Switch that shuts off when power goes out

[dullard]

I'm looking for a power switch for a machine. For safety reasons, I'd like it to default to off anytime the power is cut (say a power outage occurs). I've seen these things everywhere on machines that could be dangerous (like a table saw). That way when the power comes back on you might not have your hands in the way of the blade. Then of course there needs to be a button to push manually when you want to start the machine up again.

Edit for clarification: Ideally I'd like a pushbutton power switch (like on a computer). The user uses it to turn the machine on/off. But the machine defaults to off whenever power is cut. Then the user needs to press the power button again to turn it on.

Unfortunately I don't know where to look. I've tried google searches but I don't know the proper terminology to search for. I find lots of links to switches that turn off when there is a key turned, a pressure mat stepped on, a light change, a chain pulled, a door opened, etc - but not a single one for when power is cut.

Do any of you know where I can buy a switch that does it, or a company that makes them, or a catalog that has them in it, or the proper words for a google search, an electrical forum where I could find a better answer, or even just a book that discribes the terminology? Anything at all would be a great help.

 

[HokieESM]

dullard--some circuit breakers do this. My dad's woodworking shop is wired this way--when the power goes out, all the breakers are tripped. You might want to look into this area... as its not really a "switch".

Sorry I don't have more specific information.... I know how frustrating finding an obscure part can be! Good luck.

 

[dullard]

Originally posted by: HokieESM
dullard--some circuit breakers do this. My dad's woodworking shop is wired this way--when the power goes out, all the breakers are tripped. You might want to look into this area... as its not really a "switch".

That is one of the many "close" things I've found. A $300 bulky circuit breaker is really overkill for my use. I've considered it, but I know there has to be a better solution since I've seen them all over the place.

 

[FeathersMcGraw]

You want a relay.

 

[dpm]

How about SKU # 454480 at homedepot;


<P class=tenbold>White 6-Side Outlet Wall Tap with Breaker for $4.97?

 

[dullard]

Originally posted by: FeathersMcGraw
You want a relay.

I don't know much about electrical parts but, as I understand it a relay is just a type of a switch. Put a voltage on one set of terminals and the other set is opened/closed depending on the relay. This is close to what I need, but I don't know of any relays that default to off and won't turn back on until the user pushes a button. Do you know of a specific relay that will to that?

Edit for clarification: Ideally I'd like a pushbutton power switch (like on a computer). The user uses it to turn the machine on/off. But the machine defaults to off whenever power is cut. Then the user needs to press the power button again to turn it on.

White 6-Side Outlet Wall Tap with Breaker for $4.97?

What exactly does that do? I cannot tell from the website. It is quite bulky though at 8"x5".

 

[Demon-Xanth]

It's basically a relayed switch, hardware stores have them for mounting on table saws and the like.

 

[Harvey]

It would be easy to design and build circuits to do this, and it sounds like a good safety feature, but I don't know if anyone makes a commercial product that does this.

The easiest circuit to build would include a separate "Start" button that required mechanical actuation after the power switch was turned on so when power was restored after an outage, the machine would not start again until the button was pushed. Another approach would be to add a logic circuit that controls a relay in the power line. This circuit would be wired so that it could not work unless the main power switch started in the "Off" position.

 

[arcas]

You can construct this type of switch from a normally-open latching relay. Control the relay with a momentary pushbutton switch. Once closed (ie. turned "on") the relay will remain closed so long as power is applied. This means you'll need a separate toggle switch to kill power to the machine when you want to turn it off. Prices for 120VAC latching relays range from around $20 to $60 depending on features, number of coils, etc. Make sure you pick a relay capable of handling the amount of voltage required by your device.

You can also construct solid-state versions but a momentary switch + latching relay is probably the simplest way to do it.

 

[dullard]

It would be easy to design and build circuits to do this, and it sounds like a good safety feature, but I don't know if anyone makes a commercial product that does this.

I would think it would be a very useful device for someone to make... But I don't mind making it myself.

Originally posted by: arcas
You can construct this type of switch from a normally-open latching relay. Control the relay with a momentary pushbutton switch. Once closed (ie. turned "on") the relay will remain closed so long as power is applied. This means you'll need a separate toggle switch to kill power to the machine when you want to turn it off. Prices for 120VAC latching relays range from around $20 to $60 depending on features, number of coils, etc. Make sure you pick a relay capable of handling the amount of voltage required by your device.

You can also construct solid-state versions but a momentary switch + latching relay is probably the simplest way to do it.

Ok that sounds more like what I need. If I understand it you say there will be a 'main' on/off switch to control the power to the machine. This current goes through a latching relay that is closed so current travels thorough. When power is cut the 'main' on/off switch will remain in the on position but the relay will revert to its normally open position, therby cutting power to the machine. A momentary 'reset' button can be pressed to reset the latching relay back to the closed position. Is that what you were talking about?

 

[arcas]

Yes, exactly. The momentary pushbutton is a 'reset' button which powers the relay long enough for it to latch closed.

Even though it's a straightforward circuit to build, I'd still recommend buying a pre-built commercial latching relay switch if you're going to use it with a machine that could kill/maim or if it requires a lot of power. It gives you someone to blame other than yourself if it fails

 

[dman]

How about buying a SmartUPS. Once setup, The machine will shutdown automatically when the power goes off and the battery is low (only if you get a 'smartups'). If it's ATX and somewhat recent, You can set the bios to not automatically turn on when the power comes back on (too). That should effectively minimize downtime to poweroutages, unless you live somewhere they are constantly switching off power.

 

[dullard]

Originally posted by: arcas
I'd still recommend buying a pre-built commercial latching relay switch if you're going to use it with a machine that could kill/maim or if it requires a lot of power. It gives you someone to blame other than yourself if it fails

The main issue with my machine is that it will have dual 300 W heaters to heat air running through them. If there is no air, the heaters overheat, bend out of shape, and eventually could pose a significant problem. The heaters will be in their own insulated box, but you still don't want a 600 W heater sitting on your desktop non-stop. I don't think it would maim or kill, but it is a potential hazzard. Do you have any clue where I could get a prebuilt commecial latching relay switch?

Originally posted by: dman
How about buying a SmartUPS.

That would work, but I didn't tell you enough information. I'll be attaching a lot of other safety devices (the power outage was just one example). Each of them will be a relay that cuts the main power if there is a problem (such as the heaters overheat or the computer which the machine is connected to shuts off). So with just one main switch that stays off with the power off, all these power cutting safety devices will work together flawlessly.

A UPS would have to be internal to the machine (which is possible but is a nightmare to allow the user access to it). Or I'd need an external UPS with an additional external box containing all the safety relays which is hooked up to the UPS which is hooked up to the machine which is hooked up to the safety relays all of which is hooked up to a computer. And since I need the whole machine to be lightweight and portable, all these external boxes and wirings just won't be practical. Plus the shear size of a UPS won't work well for a small portable machine.

 

[arcas]

Off hand, I don't know of any commercial off-the-shelf versions though I'm sure you can find them in electrical-supply stores or carpentry tool stores. They might sell them as "magnetic starters." If all you're doing is driving 600W worth of heaters (basically a resistive load), you shouldn't have a problem and I'd be tempted to just build it myself. If you were controlling 3 phase motors or saws or something, I'd go commercial. Heck, you could probably scrounge one from an old clothes dryer