are notorious for causing injuries by way of kick back. The wood gets lifted off by the spinning blade and thrown back at the operator. There are ALREADY many ways to prevent this. One more way can't hurt. My switch would change state if the wood lifts off the table. The motor would shut down. The blade would still have some inertia but the injury might be far less severe. This would actually be a kick back "mitigator" rather than a kick back "preventer". I just don't know where to get such a switch and how to wire it up. I do understand the workings of the main magnetic switch and I figure the spring loaded switch would do the same thing as the big red switch: kill power to the magnet. But it would have to do this NOT when it's pressed but when it's released. Thanks for any help.

author: david77
author_rank: Frequent Contributor
timestamp: March 04, 2011, 01:39:32 pm
content: All you need is a momentary push button, or maybe some sort of microswitch.But:If you simply wire your safety switch in series with the big red STOP button the saw won't startunless your safety switch is also pressed.I don't know of any simple mechanical switch that has the desired function.David

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 04, 2011, 03:29:41 pm
content: If you simply wire your safety switch in series with the big red STOP button the saw won't startunless your safety switch is also pressed.That would be okay. The operator would simply place the lumber on the button before turning on the saw. I would also want a feature wherein the operator could disable the device for certain cut.

author: Jimmy
author_rank: Regular Contributor
timestamp: March 05, 2011, 08:11:57 am
content: http://www.break.com/usercontent/2007/5/17/stop-saw-295789Something like this would be better

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 06, 2011, 04:43:45 pm
content: Quote from: Jimmy on March 05, 2011, 08:11:57 amYes. The Saw Stop is well known as a high-dollar way to preventcuts. But even the Saw Stop mechanism is not intended to prevent kickbacks. It does have pawls for kickback. As I said in the beginning of the string-- this would simply be one more device. But apparently there is no such switch available.

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 06, 2011, 06:53:47 pm
content: I think David's right - all you need is a micro switch. It would likely have to be used with a relay of some type because you probably won't find anything with the amp rating you need for the saw motor, but it could be programmed to work a number of ways. I think you can find a relay that will allow you to start the saw without the safety switch closed for a preset time, which would give the user the time needed to get the material on the table. You should talk to someone who really knows industrial controls.

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 06, 2011, 09:46:45 pm
content: Quote from: PetrosA on March 06, 2011, 06:53:47 pmThanks for the encouragement. I'll see if I can find a forum for such a thing.

author: RayJones
author_rank: Frequent Contributor
timestamp: March 06, 2011, 10:46:36 pm
content: If your switch is in front of the blade, won't the saw stop just as you are finishing the cut, and most likely result in an incomplete cut?

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 07, 2011, 02:55:07 pm
content: Quote from: RayJones on March 06, 2011, 10:46:36 pmGood point. Hadn't thought of that. It would have to be mounted a little forward of that point so that the cut would complete. The switch would be installed in the throat plate, possibly marketed together.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 12, 2011, 06:53:35 am
content: sealed, splashproof, 10A, NO & NC connections so you can easily interface with whatever logic you choose.cherry e70-sj

author: sonicj
author_rank: Frequent Contributor
timestamp: March 12, 2011, 07:16:25 am
content: does it kick up then back?heres how i imagine it working...-micro switch strip is adjustable on the z axis (automatic or manual) to proper height for board. board does not engage the switch during normal use.-start button triggers latching relay on-mushroom cutoff & all micro switches are wired in parallel from the NO contacts to the off terminal on the latching relay.-if any switch is engaged, either by a bucking board or the operator kill switch, relay will cut power until the start button is re-engaged-this will allow for the board to finish its complete pass without tripping a kill switch.-any number of kill switches will work-relay carries load, not the switches, so any size switch will workmake any sense?-sj

author: FreeThinker
author_rank: Frequent Contributor
timestamp: March 13, 2011, 04:27:19 pm
content: You could probably configure a 2 channel pilz unit to do what you require.Not cheap thoughhttp://www.pilz.co.uk/products/control_communication/safety_relay/f/pnozelog/index.jspKickback video here

author: Zero999
author_rank: Super Contributor
timestamp: March 13, 2011, 04:44:00 pm
content: Yes, it needs to be a specially approved safety relay and switch. Don't trust an ordinary microswitch and relay to reliably cut off the power.

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 13, 2011, 09:06:08 pm
content: Sorry to be slow in responding. Have been away. I do appreciate the responses. I especially appreciate the drawings. Your idea(s) may be better than mine. In my minds-eye the micro switch(es) would be mounted on the underside of the wood throat plate of the saw. The roller would protrude up through a hole. It would be very inexpensive and easily replaced as the throat plate wore out. I think it's impossible to know whether a board is going to lift up or merely traverse backward without lifting. I suppose it could happen either way. People have been killed. I do not know if chopping the power would be of any benefit. The kickback happens so quickly. But I'm thinking it couldn't hurt. It might prevent an injured operator from falling into a blade (still under power) for further injury. Certainly the Saw-Stop patented blade "jammer" could be triggered so that the Saw-Stop table saws would actually have two modes of protection rather than one. I think I need to take your suggestions, order some switches, wire it up and evaluate from there. Thanks to all.

author: sonicj
author_rank: Frequent Contributor
timestamp: March 14, 2011, 02:58:03 am
content: wow, that happens with the quickness! i think you might be right easilyconfused, i don't know that a switch will do much good. the launch event will have already started by the time the switch is engaged and a 1/2 or 1hp motor is a lot of momentum to deal with. i think a conveyor with some fingers that spring up when the end of the board meets the table might be a more practical approach. the operator could step to the side as the conveyor finishes the cut.-sj

author: easilyconfused
author_rank: Regular Contributor
timestamp: March 14, 2011, 05:10:46 pm
content: wow, that happens with the quickness! i think you might be right easilyconfused, i don't know that a switch will do much good. the launch event will have already started by the time the switch is engaged and a 1/2 or 1hp motor is a lot of momentum to deal with. i think a conveyor with some fingers that spring up when the end of the board meets the table might be a more practical approach. the operator could step to the side as the conveyor finishes the cut.-sjYou might have something there. You're quite the arteest.