Need to make a dead mans switch

[bigal_scorpio]

Hi to all,

I need to make a safety switch for my newly made 12" disc sander.

Most workshop machines now come with a switch that kills the supply if there is no power to the machine, like after a power outage. As I have just built the sander I thought I would incorporate this feature but I just discovered that ALL my relays are at most 24v dc coils.

My relays are fine for carrying the voltage and current needed on their contacts its just the problem of tham needing low dc voltage for the coils.

Is there an accepted way of using a low coil voltage relay to switch mains AC in this type of setup?

I am looking for a simple design if possible, hopefully not requiring a separate transformer but I have never made this type of switch before so I don't know all the implications.

I have various push button switches that are suitable for either AC or DC at mains voltage, its just the coil voltage problem I need to address. Any suggestions guys?

Thanks Al

 

[Reloadron]

Sure if you have a N/O and N/C push button switch and a 24 volt source for the relay coils. Just a matter of making a relay latching circuit. I can give you a basic drawing if you would like. You will need a power supply for the 24 volt relay coils and at least a DPDT relay in the lot.

Ron

 

[Nigel Goodwin]

However, this isn't a "dead mans switch".

 

[crutschow]

Do you mean "dead mains switch?

 

[Reloadron]

Based on the description I doubt a dead man's switch is what the OP actually wants. However, I did assume (maybe wrongly) they want a simple latching relay circuit.

Ron

 

[gary350]

THIS IS EXACTLY WHAT YOU NEED. It is all wired with relay, over loads and push buttons all you do is connect your machine to it. Cheap and simple. Here is a link to one on ebay. $20 used on ebay about $180 if you buy it new. **broken link removed**

We do this in industry all the time. You need a 120 VAC relay, 1 push button momentary on N.O. switch and 1 push button momentary off N.C. switch. You wire the RED push button N.C. switch in series with the N.O. relay contacts and in series with the 120 VAC relay coil connect it to 120 VAC. Next you wire the GREEN N.O. switch in series with the 120 VAC relay coil then to 120 VAC. Both momentary switches are spring loaded in the off position. You push the green button and release it, the relay comes on and stays on. You push the red button the relay goes off. Now you wire your machine across the other set of N.O. relay contacts. Push the green button the machine comes on and stays on. Push the red button the machine goes off.

Industry uses a motor starter with over loads. The over loads are circuit brakers if the motor pulls too many amps it trips the dead man switch and the motor turns off. Over loads are held in with 2 screws you can swap them for any amp rating you want depending on the size motor starter you are using.

If your equipment runs on 240 volts use 1 of the hot wires and ground to get 120 VAC to the coil. If your equipment runs of 480 VAC swap the 120 VAC coil to a 240 VAC coil. If your equipment has a control circuit transformer 240/480 step down to 120 VAC use the control circuit voltage for the 120 VAC relay.

Here is a USED motor starter on ebay. **broken link removed**

Each company makes their own type of over loads so you need to buy over loads for the size and type motor starter you have. Each company makes different size motor starters too, I am doing this from memory I think a size 0 is rated up to 15 amps 480 VAC with a 120 VAC coil.

I have forgotton the rule of thumb I believe it is 1 amp per horse power for a 480 volt AC motor. Amps will be double for a 240 VAC motor. Rule of thumb is NOT accurate you need to look at the AMP chart on the inside cover of the motor starter metal box. If you have no box or the chart is damaged go to your local industrial electrical supply they can look it up for you. A 480 VAC motor 40 HP will be about 35 to 42 amps depending on the RPMs, motor frame size and the manufacture full load running current. Idle current will be about 8 to 12 amps.

We scrapped some old equipment at work, removed all the parts we wanted the rest went to the scrap yard. I have probably 7 or 8 old motor startes, these are all electrical panel mount type, no box and no push buttons. I will give you one free if I have one you can use and if you pay $12 flat rate postage to get it. You will have to build a box, buy 2 push button switches, and the correct over loads about $75 new, much cheaper on ebay.

 

[Reloadron]

It would look like the attached. Since the OP is UK I would assume 220 VAC mains. Sorry about the PB switches both looking N/O as I didn't have a good looking N/C switch.

The attached assumes that RY1 has multiple contacts.

Ron

 

[Nigel Goodwin]

Since the OP is UK I would assume 220 VAC mains.

It's 240V over here

220V is mainland Europe.

But both are nominally 230V now, so equipment is made to work correctly with both.

 

[alec_t]

Is there an accepted way of using a low coil voltage relay to switch mains AC in this type of setup?

If you are asking is there an accepted way of driving a low coil-voltage relay and you can find a suitably rated filament bulb you could simply connect it (or perhaps 2 bulbs) in series with a diode bridge driving the relay coil. This assumes bulb current when hot = relay pull-in current. For example, a 15W bulb has a hot current of ~ 65mA. You might need a capacitor across the coil to prevent relay drop-out at mains zero-crossings. The bonus is you get a work-light as well!

 

[canadaelk]

Should it not be a switch that, when let go-off, it turns off (spring-loaded) by itself? If the OP is comfortable with 230VAC, why not a poor-mans power supply: capacitor, rectifier, DC relay? E

 

[alec_t]

Here you go :-

 

[ericgibbs]

hi alec,
I guess the Common [Gnd] symbol is only there to keep LTS happy.???
E.

 

[Mikebits]

I had a dead man switch on my dirtbike, it was just a cord that killed power in case I crashed. It was a simple cord attached to power that would disable if I fell off the bike. No circuits needed.

 

[Nigel Goodwin]

Should it not be a switch that, when let go-off, it turns off (spring-loaded) by itself?

That would be a 'dead mans switch' but as I pointed out earlier, that's not what he's wanting.

 

[alec_t]

I guess the Common [Gnd] symbol is only there to keep LTS happy.???

Dead right!

 

[ericgibbs]

Dead right!

You missed my point, as sure as eggs someone will copy that circuit and it will appear somewhere else,with that Common pin in place, I thought you may want to delete it from the drawing.

 

[alec_t]

I thought you may want to delete it from the drawing.

GND symbol deleted.

 

[Nigel Goodwin]

GND symbol deleted.

That's much better

 

[bigal_scorpio]

Hi Guys,

Sorry for the late reply, been a bit poorly the last few days.

Alec, I probably could use your design and the light may well be helpful at times!

What I would need though is an off switch built in too, as I will be using the circuit to both turn on and off the sander. I reckon a N/C switch in the relays coil line would do the trick nicely though and I have some!

I may have caused some confusion about the original thread name as I didn't know the correct terminology for the setup. Some of my other workshop machines, table saw and router table have this setup and they just have a green on button and a red stop one, but as I said earlier it also serves to make sure the machine does not start when plugged in UNTIL the start button is pressed.

These switches are really handy as I once while at work saw a mate plug in an industrial side wheel grinder which is a bit like a stihl saw, and it was already turned on! WOW did it go! Robot wars had nothing on how that bugger moved, sparks and mayhem followed until it eventually cut its own cable, luckily before anyone got in its way.

Thats one good reason for the switch I'm trying to make.

I also realise that it would be a lot simpler with a mains voltage coil AC relay, but I have loads of DC relays and money is tight, hence making the sander in the first place, which in total has cost me about £20 instead of the £160 at the cheapest place I found (Machine Mart).

Anyway thanks for all the help.

Just one final question. What would be the maximum current available to the relay coil with the components shown? I have a pretty large 24v DC DPCO relay in mind. Its a Schrack RMB05024 rated 25A at 250v AC and the coil looks pretty meaty.

Thanks again Al

 

[ericgibbs]

hi Al,
From what you describe, you need a mains powered relay that applies power to the tool via its contacts.
The Power On switch energises the mains relay and a contact on the relay applies latching power to the coil.

The OFF switch breaks the relays self latching contact path.

If you need a sketch, let me know.

OT: Sorry to hear you are not too well at the moment.
Eric