I'm modernising an old bench saw, and I want to replace the switch it has with a safer one, so that the tool will not start if the power is disconnected and then reconnected. The usual way to do this with these sort of tools is a magnetic switch, basically a relay that has the coil connected to the output. The disadvantage of mechanical relays is that they can be accidentally triggered by a whack with a bit of lumber, and the cost. Industrial safety switches are too expensive for my home workshop.

So I figured out this circuit using this SSr from ebay. Power is off until the start button is pressed, which then "closes" the output side of the SSR. This energises the input side when the start button is released, so that it will latch on until the stop button is pressed. If power is interupted the SSR will switch off so that the machine won't start again until the user pushes the start switch.

schematic

simulate this circuit – Schematic created using CircuitLab

Just mentally replace the mechanical relay with an AC-AC SSR in the circuit above.

My question is: will running the output from the SSR back to the input work? It would on a mechanical relay, but I don't know if an SSR will work exactly the same. Also, will I need a varistor on this kind of SSR, or is one built in? enter image description here

EDIT So this is the new circuit I propose. It has the start and stop buttons controlling the relay from a low voltage source, which has the benefit that I don't need to run mains wires around the cast-iron body of the saw to the switches, and I can add an indicator lamp to show that the power is on.

schematic

simulate this circuit

  • You can make it work but it would most likely not qualify under some machine directive as a safety start. If I were you I would just get myself a commercial safety start. – winny Jul 24 '16 at 13:56
  • This is for my shed, not an industrial application. What would be the safety concerns? – stib Jul 24 '16 at 14:01
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    Simple answer is: NO WAY. – Marko Buršič Jul 24 '16 at 14:05
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    That SSR going on by itself, either due to short in it or mistriggering by the external circitry. A common fault mode of SSRs. Don't get me wrong, I gladly violate rules to get things done when it only concerns me, but you asked for safety + SSR and these two don't mix. – winny Jul 24 '16 at 14:41
  • @stib WARNING there are a huge amount of counterfeits (fakes) of that specific SSR. You have to be careful which one you buy. Take a look at UL's warning: ul.com/newsroom/publicnotices/… – Bradman175 Jul 25 '16 at 3:17
up vote 3 down vote accepted

That should work for you depending on the details of the internal circuitry. There is a zero-cross detection circuit built into the output so it can only turn on as the mains voltage crosses the zero line and this minimises electrical interference from the switching. Since the input is powered from AC and is most likely on the same phase the "input circuit" must rectify and store enough charge to keep the LEDs on through the zero-cross.

The problem, however, is one of safety. The SSR (solid-state relay) uses a triac as the switch.

enter image description here

Figure 1. Innards of the AA (AC/AC) version Fotek SSR with the switching triac highlighted.

The problem with any semiconductor device in a safety critical circuit is predicting failure modes. In general you can't: the triac in this case could fail open circuit (safe) or short-circuit (unsafe). There are ways around this but they are not simple in a power circuit such as yours.

A second problem is that triacs can be erroneously triggered by a sudden change in applied voltage (a mains transient or restoration of mains power while at its peak) across its terminals even without any trigger voltage. This may be listed as maximum \$ \frac {dV}{dt} \$ that the device can stand before turning on. In normal circumstances this would cause the triac to conduct for one half-cycle and this might be harmless. (You wouldn't see a lamp blink, for example.) In your case, since you have a latch the circuit might remain on and start the motor.

Using an SSR for a safety-critical device is not recommended. Relays with air-gaps are far safer. You could add it in series with a mechanical relay to solve the "whack" problem but then you really need to think about cycle-testing the device on each use. i.e. Some means of detecting that the SSR has stuck on. Indicator lamps might be sufficient.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 2. Dual safety circuit with indicator lamps.

Further reading on this site: Self Checking Circuit

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    So, basically for simplicity's sake I should just replace the SSR in my circuit with a mechanical relay, and then maybe mechanically shield it from knocks, eg. put it somewhere out of reach inside the bench saw instead of near the actual switch. – stib Jul 24 '16 at 14:11
  • @stib since you'd want to mechanically shield the SSR-based circuit, too... yes, that sounds like the best option – Marcus Müller Jul 24 '16 at 14:12
  • Yes. Do the right thing. The ability to knock one on suggests that its air-gap isn't large enough. Wire your new one up to a lamp and whack it to test! – Transistor Jul 24 '16 at 14:13
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    I read about the knock-on problem on some power tool forum, I've tried on a more modern bench saw and couldn't get it to happen, but I was thinking that something solid-state might be more reliable (shows how much I know). Given that the bench saw that I'm modernising weighs around 200kg I think putting the relay inside somewhere should be safe. If anything can get that machine to move quickly enough to activate the relay I've probably got a bigger problem. – stib Jul 24 '16 at 14:16
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    @transistor "Using an SSR for a safety-critical device is not recommended" especially when the OP is buying a counterfeit! – Bradman175 Jul 25 '16 at 3:27

Do NOT use Fotek solid-state relays from eBay or Amazon. All of them are counterfeit. See this Underwriter's Laboratories warning notice: http://www.ul.com/newsroom/publicnotices/ul-warns-of-solid-state-relay-with-counterfeit-ul-recognition-mark-release-13pn-52/ Not one picture of a Fotek relay on eBay or Amazon matches UL's picture of an approved relay. Most of the fakes fail at much lower current levels than their markings would indicate, and sometimes they fail in the ON state.

The proper answer is a motor starter with contactor, but those are expensive.

Consider something like this 30 amp "Relay in a Box" unit: http://www.functionaldevices.com/building-automation/display.php?model=RIB01P30 These come in an electrical box with UL approval, which is good around a saw where you need to protect electrical parts from sawdust. About $20.

  • Watch this very informative teardown of a Fotek SSR-25 DA (counterfeit?) at youtu.be/DxEhxjvifyY. While its triac is rated 12A (not 25A), its look acceptable if you keep current low. – user83628 Apr 21 '17 at 17:32

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