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Forum Index : Microcontroller and PC projects : Interfacing Solid State Relay
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| Phil23 Guru  Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
Hi All, Does anyone know what's inside a Solid State Relay regards it's input interface? Wondering whether I can directly connect to this, or if I need additional circuitry. Thanks Phil 
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6269 |
It's probably similar to the Jaycar offering. http://www.jaycar.com.au/solid-state-relay-3-32vdc-input-240vac-40a-switching/p/SY4084 Note: 'probably' means that caution should be taken if you are unable to find the datasheets for the device you are looking at. Jim VK7JH MMedit |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
Thanks Jim, Haven't managed to turn up a Data Sheet yet. $7.50 off ebay Vs $47.95 for the above. Typical load will be about 3 amps, Max about 5. Don't know about compressor lockup load. Phil. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
Be careful buying cheap ssr's off ebay, I've been stung with a fake in the past. Looking at your picture, I would suspect for that price its a fake. It will work for an amp or two, but quickly fail after that. You might get lucky. Expect to pay about half Jaycars price, around $20. Any cheaper and your taking a gamble. What are you going to use this for? To give us an idea if you need any input circuitry. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
I was planning on using it to control the compressor in the heat pump. Driven by an MX170. I have 2 of them & there are 2 MicroMites involved. One is installed in the heat pump to monitor it & provide protection. It's main purpose is a safety cut out type device. At this stage it's only output is back to a console over Wifi. Tho other MicroMite is my main controller, that controls the entire Spa heating process. It Monitors all the temps & controls the "Stuff". It has the board with the six relays shown in this thread. Just thought Solid State might be preferable to the cheap mechanical ones. And a bit of the necessary "Parts Shopping Therapy". Phil |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
Often (not always! check datasheet) the input is the LED side of an optocoupler. You can drive this with a pin directly by adding a small resistor to limit the current to maximum 20ma which will fall within the limits of the pic32. I often use a 330 ohm resistor to get 10ma and that works. If not you can go a bit lower resistance. Microblocks. Build with logic. |
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| LouisG Senior Member  Joined: 19/03/2016 Location: AustraliaPosts: 129 |
While on the subject of optical isolation, does anyone have any experience in putting a LED in series with the input of an optocoupler and operating the combination from switched 3.3V from a Micromite? The LED (e.g. red LED) would give visual indication that the output is at logic 1. The series combination voltage drop might be a bit close to the available 3.3V. I believe the voltage drop of the IR LED in an optocoupler is about 1.2V. If workable, what is the resistor used? |
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f1fco Senior Member Joined: 18/03/2012 Location: FrancePosts: 155 |
Phil, you can try your ssr with a +5 or +3.3 V in input (with a resistor to adjust to limit the current) and you pump (or a load of the same power...) on output if all is ok (no over heat on the ssr) you can command the ssr by the micromite ! try with safe (220 V !!!  )
Pierre. 73s de F1FCO |
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| f1fco Senior Member Joined: 18/03/2012 Location: FrancePosts: 155 |
look at this "autopsy" https://www.youtube.com/watch?v=DxEhxjvifyY and schematic... Pierre. 73s de F1FCO |
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Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
Yes, the voltage-drop of two LED's in series like this at 3v3 is a concern. Not saying it would not work though - you'd have to try it out. Me personally, I often put LED's on the MM outputs for relay drivers(and opto-couplers), and I do this not by putting the LED in series with the driving one, but as a standard LED+resistor in parallel with the output. I tend to use about 1k for LED's. Even at 3v3, 1k lights up most LED's quite brightly and only adds a few mA of load to the output pin, which, combined with the relay-driver or opto-coupler, is still well under 50% of tolerance for the I/O pin in most situations. Smoke makes things work. When the smoke gets out, it stops! |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
Thanks for that, Very interesting. Mine don't seem as light & hollow as he describes, so may be fully potted. Other thing I notice is these ones have 2 screws showing thru on the back plate, so might have a better rated device. Cheers. |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5116 |
For $7.50, I would say its a 90% chance they are fakes. But you wont know until you start to use them, they might be fine. My fake SSR was a 60amp model, worked fine up to about 3 amps, over that and it started to get hot, dam hot. At 10 amps it was in the catch on fire territory and started smelling bad. The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
Further to Gizmo's comments, yes, be careful of any high-current things on eBay. Often, high-power ability with extremely low cost should set off alarm bells in your head. Low-power stuff(anything under three amps or so) always seems to work just fine, but the higher-power things are where you can run into the Chinese Fakes problem. Not always the case, just sometimes, and more often with the higher-powered stuff. IE: You are more likely to be bitten by the pit-bull terrier that is the Chinese Fakes market, if you are paying peanuts for something with an advertised very high power rating. Take your SS relays - they might re-label 5000 10A units as 150A. They all blow-up if you put 100A or so on them, but by that time, they have made their money selling all the fakes to buyers all across the world. It's a huge business, the Chinese Fakes, and they are in EVERY market you can think of from Electronics to leather bags and shoes. Your chosen SS relay above may in fact be OK - you'd only know by testing it on the bench and ramping up the current on it to see if you could get it to survive a 50A load. However, at 50A, seven bucks seems suspiciously like it would not survive much more then 10A or so. My humble opinion only. Because you know for a FACT you are only going to be running 3-5 amps on it, the cheap $7 unit will PROBABLY be fine in YOUR case - just bench test it at a good 6-7 amps first to see if it gets hot like Gizmo mentioned above. If it does, it's most likely rubbish. It certainly costs more to buy from genuine stockists, but you can be 99% sure the part will be the genuine article capable of the stated ratings. Smoke makes things work. When the smoke gets out, it stops! |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
So what's a good bucket test procedure? Hook it up to a fan heater and measure it's temp rise at 1200 & 2400 watts? What should I exodus to see for safe operation? My working load is 700 to 1000 watts max. Phil. |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
In YOUR case, you are probably OK with the SS relay you mentioned. I doubt it will deal with a 50A load, but as you are not going to be pushing that much through it anyway - and know that - then the much lower load of 3-5A you mention is probably going to be just fine. Just don't take the 50A figure too seriously! 
If you hooked it up to a 1500W heater and left that running for half an hour or so, while monitoring the temperature of the SS relay, you'd get a ballpark reference point. That would be a 5A load @ 230v...... If it handled that OK, you could step up to 2400W as a test, but I would expect this one to start heating up quite alarmingly like Gizmo said his ones did, once you get up to 10A or so. Again - as you KNOW you are only going to be sucking 1kW through it max, then I am sure that even as an over-rated fake, it would still work OK, but you just need to bench-test it first is all.  Smoke makes things work. When the smoke gets out, it stops! |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2431 |
note that these relays are designed to be fitted to a heatsink when used with high currents. if there is a 1 volt drop across the relay 'contacts', then that is 1 watt of heat generated per amp of current flow. less than 10 watts of heat (10 amps) should be pretty much ok with the package just as it is, but 50 watts (at 50 amps) would require as much of a heatsink as a 50 watt audio amplifer. this is unlike a mechanical relay, where there is negligible heat generation. i'm with grogster, at 10 amps or less you're probably ok on all fronts, but any more is taking a punt. common sense economics should alway be applied, although i am sure you could pay $50 for a fake if you tried. and, of course, this does all beg the question: who would want to switch more than 10 amps at 240 volts anyway? more than 10 amps is going outside the realm of domestic applications, and in commercial mains AC power situations there are more than just saving a few dollars at stake. if you are switching low AC voltages (50 volts or less) at high currents (over 10 amps), you should be making your SSR's out of discrete components. cheers, rob :-) |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1667 |
Only ever envisaged switching 4 or 5 amps with this device. Presuming at that level heat sinking would be negligible. Got caught out about 20 years back controlling a 5kV heating element with an un-sinked SSR; boiled a 2000 litre tub of fish... Plan B was a contactor. Phil |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9590 |
Nice points, Rob, well made. 
I think you'll be just fine with that SSR, Phil, knowing you are not going to switch anything like the "Rated" current with it. Contactors are probably preferred over SSR's I would expect for larger loads. Some of those big power-station contactors are impressive things to behold and to hear the noise they make when they switch! They are pretty much all oil-filled though I think, so you don't see any "Frankenstein Arc" stuff going on with them, just a hell of a loud and very serious clunk noise! 
My guess is that you could do without one of those though.  Smoke makes things work. When the smoke gets out, it stops! |
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mikeb Senior Member Joined: 10/04/2016 Location: AustraliaPosts: 174 |
Hi Phil, Do NOT use a solid state relay on its own to power the compressor, or more importantly, an electric heater. The most common failure mode for an SSR is short circuit.In your application these SSR's would be more trouble than they are worth. To control a heater, and stay within the AS3000 regulations you are going to need a mechanical relay anyway with a high limit safety thermostat in series with the coil when controlling a heater.You also need a high speed 'semiconductor' fuse to protect the SSR from switching into a short circuit load. No other circuit protective device is fast enough. To get the stated power rating, which is highly dubious, you need to mount the SSR on a substantial heatsink. If the thing goes short circuit your compressor is dead in the water as you won't be able to stop it under any circumstances. Use the same type of relay as used on the original board. You will also need to use the same type of snubber network, as used on the original board (res/cap combination near the output wiring plug) to protect the relay contact from arcing whilst switching the inductive compressor load.Unfortunately switching an inductive, or capacitive load,is far more difficult than switching a resistive load. Contact derating is paramount. High line transient spikes, as experienced regularly, on the council network are highly detrimental to SSR's, particularly when they are used close to their maximum ratings. I haven't got the photo, you posted of the original, of the original board but from memory one of the output channels was 'toasted'. You will need to check the load that is connected to that channel as it may be short circuit. BTW. Hope your parts have arrived by now. There are 10 kinds of people in the world. Those that understand binary and those that don't. |
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