SSR for emergency stop switch circuit (cnc router) - Page 1
Forum section: electronics / beginners

author: RedTeamGo
author_rank: Newbie
timestamp: July 07, 2012, 02:44:01 am
content: This is my first mains voltage project (outside of basic household wiring). I am assembling the electronics for aCNC milland I want to include an emergency-stop button for the business end of the mill. Since this is in the name of safety, I am interested in avoiding stupid mistakes that introduce more problems.My main electronics enclosure (which I will post about some other time) includes an off-the-shelf 5V switching power supply. The plan is to wire 5V through a red-mushroom-cap e-stop switch (NC) to the input of an SSR. The SSR will sit in a suitable aluminum box, connected to a 120v socket. My CNC "spindle" (common power-tool router) would plug into that outlet.I will add a 10A fuse in line with the mains supply line. Here are my questions:Would an SSR likethis one (Crydom D4825)be suitable?Do I want a zero-crossing version? Or do I specifically NOT want a zero-crossing version?I have heard that relays need to be derated for inductive loads. However, the datasheet for that SSR doesn't mention that. In fact, it mentions inductive loads specifically. So, would 25A be enough for my purposes (common 15A household circuit)?Do commercial power tools already include circuitry to suppress flyback voltage and other problems associated with switching off inductive loads?The SSR I linked above appears to include a snubber circuit and current-limited input. Did I understand that incorrectly? Can I just wire my 5V to input, and my mains to the load and be on my way?

author: codeboy2k
author_rank: Super Contributor
timestamp: July 07, 2012, 05:24:05 am
content: Quote from: RedTeamGo on July 07, 2012, 02:44:01 amseems okfor inductive loads you probably don't want a zero crossing versiona 25A SSR is more than enough for a 15A home circuitI am not an expert on commercial power tools, or most power tools for that matter. But I doubt they contain anything to suppress anything. An RC snubber is pretty simple. A 1k resistor in series with a 0.1uF X1 or X2 class capacitor. Put it across the line and neutral, at the load. You can also put a 400V bidirectional TVS diode at the load if you think the RC might not work (some people say RC snubbers don't work when the load is more than 2-3 Amps)yep. wire it up. SSR's are designed to be easy to use. Although there's a snubber on the SSR, snubbers close to the load work best. So you might decide to protect your $50 SSR with the $2 in parts listed in #4 aboveFinally, why choose an SSR? they are expensive. You can simply use a mechanical relay. Are you are concerned about contact arcing and wear?Emergency off buttons are best left as simple devices; even simpler still is a 15A - 20A physical emergency off button, no relay, no SSR.

author: T4P
author_rank: Super Contributor
timestamp: July 07, 2012, 05:30:06 am
content: And how big should be the 1k resistor be ?

author: pickle9000
author_rank: Super Contributor
timestamp: July 07, 2012, 05:30:49 am
content: A real estop switch is inexpensive and reliable. You should be able to find one at a local woodworking or machine shop supply mounted in a box and ready to roll....mike

author: SeanB
author_rank: Super Contributor
timestamp: July 07, 2012, 05:44:36 am
content: Most Estop units have a pair of switched in the bottom, rated at 16A or more. One is normally closed, the other is normally open. Wire the power line incoming through the normally closed contact, so that pressing the button removes power to the system. The normally open switch can be wired across the spindle motor terminals to stop it fast, as normally the motor is the highest inertia load.Place a software stop button, as this big emergency stop has the ability to corrupt the firmware, as it will stop any operation in progress, even a eeprom write.The stop button does not need a snubber, but you can place a few 0.22uF 275VAC class X2 capacitors across the input to the CNC, or use a premade 15A mains filter unit. A few big MOV units rated for the local mains supply will help with the noise reflected back to the mains from the CNC as well.

author: Psi
author_rank: Super Contributor
timestamp: July 07, 2012, 07:02:53 am
content: Just be aware that e-stop systems should be designed so a hardware failure causes the device to enter the stopped state. (as opposed to a running state with no working e-stop button)Do solid state relays fail in open or closed state? i cant say i knowAt least with a mechanical relay you can be pretty sure it's going to fail to its non energized state.

author: Bloch
author_rank: Unknown
timestamp: July 07, 2012, 07:49:01 am
content: Quote from: RedTeamGo on July 07, 2012, 02:44:01 amQuoteSTOPThat you are making is not a "emergency-stop button".First you have to find out that kind ofSafety categorythe complete machine is required.Here is a link to you about the EN ISO 13849-1 standardhttp://www05.abb.com/global/scot/scot209.nsf/veritydisplay/f282e8fb773fa733c1257996004307a6/$file/en_iso_13849-1_2tlc172003b02002.pdfI am not sure of you local law. But here it is very expensive to not comply with machinery safety rules.If you say it will be to expensive to comply with the EN ISO 13849-1.Then dont use a red stop button (Because the implies that it is a safety stop button.)Just use a normal on/off breaker.

author: GeoffS
author_rank: Unknown
timestamp: July 07, 2012, 08:05:34 am
content: For my CNC router, I just have a large mushroom type mains switch.Normally closed contacts open when the button is pushed, you twist the top of the switch to release it.It's probably a good idea to have an NVR (no volt release) switch included as well.I bought a few ofthesewhich combine both functions in a single unit

author: codeboy2k
author_rank: Super Contributor
timestamp: July 07, 2012, 08:07:31 am
content: I agree with the posts here, that the safety system should not be electronic. A physical switch is much better.With that said, I'll answer DaveXRT's question about the snubber resistor:Quote from: DaveXRT on July 07, 2012, 05:30:06 amFor the snubber, it really doesn't need to be that big at all. A safe size would be 2W or 5W. The transient will be so short lived, that there will be no time for heating. The instantaneous power is immense, but it dissipates in nano-seconds. Lets do some math..Depending on the actual inductance and resistance of the load, then the instantaneous power could be up into the terawatts, but the average power will be just a few kW, and that's only for 10ns or so. 2kW for 10ns will cause Q = 2000W*10ns = 20e-6 Joules of heat. The heat rise depends on the mass (m) and specific heat capacity (c) of the material that makes up the resistor. Some really good high-energy resistors have specific heat of 0.23 cals/gm K. The specific heat capacity of resistors varies from manufacturer to manufacturer, but let's assume two orders of magnitude worse than that really good one, and say we have a specific heat of .0024 cals/g K. That's 10 J/kg K. assume a 5W ceramic resistor has mass 4g (.004kg); hmm that's probably realistic, but lets make it lighter. Say 1g or .001kgSo the temperature rise is given by ?T = Q / mc?T = Q / mc = 20e-6 J / (.001 kg * 10 J/kg K) = 0.002 °K temperature rise = 0.002 °C.That's so small, it would not change in temperature at all. You could probably use a 1W resistor, or stay safe at 2W.

author: Bloch
author_rank: Unknown
timestamp: July 07, 2012, 08:22:51 am
content: Quote from: GeoffS on July 07, 2012, 08:05:34 amI just want to point out that thing have nothing to do with safety!

author: GeoffS
author_rank: Unknown
timestamp: July 07, 2012, 08:35:22 am
content: Quote from: Bloch on July 07, 2012, 08:22:51 amYes it does. It provides a way for the operator to remove power quickly (safety)It also prevents the machine from starting without operator intervention after a power failure (safety)

author: Bloch
author_rank: Unknown
timestamp: July 07, 2012, 08:47:23 am
content: Quote from: GeoffS on July 07, 2012, 08:35:22 amIf it not failed in the mean time.Or much worse the normal user off the mashin do know it broken ( allways on) so he turn the mashin off at the mains plug. Some day an other person turn on main plug. Call 911 ......See picture. If there are an fail at contactor 1 or 2 or even a wirer failure at the emergency stop button. It will not start.

author: G7PSK
author_rank: Super Contributor
timestamp: July 07, 2012, 09:09:35 am
content: A no volt contactor is an absolute must on any machine tool and preferably with more than one emergency stop buttons fitted. At least one on the machine where the operator can get at it easily and another say near the door so that some one coming to the assistance of the operator can quickly turn of the power without getting near the machine.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 07, 2012, 10:03:06 am
content: Power semiconductors tend to fail short (unless there is enough energy to physically disintegrate them), so using them in a safety stop is a bad idea.No reason to reinvent the wheel here...

author: RedTeamGo
author_rank: Newbie
timestamp: July 07, 2012, 03:50:51 pm
content: Quote from: codeboy2kI was going to house the relay in a box separate to my main electronics enclosure (where the e-stop would be). I could plug the outlet box into a different mains circuit (if I wanted), by just plugging it into a separate wall outlet. I needed a relay to be able to control this separate circuit.The idea to use an SSR came about because (from what I understand), they offer opto-isolated inputs and come in a package that wouldn't require additional components. Also, I have not yet seen many e-stop buttons rated for more than 10A.However, based on all these comments, I agree that a simpler solution would be better and I plan to do a better search for emergency off buttons.Quote from: BlochYou make a good point about taking time to do a risk assessment. This is a home-built CNC machine, of pretty modest size, that will be located in a residential garage/workshop. I am okay with there being a more operator risk than there might be in a commercial environment.Quote from: GeoffSI agree.Quote from: GeoffSQuote from: BlochThe following links were helpful to me in explaining your diagram:http://www.pilz.com/au/company/press/article/sub/technical_editorials/articles/02367/index.jsphttp://safety-relay.com/monitoring_relay.htmlHowever, I can't say that I really understand how to hook something like that up at this point.

author: SeanB
author_rank: Super Contributor
timestamp: July 07, 2012, 04:07:45 pm
content: TRhe emergency stop is just a switch contact block that fits an industrial panel cutout, with standard industrial switches behind it. Go to a big electrical wholesaler (look in the phone book) and look at the range from Cutler Hammer, Merlin Gerlin and such. These are upgradable, to allow you to have a 32A contact to carry the mains input and break it when the switch is pressed, though the common recommendation for all these is that it must not be the main power switch off mechanism, but the last resort mechanism.