[The Wizard]

Hi Folks,

I am trying to solve a basic electrical problem but unfortunately my disability (ME/CFS) is acting up tonight and I am feeling decidedly 'brain fogged' and confused . Can someone please check my 'thinking' to make sure I am heading in the right direction? The problem I face is that I promised to make a number of 'trophies' for the annual charity kite festival for the kite club I belong to. The festival is the weekend after next and I need to get my system up and running ASAP if I am going to get them made. Unfortunately my original motors/board were not fast enough for my needs and I have upgraded my system to a Hobby CNC board, larger motors and a bigger power supply. I have the board built and I am awaiting delivery of the transformer so that I can build a larger power supply. In the meantime I am trying to get everything else sorted and the wiring figured out so that when the transformer arrives I can simply solder everything together.

Now, my original idea was to have an e-stop button controlling the coils of two relays, the switch would be wired NC so that it 'pulls' the contacts 'in' when in the 'up' position and when the e-stop is pressed the relays would drop out thereby killing the power. One relay would switch the 110VAC line going to the router motor and the other would switch the DC voltage going to the board and therefore the stepper motors. With the increase in DC supply voltage I am now confused as to the best method of reducing the voltage so that I can 'feed' the coils of the relays.

The information I found on the net for the relays in question (Potter & Brumfield type KRP11DG) says the coils are rated at 24VDC, with a resistance of 472 Ohms and a nominal coil current of 51 mA. My transformer is expected to put out 25VAC at 10A so by the time it has been through the bridge rectifier and smoothing cap I will be looking at 35.35VDC available to the board. In order to obtain 24VDC to feed to the relay coils can I simply insert a resistor into the feed line to the coils (if so what size/wattage) or do I need to build a voltage divider using two resistors with a 'tap' point between the two (and if so what size/wattage)?

I am rather embarrassed that I can't figure this out myself as I am a qualified ham radio operator and used to be able to do this stuff with no problems before being struck down with this 'damn disease'!

As a side note, my new board has the ability to take an e-stop signal and feed it back to my controller (Turbo CNC). My previous board did not which is why I was killing the DC voltage to the board to stop the stepper motors in an emergency. I was hoping that TCNC would see the loss of signal on the limit switch line as a 'hit limit' and stop the program otherwise the program would continue to run even though everything else had stopped. Does anyone have any ideas on how I can supply a signal to the board with the set up described above bearing in mind the e-stop circuit is likely to be on 24VDC and I am guessing the board/TCNC is looking for a change in state on a 5VDC signal.

If I can solve these problems before my transformer arrives on Friday I might be able to meet the deadline although we do have a contingency plan to produce 'certificates' if I can't get the trophies made in time.

Any help greatfully appreciated.

Larry

[ESjaavik]

Try 200ohm 1W.
But you may check the relay specs as they do not require 24.00V but rather 24V +- something. I would not be surprised if 35V is OK. Or 17.5V as you get if wiring them in series. 17.5V will probably justmake them engage a fraction of a second slower, but in your case it is the release time that should be fast anyway. And that will not be slowed down.
For your feedback to the PC use a photocoupler. Then your PC will be protected from current loops, voltage spikes and other nasty things in your power electronics. Use 1800 ohm to limit the current from 35V into the diode of your optocoupler.

[CNCgr]

One other point: using a switch for e-stop is not a good idea, it's better to use a Start and a Stop button.

What I did was use a relay with a 220V coil and four contacts (only used three). The Start button is N.O. and the Stop N.C. and they are wired in series to the coil. One set of contacts is wired parallel to the Start button. So, if I press the Start button current flows through it, the stop button and the coil so the relay is armed. When I release the button the relay stays armed because the parallel contact acts as if I was still holding the button. If I press the Stop button I will open the circuit and the relay will reset.

The same will happen in a power failure, and this is the advantage against using a switch: say you're using a switch and there's a black out. You wait for the power to come back up but get bored and decide to call it a day. You try to remove the workpiece or tool and suddently the power comes back up. Ooops!!! If you have the setup I mention nothing will happen unless you press Start again.

The second condact is used to notify the PC that the e-stop was pressed. It doesn't make any sense now since the PC wouldn't be open in a power failure but 1) You might be using a UPS 2) The power failure might be in the machine (e.g. a wire was disconnected)

The third is of course the power output which is connected to another relay controlled by the PC (with the M3 command)

[lerman]

For an ESTOP switch I recommend a BIG RED BUTTON with normally closed contacts.

There are special switches made for this. Some are pull to reset; others are turn to reset.

www.mcmaster.com has five pages of these things.

Allied Electronics also carries them.

I bought one in an oil tight box for around $45.00. If you buy just the switch, you can save some money.

Ken

[Al_The_Man]

I agree with Einar that the relays can in all probability take the higher voltage, this would cause a 76ma current which they should be able to handle, also the 'latched' on/off/e-stop circuit is a good idea. Is there an enable input on the amplifiers? as this is sometimes a really good way to disable the motors immediately, if you drop the AC power out to the amps only, then the residual charge in the DC supply caps can in some cases cause the motors to keep on moving until the charge has decayed.
Al.

[The Wizard]

Try 200ohm 1W.
But you may check the relay specs as they do not require 24.00V but rather 24V +- something. I would not be surprised if 35V is OK. Or 17.5V as you get if wiring them in series. 17.5V will probably justmake them engage a fraction of a second slower, but in your case it is the release time that should be fast anyway. And that will not be slowed down.

I have checked the data sheet (it can be seen here... http://relays.tycoelectronics.com/da...ts/KRPA_DS.pdf ) and it only shows the minimum required coil voltage and not the maximum it is safe to use.

For your feedback to the PC use a photocoupler. Then your PC will be protected from current loops, voltage spikes and other nasty things in your power electronics. Use 1800 ohm to limit the current from 35V into the diode of your optocoupler.

I will look into this option, thanks for your help.

Larry

[The Wizard]

One other point: using a switch for e-stop is not a good idea, it's better to use a Start and a Stop button.

I will be using a commercial, latching, E-stop button.

What I did was use a relay with a 220V coil and four contacts (only used three). The Start button is N.O. and the Stop N.C. and they are wired in series to the coil. One set of contacts is wired parallel to the Start button. So, if I press the Start button current flows through it, the stop button and the coil so the relay is armed. When I release the button the relay stays armed because the parallel contact acts as if I was still holding the button. If I press the Stop button I will open the circuit and the relay will reset.

The same will happen in a power failure, and this is the advantage against using a switch: say you're using a switch and there's a black out. You wait for the power to come back up but get bored and decide to call it a day. You try to remove the workpiece or tool and suddently the power comes back up. Ooops!!! If you have the setup I mention nothing will happen unless you press Start again.

I am very aware of the problems of having control systems controlling motion on machinery. I would never get my hands anywhere near the cutter/machine while it was powered up. I also live at the top end of 'Tornado Alley' here in Canada and this county is the Canadian capital for severe thunderstorms during the summer and outages are fairly common. I always disconnect any electrical equipment during an outage as I had a problem with 'surge' when the power came back on once and it fried my computer power supply and a video card.

The second condact is used to notify the PC that the e-stop was pressed. It doesn't make any sense now since the PC wouldn't be open in a power failure but 1) You might be using a UPS 2) The power failure might be in the machine (e.g. a wire was disconnected)

The third is of course the power output which is connected to another relay controlled by the PC (with the M3 command)

Thanks for the info.

Larry

[The Wizard]

For an ESTOP switch I recommend a BIG RED BUTTON with normally closed contacts.

There are special switches made for this. Some are pull to reset; others are turn to reset.

www.mcmaster.com has five pages of these things.

Allied Electronics also carries them.

I bought one in an oil tight box for around $45.00. If you buy just the switch, you can save some money.

Ken

Thanks Ken, I do have a commercial E-stop button and box for it. I searched long and hard for an affordable solution and got a tip on here that a Canadian auto supply outlet had them for about $8.50 CDN ($6.80 USD) with boxes for $5.50 CDN ($4.40 USD). This has a large, latching, red button with turn to release. The one I chose has both NO and NC contacts although they do offer a basic NC one too. I will be wiring the NC contacts to the relays and the NO contacts are wired to a flashing LED in the box as a reminder that the button is latched.

Larry

[The Wizard]

I agree with Einar that the relays can in all probability take the higher voltage, this would cause a 76ma current which they should be able to handle, also the 'latched' on/off/e-stop circuit is a good idea.

Thanks Al, the datasheet can be found here..... http://relays.tycoelectronics.com/da...ts/KRPA_DS.pdf ...... but it doesn't give a max. coil voltage, just a min. value. I will be using a latching commercial e-stop button (see the other replies). I quess all I can do is try it....lol. The relays were given to me so I won't be losing a lot of money if I fry them .

Is there an enable input on the amplifiers?

I don't believe there is.

as this is sometimes a really good way to disable the motors immediately, if you drop the AC power out to the amps only, then the residual charge in the DC supply caps can in some cases cause the motors to keep on moving until the charge has decayed.
Al.

I would be using one of the relays to kill the DC power to the board after the bridge rectifier and the filter cap but before the control board so the residual charge in the cap shouldn't be a problem. I still need the DC power to be on for the flashing LED I have fitted in the E-stop button box which will be lit when the button is down as a reminder.

Larry

[ViperTX]

Current limit the 35 VDC to the relay coil to the power rating of the coil and you should be okay. You want to limit the current to under 50 ma....probably somewhere around 40 ma. If you don't want to experiment then add a voltage divider for each coil.

[Al_The_Man]

I would be using one of the relays to kill the DC power to the board after the bridge rectifier and the filter cap but before the control board so the residual charge in the cap shouldn't be a problem. I still need the DC power to be on for the flashing LED I have fitted in the E-stop button box which will be lit when the button is down as a reminder.
Larry

One problem with switching high current DC with these relays, if you break at maximum current you may see something across the contacts resembling a Plasma Cutter arc Believe me I have done it, relays that switch high current DC are fitted with Arc-Blow-out magnets for this reason.
Al

[The Wizard]

Current limit the 35 VDC to the relay coil to the power rating of the coil and you should be okay. You want to limit the current to under 50 ma....probably somewhere around 40 ma. If you don't want to experiment then add a voltage divider for each coil.

Can I do that with a resistor or would I need to get a current limiting diode?

[The Wizard]

One problem with switching high current DC with these relays, if you break at maximum current you may see something across the contacts resembling a Plasma Cutter arc

Ooo....errr........ :frown:

Do think that is likely in my case with three motors rated at 3A?

Believe me I have done it, relays that switch high current DC are fitted with Arc-Blow-out magnets for this reason.
Al

Point taken but if I can use these relays it would help me out as funds are extremely limited here and time is against me at the moment.

Larry

[Al_The_Man]

If you had all three drawing 3amps you might get it to light up otherwise with probabally
switching somewhat less than that you would be Ok. BTW Those relays usually give the DC contact current rating for a resistive load.
Al.

[ViperTX]

Can I do that with a resistor or would I need to get a current limiting diode?

Use a 1k or 1.2K ohm resistor for current limiting just put it in series with the 35 VDC that you'll feed to the relay coil....if you find that the relay is not picking at that current reduce the value of the resistor slightly.

[Al_The_Man]

I make it around 240 ohm if you want to drop the 12v and maintain the 51ma,
.6w (1watt) resistor.
Although relays are pretty durable and imprecise components, so I would have thought they would handle the extra bit of dissipation.
Al.

[The Wizard]

If you had all three drawing 3amps you might get it to light up otherwise with probabally
switching somewhat less than that you would be Ok. BTW Those relays usually give the DC contact current rating for a resistive load.
Al.

Hi Al, according to the spec sheet I found online the DC contact current rating for these relays is 10A @ 24VDC. It is possible that all three motors could be spinning at the same time as the e-stop is activated but they may not all be drawing maximum current at that time.

Larry

[The Wizard]

I make it around 240 ohm if you want to drop the 12v and maintain the 51ma,
.6w (1watt) resistor.
Although relays are pretty durable and imprecise components, so I would have thought they would handle the extra bit of dissipation.
Al.

I ran the figures through an online calculator designed for calculating the current limiting resistor for LEDs (I just pretended the relay coil was a LED ) and it came up with 220 ohms at 0.55W so we are pretty close :cheers: . I just checked on the preferred values tables too and it would appear that both 220 Ohm and 270 Ohm are available as 'standard' sizes.

Larry

[Al_The_Man]

Yes, like I mentioned earlier you probabally can get away with it, but I think you will find that the 10A at 24vdc is a resistive load which is quite different from an inductive load which is what you will have. The Inductive rating is often a fraction of the resistive rating.
Al

[The Wizard]

Yes, like I mentioned earlier you probabally can get away with it, but I think you will find that the 10A at 24vdc is a resistive load which is quite different from an inductive load which is what you will have. The Inductive rating is often a fraction of the resistive rating.
Al

Thanks Al. I think at least half of my problems have been solved with a posting on another forum. It looks like I won't need a relay on the DC side of things after all now as my Hobby CNC board has the capability of accepting an e-stop signal and my Turbo CNC software will recognise one too. Apparently I can just run a line from the board up to the switch and back to the common ground and TCNC will then sense the e-stop button and stop the steppers and the program. That just leaves the relay to control the AC line to the router head which I guess I will feed at 24 VDC with a resistor in the feed line to bring the voltage down from the 35.35 VDC from the supply.

Thanks for all your help (and everyone else's too) :cheers:

Larry