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Topic: Mechanical relay vs solid state relay vs octocoupler (Read 534 times) previous topic - next topic

cornestrijkert

Oct 17, 2018, 09:01 pm Last Edit: Oct 17, 2018, 09:05 pm by cornestrijkert
Hi all,

I'm really confusing about which type of relay I should use to regulate my central heating system. There are two types of connection to my central heating possible: on/off system and communication via OpenTherm protocol. OpenTherm is more complicated, so I will start with on/off system.

The boiler will be activated when the on/off circuit shorts. The voltage is 12V.

I would like to control this on/off process using my Arduino, so I need some type of relay to make that possible.

What I have read was that it's really important to isolate the high(er) voltage system (central heating) to the low(er) voltage system (arduino). Then a relay with octocoupler should be used?

But the 12V central heating system isn't that much that I'm not sure if an electromechanical relay is really needed. What's the max count of on/off switches an mechanical relay can handle? Because the central heating is activated each time when the current temperature is lower than the set point, the relay should be able to handle that.

Other type of relay I heart about: the solid state relay. This type is not based on mechanical switches, so that sounds great, because of the load of on/offs. But... is this type suitable to control my central heating?

And what about using an octocoupler based circuit directly like this guy did: http://ihormelnyk.com/arduino_opentherm_controller
This circuit uses OpenTherm to communicatie to the central heating system.

Maybe anybody can give me some tips about this.


Perehama

Electromechanical relays are prone to mechanical breakdown due to moving parts. Because there is an electromagnet in use, they are very noisy in EMI on your circuit. Also, they make a click-clack noise when switching. A solid state relay often contains an optocoupler as it's method of switching, but it also contains the circuitry to handle higher voltages and currents. Your best option is a solid-state power relay if your project budget can accommodate it.
F=C/V=(A*s)/V=J/V^2=(W*s)/V^2=(N*m)/V^2=C^2/J=C^2/(N*m)=(s^2*C^2)/(m^2*Kg)=s/Ω=1/(Ω*Hz)=s^2/H

Bringamosa

#2
Oct 17, 2018, 11:15 pm Last Edit: Oct 17, 2018, 11:25 pm by Bringamosa
The 12v is only the signal on the switch for the central heating system right?  You won't be switching any mains voltage? Mosfet then maybe?

MorganS

The datasheet for any relay you choose will have a "mechanical life" rating. That's the rating that the manufacturer is sure it will do, before it starts to be come "unreliable". Most relays will perform many times longer than that rated life. The first datasheet I could find at my favourite supplier has a mechanical life of 1 million cycles, at 180 cycles per minute.

You aren't going to hit the relay this hard. Your heater may switch on twice per hour every hour of the day. You are also switching a very benign load - a 12V signal line is nothing like switching the main power actually going to the fan motor. So you're definitely going to get close to the manufacturer's rating. At that rate you're going to get 57 years of reliable operation out of your relay.

I think the mechanical relay is the best choice for this application. But remember only the tiniest of tiny relays can be driven directly from the Arduino pin. It's best to buy a dedicated Arduino relay shield to take care of this for you.

Solid-state relays are more dependent on knowing the exact load you are switching. An AC relay used on a DC circuit will only switch on and can never be switched off.

Optocouplers are nice and I've used them in a few projects. Their current is usually very low - you need to know if the signal you are switching is using 1mA or 10mA to set up the optocoupler correctly.
"The problem is in the code you didn't post."

outsider

12V AC or DC? How much current? Is the 12V control voltage powering a relay in the boiler control or ...?

cornestrijkert

#5
Oct 19, 2018, 02:53 pm Last Edit: Oct 19, 2018, 03:04 pm by cornestrijkert
The OpenTherm protocol says the following:

Quote
The system operates by sending current signals from the boiler unit to the room unit and voltage signals in
the reverse direction.
So, the system communicates by varying the voltage and the current levels:

Current signal High level Ihigh : 17 .. 23 mA
Current signal Low level Ilow : 5 .. 9 mA (idle state)

Voltage signal High level: 15 .. 18 V
Voltage signal Low level: 0 .. 7 V


The most on/off room thermostats are not using mechanical relays, because the lack of space and I have never seen noisy room thermostats. I think they uses something like octocouplers as well, at least something small.

For my project I will place an gateway between thermostat and central heating system doing the on/off switches, commissioned by the thermostat. There is no direct connection between thermostat and central heating unit, only between the gateway and central heating unit.

On this project http://ihormelnyk.com/arduino_opentherm_controller PC817 octocouplers are used. Maybe this type of octocoupler is suitable for my project as well?

donperry

IF simple on off and no duty cycling use the mechanical relay. You cars use these things and they last for decades.

IF you have to PWM/duty cycling the heat source and the 12DC is the "higher voltage" portion of this circuit,  then use the optocoupler.

IF you have AC current messing with then use the solid state relay.

DVDdoug

Electro mechanical relays are pretty-much foolproof because it's simply an electro-magnetically operated and isolated switch.    The are generally very reliable.   I had a horn relay  in a car fail once...     There are lots of relays in cars and they almost never fail...

My home furnace didn't have a fan-only option so I added a regular relay to switch-on the AC to the blower.  (The wires were already in the wall.  I just had to wire-up the thermostat on one end and the relay on the other.)    That was several years ago and I haven't had any problems.

The do eventually wear-out, but they are more "electrically rugged"...    Excess voltage or current will shorten it's life but (within reason) it won't "blow" instantly.    A solid state relay can die instantly if you abuse it.

Of course, you should buy from a reputable manufacturer...   If you buy the cheapest thing you can find on eBay, who knows how long it will last...

If you don't know the electrical details or schematic of what you are connecting-to, a regular-old relay is the safest choice.


A solid state relay will "last forever" if not abused.   It's quiet, smaller, and just more "high-tech".    And, if you get the right solid state relay you can drive it directly from the Arduino.  Electro-mechanical relays generally require a driver to supply the coil voltage & current.

Solid state relays are more "picky" about the application.   AC solid state relays usually don't work with DC* and DC solid state relays don't work with AC.   120V/240V SSRs often don't work with low voltages.

If you are switching 120/240VAC, industrial-type relays like this are hard to beat.  They are super-easy to mount & wire-up, and they work directly from the Arduino.

The main difference between an opto-coupler and a solid state relay is the current & voltage rating.    Opto-couplers are for signals, and relays are for power.



* AC solid state relays are usually made with TRIACs.  A TRIAC latches-on until current falls to zero.  That's fine with AC because there is a zero-crossing twice per cycle, but with DC they won't turn-off.

JohnRob

I agree with MorganS regarding the use of a relay.

I do suggest you research the On / Off signal to be sure you know what you are switching.  Would need voltage (you already know this), current and is the signal AC/DC.

If the control current is low, you might have an issue using a SSR.  They need a minimum current to remain on through the complete cycle an your control signal might not draw enough current.


My only caveat is to purchase a good name brand relay, not one of the cheap china relays on eBay.  The frustrating thing about the eBay china relays is some can be good, some not so good. You don't know (until one fails)

Note: Every electronic thermostat I have / had utilized a small relay to interface with my boiler.  I've never had one fail yet.
Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

wvmarle

So, the system communicates by varying the voltage and the current levels:
Don't think so.
It's either voltage or current that's controlled, not both. Can't be both as they depend on each other. Make sure you know the nature of the signal you deal with.

Quote
The most on/off room thermostats are not using mechanical relays, because the lack of space and I have never seen noisy room thermostats. I think they uses something like octocouplers as well, at least something small.
Those mechanical thermostats switch a small signal - on and off - to a relay or so in the main unit. That's why you won't hear the relay: it's not in the thermostat unit itself.

Optocouplers are for isolating signals, not for switching power. So an optocoupler may be suitable for isolating your signal, but based on the specifications you gave it appears to be an analog signal, and in that case optocouplers are out.

Relays and SSRs are suitable for power switching. But to know which is suitable, you have to tell us what you actually want to switch, i.e. the voltage and current rating of this heater.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

MorganS

Don't think so.
It's either voltage or current that's controlled, not both. Can't be both as they depend on each other.
Consider the furnace trying to send a current signal. The indoor unit has a switch so it can open and close that connection. If the switch is open the voltage is high. If it is closed then it is low. The furnace can detect those different voltages on its outgoing current wires quite easily.

It sounds like a very clever system if you only have 2 wires in the wall. There must be some drawbacks or it would be more common. The baud rate is probably extremely low.
"The problem is in the code you didn't post."

wvmarle

I don't really understand what you're trying to do here.

A current signal is normally detected as voltage in the first place - send that current through a resistor, measure the resulting voltage across that resistor. So the second signal would be that voltage temporarily dropping to zero or so? That's the only way I can think of to add a digital signal on top of a current signal.

Reading it won't be easy in situations where the current signal is on the low end, as that means the high state of the digital signal is also quite low.
Quality of answers is related to the quality of questions. Good questions will get good answers. Useless answers are a sign of a poor question.

JohnRob

I didn't see the step where the control function went from a simple ON/OFF to the use of OpenTherm protocol.


Regarding the voltage / current description of the communications;  My understanding is the information is sent in a digital format (I believe using Manchester code) so regardless of how the voltage and current are generated they are still on / off signals.

I also understand if the boiler is OpenTherm it will respond to ON / OFF control (i.e.as if you used a relay).


So the question is:   is your boiler OpenTherm or just ON / OFF?

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

MorganS

It didn't. The OP asked a simple question and the process of answering that question diverted into a slightly different topic.

But with no feedback we don't know if the question is answered yet.
"The problem is in the code you didn't post."

JohnRob

@MorganS

Thanks.... missed the shift to Opentherm.

Please do not PM me with thread based messages.  If your thoughts are worth responding,  the group should benefit from your insight.

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