It's been a while since I have messed with my arduino but I'm back!
I'm experimenting with a SSR switching 120vac. I have it working with a battery (9v) supplying the DC side, but when I try to switch the SSR with the arduino output pin it doesn't work. The operating LED on the SSR still turns on but it won't actually switch the load on the AC side. I'm pretty sure this is the SSR: http://www.futurlec.com/Relays/SSR25A.shtml. It says max input current of 25ma. Is this less than the arduino can supply?
Products Name
Solid State Relays SSR 24-480V AC, 25A
Outline Dimension
I don't think it is because of what you say next:-
but it won't actually switch the load on the AC side.
So that is implying it isn't turning on.
What sort of load do you have it has to be greater than 0.1A at 120V that is 12watts minimum load. Any less and it won't turn on.
How is it wired, it should be pin3 to ground and pin 4 to arduino output.
Grumpy_Mike - The load is just a fan for testing. Again it works with the 9v hooked up on the DC side. I'm actually connecting pin4 to ground (as it shows -ve on the DC side of the SSR) and pin 3 to the arduino output (+ve). This DOES turn on the LED on the SSR (the other way doesn't) but again, the load on the AC side does not switch. The LED is brighter when using the 9V (obviously 9v vs 5v) but it should switch. I'm willing to bet 3volts from 2 aa's will do the same.
Crowley - Very simple setup really. Just plugged into ground and an output pin. Thanks for the troubleshooting / faultfinding tips. I'm gonna try another pin.
Richard: Tried swaping as you've suggested but no light on the SSR. What I don't get is that if the SSR active light is coming on, then the arduino is sending it power, but it just won't switch. Since the max input is 25mA is the arduino perhaps sending too much current? (50mA) should I use a resistor to limit it?
Does it switch if you connect it to Gnd and 5V on the arduino when it is powered (not through a digital pin)? SSRs usually have current limiting resistors built in hence the markings 3 -32 volts that it says. I reckon its out of spec and doesn't turn on fully with 5V whereas it will with 9v. The maximum current will be when you feed it its maximum voltage. It will take a fraction of that with 5V. A resistor will more than likely worsen the situation.
Thanks for your help guys! I figured it out! Your assumptions were correct, it SHOULD/WILL work with everything I described. I made an error on my circuit board with the wiring, not even sure what it was. (I had other parts of the circuit that were disconnected but convoluting things). Anyway after troubleshooting and testing just the pins with LEDs, I was able to track down the problem.
If a relay will mostly be on, and very seldomly be switched off is this ok? (think of using this as an emergency off) Or will it wear out the relay / arduino constantly being in the on state? Is there a better practice using TTL so that the relay instead turns OFF the load? Or is it ok to just leave that sucker on all the time?
I'm assuming these SSRs don't need diodes like mechanical relays since there is no EMF back current from a coil? No resistor neccessary either as it has one built in hence the variable voltage input rating? It just seems way too simple having it connected right to the arduino output!
Would you recommend putting a heat sync on this SSR? It's rated up to 25amps I believe, but won't be handing more than 3amps at most. It's super overrated which hopefully will equate to a long life span.
If you do recommend a heat sync, any links to an appropriate one would be very much appreciated. Cheers and thanks for everyones help so far!
As far as the Arduino is concerned its an LED. It should be OK turned on long term. It almost certainly won't need an heatsink at 3 amps. You could suck it and see and see how warm it gets. If you can comfortably touch it when its been on a while (be careful of the contacts !!) it should be OK.
You're right, you obviously need more info to provide any good suggestions.
The project has multiple loads with relays switching them by the arduino. I wanted one 'master' relay to switch off everything in the event a sensor measures a specific temperature (risk of fire etc..).
After writing it out perhaps just turning off all the relays (already controlled by the arduino) is the exact same from a safety perspective as having the extra 'master' relay in line before them.
However, the master relay could also trip upon failure of one of the other relays, circuit components, or even processor error. Like a rundandant relay system.
I chose the SSR due to it's high load capacity, low input specs and I assumed since it wasn't mechanical it was less prone to failure. The master relay would be plugged into the main receptacle and a powerbar would be plugged into that where the other loads would be connected with their own relays. Does that make sense?
I have to add Richard that seeing that pic of the EMO switch you posted reminded me of how much I love those types of emergency switches/buttons. I'm particularily fond of when they have protective covers and keys etc... Like missle firing buttons! lol. ;D
I think I might need to implement an EMO device in this circuit! Would it essentially just be an high quality inline switch over-rated to handle the load on the wire?
My apologies, I'm not sure what else you need to know about the system. What types of loads? It is an art project and there will be 3 - 4 lights moving on little cars, potentially a heater, 2 servos and 2 electromagnets. These are the loads plugged into the main power bar.
The mechanical thermo cutoff switch looks like an amazing device!
Can I replace the SSR in my plans with the MTCS (it won't just be current passing through for the heater however, it will be the current passing for all loads plugged into the power bar). I really want to make this project as safe as possible. The idea of it cutting the power in the event of temp rise is great, esspecially with the device being isolated from everything else! Your wisdom is much appreicate Richard. Jeez you may have saved my life. No exaggeration intended.
Thanks for the detailed reply Richard.
There really isn't any special inherent danger for this project. I do appreciate you breaking down your approach. It is most helpful. The only risk I calculated was that if something shorts or if it overheats for some reason (I guess the same safety issues that surround pretty much any AC circuit). Hence the power interrupt / cutoff idea.
I don't plan on using the SSR as an EMO anymore. It guess it was never an EMO to begin with just a redundant relay I suppose (is the practice of redundant relays not used at all - is it at all a best practice or is it pointless?). From the picture you attached before it looks like traditionally an EMO is just a switch, not something controlled by DC. Is that right?
Also can you address this question?:
"Can I replace the SSR in my plans with the MTCS (it won't just be current passing through for the heater however, it will be the current passing for all loads plugged into the power bar)."
Thanks for clarifying Richard. I understand the MTCS is not to be used as a relay or a thermostat. So you wouldn't recommend using the themal cuttoff in place of the SSR to switch all loads off in the event of overheating? I was thinking that's a great extra level of protection. In fact, I'd like to use both and still use the SSR for switching (not EMO).
Are you recommending an inline fuse and circuit breaker for this AC circuit as well? I'm using a GFCI for the main receptacle/switch (the one that will have the SSR and MTCS taped in), this should protect against ground fault. Will that cover the over/under current risk?
Let me try to summerize:
The problem:
Switch 120VAC the safest possible way. (check for temp)
Solution:
GFCI, with an SSR (not for EMO purposes), and a MTSC (for quasi EMO/safety purposes).
Sorry for the newbness...Thanks for sticking it out Richard.
Please don't get frustrated. I'm sorry I haven't provided you with the the level of detail you require. It's not secret at all, it's an art project. I just didn't want to bore you with too many details. I did list the loads on each relay for you in the post. Didn't you see it?
"... there will be 3 - 4 lights moving on little cars, potentially a heater, 2 servos and 2 electromagnets. These are the loads plugged into the main power bar."
This is a general project so when I say lights I mean like a bulb of a generic commercial fixture. The servos are not crazy high amp, just a regular generic microcontroller servos. These are not high drain just the lights, heater and potentially elecromagnets (again simple electromagnets here, likely consuming well under an amp). Heater isn't particularily powerful, just a little space heater. It will be pointed at nothing. Heating nothing but the space. Again, nothing esspecially dangerous about the setup or I would mention it, it's just the actual switching of the loads I want to make sure is safe. Does that help?
Again, looking to clarify that this is in fact a good direction for safely switching multiple AC loads:
"GFCI, with an SSR (not for EMO purposes), and a MTSC (for quasi EMO/safety purposes)."
Sorry again for the confusion. I'm not the best at this. I'm really trying to give you as much detail as I thought you'd need. I don't have a circuit diagram drawn up or anything. Thanks for your patience.
Also, just to reply to the powerstrip idea. The problem with the powerstrip is:
a) it is not controllable by the arduino
b) while it is grounded and potentially surge protected I don't believe it provides the level of safety of a GFCI (remember I want to switch the AC so even if I taped the surge protector it wouldn't have ground fault protection)
c) it cannot switch off in the event of overheating (at least not deliberately)
Quick questions: Even if the small heater has a MTSC in it can I still put an additional one on the main power for the system coupled with the GFCI as an extra level of safety?
Is it overkill to use this setup for other AC switching projects (I mean the MTSC) that are potentially not high load just simple AC like a small desk lamp?
Richard thanks so much for your detailed help. I'll be looking into this more over the weekend.
Right. Hence the super high temperature rating of many of them. I'm thinking perhaps it isn't appropriate to have one of these if it is meant to be mounted on the actual device or case. I'm only considering this level of safety, well to be safe, and also because as an art piece, it won't always be supervised and thus needs to be a safe system with lots of failsafes. My issue is my know how only goes so far. I'm feeling more confident after this discussion however.
The MTSCs are fairly cheap and certainly shouldn't add any risk. Other than the added risk of one more splice into the live wire. Perhaps if it is mounted to the power bar. Or what do you think about mounting one to the project box holding the arduino and components? Just as a secondary safety protection device, as you've outlined.
I must admit I'm beginning to think this whole scenario is a bit overkill. In any case, I have found it an enlightening investigation and incredibly productive to my understanding of these devices and safe switching in genereal. It's neat how the thermal cutoffs are used everywhere and really integral not only to safety but even functioning of so many appliances.
Thanks for your detailed responses Richard. You clearly are drawing from a lot of experience.
Thanks Richard,
So it's safe to say that you've never heard of anyone using an MTSC for thermal safety protection of the actual circuitry and/or power source? Only for devices like curling irons and heaters that create heat?
-cyboman
