Pushbutton Switch with Power Failure Cutoff

[jakerobins8]

Hey guys I am new on electro tech,

I'm building a test unit for a personal project but it could be potentially dangerous if left unattended which is why I am trying to figure out a way to make it more safe in the event of a power failure,

I was wondering if there was such a thing as a pushbutton switch that turned itself off if power was cut to the circuit (that sounds confusing)

Essentially what i am getting at is, if you have a saw (this is not actually for a saw, but a unit i built from scratch) and it is on and then there is a power outage, the saw's switch is still on, but the saw is not powered or working. When the power comes back on the saw will start up and may cause injury. To fix this problem the aforementioned switch would ensure that when power was returned to the saw it would not turn on.

This is not the exact application but something similar.

Are there products out there for this idea, or would I have to build something myself like with relays and electromagnetics and stuff.

I talked to a store owner and he had a really bad accent
and he said something about latching relays and a momentary push button, and it sounded pretty cool but I had no idea what it was about

(some things i remember from the conversation was that there are a couple or a few relays and 1 controls the other)

that solution seemed like I would just need to wire it up but at the moment i really don't know how i would do it,

do you know what he was talking about?

Lemme know what you think,

Thanks,

Jake

 

[crutschow]

A simple way to do that is to use a momentary PB switch to power a DPDT relay (with a 120V or 220V AC coil if it's to be controlled from line voltage) . You connect one of the contacts to the relay coil power and its corresponding NO contact to the relay coil. That way when the PB energizes the relay it will stay energized through the relay closed contact. But if the power is interrupted, the relay drops out and will not energize again until the PB is again pressed. A separate momentary PB is put in series with the contact to allow turning off the power without having to remove it. The other relay contact is used to power the load.

See this schematic as an example. It shows 8-14V controlling the relay but that is determined by the relay coil rating (not contact rating) that you buy.

 

[jakerobins8]

Thanks for the response, Much appreciated.

A couple of questions.

Couldn't you use a SPDT relay instead of a DPDT relay because in the image you showed me only one of the poles is used.


Also

When you said

The other relay contact is used to power the load.

Where in the schematic would the load be wired to for it's neutral and hot line.

 

[ChrisP58]

The hot line wire connects to relay terminal 1C, the hot side of the device to be powered connects to relay terminal 1NO.

The neutral wire is not switched.

You need a DPDT relay if it's coil voltage is different than that of the powered load.

You can use a SPDT relay if it's coil voltage is the same as the powered load. In that case, the load is wired in parallel with the relay coil. K1 and K2 terminals.

 

[JimB]

Just an added bit of information.

This type of circuit is gnerally known as a No Volt Release (NVR).

All machine tools such as lathes, milling machines and drilling machines have a circuit like this to kill the machine in case of a power interruption.
This is not just a safety feature to protect the operator and machine when the power is restored, but also it prevents an enormous current surge when many induction motors all try to start together when the power comes back to the whole machine shop at once.

JimB

 

[crutschow]

.............................
You can use a SPDT relay if it's coil voltage is the same as the powered load. In that case, the load is wired in parallel with the relay coil. K1 and K2 terminals.

That's true only if the PB switch is rated to momentarily handle the startup current of the load. Otherwise you could fry the PB contacts.

 

[KeepItSimpleStupid]

I have seen a device advertised in a trade publication where the ON/OFF, tagout/lockout functions were incorporated into a single switch. Nothing external required.

 

[jakerobins8]

I have seen a device advertised in a trade publication where the ON/OFF, tagout/lockout functions were incorporated into a single switch. Nothing external required.

I feel like such a noob asking all these questions but I am actually a mechanical engineer with a very shallow history in electrical

If I was running at regular 110 V would such a switch exist?
Is it the No volt release switch that was mentioned earlier?
Would you be able to link me to one?

I am assuming that the switch box just steps down the voltage to the coil so that the relay they use wouldn't be as costly.

 

[Reloadron]

Attached is a circuit example along the lines of what has been posted. The relay used just happens to be a 3PDT. Many times circuits like this use additional relay contacts for lamps such as ON / OFF lamps. Anyway, it is a simple traditional relay latching circuit. Pretty much what has been presented.

Ron

 

[ADWSystems]

Attached is a circuit example along the lines of what has been posted. The relay used just happens to be a 3PDT. Many times circuits like this use additional relay contacts for lamps such as ON / OFF lamps. Anyway, it is a simple traditional relay latching circuit. Pretty much what has been presented.

Ron

Reloadron has it 100% right. By the way, they are also referred to as a "magnetic starter" and are commonly available. Check Grizzly.com and put serach "magnetic starter".

 

[ChrisP58]

That's true only if the PB switch is rated to momentarily handle the startup current of the load. Otherwise you could fry the PB contacts.

You are right there crutschow. I hadn't considered that aspect. Thanks for catching my oversight.

 

[jakerobins8]

Reloadron has it 100% right. By the way, they are also referred to as a "magnetic starter" and are commonly available. Check Grizzly.com and put serach "magnetic starter".

I'm assuming this one would do the trick:

https://www.grizzly.com/products/Magnetic-On-Off-Switch-110V-10-Amps/G8990

 

[Reloadron]

Looks to me like the linked switch is a magnetic latching switch. When the ON is pressed it magnetically latches on and if power is lost the switch closure drops out. I would say it would work. Rated for 120 VAC at 10 Amps. That is speculation on my part but think that is how it works.

Ron

 

[jakerobins8]

Sweet, now I just need to find one in Canada,

Thanks for the tips,

Jake

 

[ChrisP58]

They have a FAQ about shipping to Canada. **broken link removed**