# Variable Relay



## dronai (Apr 11, 2011)

First time I've come across a relay that has a variable control for heat temperature control.

The relays are on the sides of this Heater control cabinet.

I have a bad one I have to find a replacement for. What brand is it ?


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## John (Jan 22, 2007)

It's just a solid state relay .....google is you friend

http://www.galco.com/buy/Carlo-Gavazzi-Electromatic-Controls/RM1A48D50

That will be $120.00 for a 24/7 service call. :whistling2::jester:


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## John (Jan 22, 2007)

WW Grainger is also your friend..:whistling2:

http://www.grainger.com/Grainger/MAGNECRAFT-Solid-State-Relay-6CVD0?Pid=search

When ya got a real problem.....just let me know.


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## dronai (Apr 11, 2011)

John said:


> *It's just a solid state relay* .....google is you friend
> 
> http://www.galco.com/buy/Carlo-Gavazzi-Electromatic-Controls/RM1A48D50
> 
> That will be $120.00 for a 24/7 service call. :whistling2::jester:


 
:thumbsup: Thanks ! I tried Goggle with the part number, and got nothing last night ? This morning everything  

*Just a relay*, How is it varying the amount of temperature to the heater ? Is it because it's solid state ? I see the DC rating of 4 -36 V


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## JRaef (Mar 23, 2009)

Can't read it for absolute sure, but the triangle symbol on the right side looks like the logo for Carlo Gavazzi, they brand label various lines of control products such as this, it's probably their part number, although it's also likely made by another company called Celduc.


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## dronai (Apr 11, 2011)

JRaef said:


> Can't read it for absolute sure, but the triangle symbol on the right side looks like the logo for Carlo Gavazzi, they brand label various lines of control products such as this, it's probably their part number, although it's also likely made by another company called Celduc.


Thanks JReaf, I found them, How does it vary the amount of current to the load ?


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## toklover (May 7, 2012)

it has to be solid state, its probably a pulsed signal thats controlling the relay, meaning the amount of times it switches on & off to maintain setpoint is far beyond the limits of a standard latch relay.

The heater will be tied back to a temp sensor, and try and maintain a certain setpoint, 
example: setpoint(SP) = 20 Celsius
1.2.3..20c...okay
1.2.3..20c...okay
1.2.3..19c...hmm
1.2.3..18c...sh*t sh*t sh*T! - TURN ON!
1.2.3..18.5c...sh*t sh*t sh*T! - STAY ON!
1.2.3..18.5c...hmm - STAY ON!
1.2.3..19c...hmm - STAY ON!
1.2.3..20c...okay - TURN OFF!
edit: so imagine this happening a few hundred times a second, it doesnt limit current, only duty cycle

so more than anything its a mechanical aspect, because the solid state has no moving parts it has a helluva lot more durability when it come to switching speed and longevity, also some pulses can be quite fast thats its almost like a buzz,


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## dronai (Apr 11, 2011)

Thanks !


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## John Valdes (May 17, 2007)

Solid state relays are used in these applications because they have no moving parts. When the temperature controller is fine tuning or operating the process temp, the relay can be taxed by the amount of on/off cycles it see's.
The relay is nothing more than a relay if it is controlled by a temperature controller.
The controller does all the work. The relay is either on or off. Nothing more.
Toklover is on point.
It's not a variable relay Dorian.


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## ace24wright (Jul 10, 2012)

We have hundreds of them at work, 120vac or 24vdc signal (a1-a2) comes fron the controller, ussually a eurotherm, which uses PID (propotional integral derivitive)(combines old signals, displayed signal and predicts the set temp.hence propotioning)


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## JRaef (Mar 23, 2009)

As the others have said, this is a simple on-off relay that does nothing but respond to the control signal given to it by the temperature controller, wherever or whatever that is in your system. I might look something like this (sticking with the Carlo gavazzi theme):










It could also be a PLC, a PC or any number of different control systems.

That particular SSR is what's called a "Zero Cross" controller, so when it gets a command that says "Turn On" from that controller, it waits until the line voltage supply is at zero volts (as in the Sine wave crossing zero) before it turns on, that way the electrical noise caused by switching On is reduced to nearly nothing. But once it is on, it is going to stay on until the command is removed, at which time current will cease to flow when the sine wave crosses zero again.


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## dronai (Apr 11, 2011)

...... :thumbsup:


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## ace24wright (Jul 10, 2012)

Yupper


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## dronai (Apr 11, 2011)

ace24wright said:


> We have hundreds of them at work, 120vac or 24vdc signal (a1-a2) comes fron the controller, ussually a eurotherm, which uses PID (propotional integral derivitive)(combines old signals, displayed signal and predicts the set temp.hence propotioning)


 
What I have is a PID loop ?


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## ace24wright (Jul 10, 2012)

The eurotherms I deal with do, not sure about your contoller, I would think so though. It would mention it in your manual


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## toklover (May 7, 2012)

maybe.

Its a math formulae that sorta controls how the heater reaches that setpoint.

So as stated before, because the solid state relay doesn’t regulate the power, it regulates duty cycle to achieve setpoint, the PID is what controls the duty cycle to reach that setpoint.


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## dronai (Apr 11, 2011)

toklover said:


> maybe.
> 
> Its a math formulae that sorta controls how the heater reaches that setpoint.
> 
> So as stated before, because the solid state relay doesn’t regulate the power, it regulates duty cycle to achieve setpoint, the PID is what controls the duty cycle to reach that setpoint.


This are not digital T-Stats. Analog controlled. I wonder if they make this type with the math formula converting voltage amounts to temp ?


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## guest (Feb 21, 2009)

Oh, and remember to clean the old heat sink compound off of the mounting surface for the relay and apply fresh compound......or you're gonna be back changing it again soon. 

You can get the compound at Radio Shack or Fry's.


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## dronai (Apr 11, 2011)

mxslick said:


> Oh, and remember to clean the old heat sink compound off of the mounting surface for the relay and apply fresh compound......or you're gonna be back changing it again soon.
> 
> You can get the compound at Radio Shack or Fry's.


Good one Tony ! I did notice that these were attached with a compound. I was wondering why they were not bolted or screwed to the panel.


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## guest (Feb 21, 2009)

dronai said:


> Good one Tony ! I did notice that these were attached with a compound. I was wondering why they were not bolted or screwed to the panel.


Hold on a minute then....the regular heat sink compound is a white, silicone based "grease" which has no adhesive properties. 

It is possible in your case that the OEM used (hopefully) heat-conductive epoxy instead. If they used the wrong kind of epoxy or glue then it is no wonder it failed. 

Is there a way to safely screw or bolt the replacement relay in place? (I.e. no wires/electronics/components on the other side of the bulkhead.)


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## dronai (Apr 11, 2011)

mxslick said:


> Hold on a minute then....the regular heat sink compound is a white, silicone based "grease" which has no adhesive properties.
> 
> It is possible in your case that the OEM used (hopefully) heat-conductive epoxy instead. If they used the wrong kind of epoxy or glue then it is no wonder it failed.
> 
> Is there a way to safely screw or bolt the replacement relay in place? (I.e. no wires/electronics/components on the other side of the bulkhead.)


 
It failed because there was a short in the heater wiring compartment. Wirenuts were melted and fused to EGC. Installers were not very carefull.

There is a white hard compound I saw squashed out from the back of the relays. Have to look into what to use.


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## John Valdes (May 17, 2007)

dronai said:


> There is a white hard compound I saw squashed out from the back of the relays. Have to look into what to use.


That is heat sink compound. The new relay will most likely come with a thermal gasket used in lieu of the compound.
Meaning. You just clean the surface real good, apply the gasket to the back of the relay and bolt firmly into place.
If it does not come with the thermal gasket, buy a tiny tube of compound and use that.


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## Splash (Apr 11, 2008)

John Valdes said:


> That is heat sink compound. The new relay will most likely come with a thermal gasket used in lieu of the compound.
> Meaning. You just clean the surface real good, apply the gasket to the back of the relay and bolt firmly into place.
> If it does not come with the thermal gasket, buy a tiny tube of compound and use that.


I think it's just gray silicone being used as an adhesive instead of bolting through the side of the enclosure. It's just a relay, the temperature sensing is being done elsewhere.


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## guest (Feb 21, 2009)

Splash said:


> I think it's just gray silicone being used as an adhesive instead of bolting through the side of the enclosure.


I think you are right. Which is stupid, for the reasons in the next part of my reply:



Splash said:


> It's just a relay, *the temperature sensing is being done elsewhere.*


Correct, BUT the heat sink compound is to make a good thermal connection between the solid state relay and the mounting surface, which is super critical in that case. It's NOT "just a relay", it is a triac based electronic switch that WILL generate heat depending on the load current it passes. If you don't get rid of that heat the relay WILL burn up.


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## ScooterMcGavin (Jan 24, 2011)

> maybe.
> 
> Its a math formulae that sorta controls how the heater reaches that setpoint.
> 
> So as stated before, because the solid state relay doesn’t regulate the power, it regulates duty cycle to achieve setpoint, the PID is what controls the duty cycle to reach that setpoint.


A PID is a proportional, integral, derivative controller. As previously stated it's a math problem that does its best to eliminate the error between your setpoint and your current value (or process variable) The proportional is the "how big is the error", the integral is "how long has the error existed", and the derivative is the "how fast is the process variable moving" Although the term PID is often used to describe these controllers, but depending on the system you won't necessarily use all three every time. The final output will be the result of adding these three portions together. By changing settings in the formula you can change how quickly or slowly the PID reponds to changes. 

The PID is able to control the temperature through time proportioning. So for a given time constant (say 1 second) the PID will turn the relay on for a portion of that time based on the output of the PID. So when you are starting cold the error will be large and the output will be 100% and the relay will just be on. 

As the temperature increases the error will decrease and the PID will starting lowering the output to say 75% or (.75 of a second on, and .25 of a second off) eventually you will hit the setpoint and the PID output will be just enough to maintain you at your current temperature at say 25% (.25 of a second on and .75 second off).


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## dronai (Apr 11, 2011)

scameron81 said:


> A PID is a proportional, integral, derivative controller. As previously stated it's a math problem that does its best to eliminate the error between your setpoint and your current value (or process variable) The proportional is the "how big is the error", the integral is "how long has the error existed", and the derivative is the "how fast is the process variable moving" Although the term PID is often used to describe these controllers, but depending on the system you won't necessarily use all three every time. The final output will be the result of adding these three portions together. By changing settings in the formula you can change how quickly or slowly the PID reponds to changes.
> 
> The PID is able to control the temperature through time proportioning. So for a given time constant (say 1 second) the PID will turn the relay on for a portion of that time based on the output of the PID. So when you are starting cold the error will be large and the output will be 100% and the relay will just be on.
> 
> As the temperature increases the error will decrease and the PID will starting lowering the output to say 75% or (.75 of a second on, and .25 of a second off) eventually you will hit the setpoint and the PID output will be just enough to maintain you at your current temperature at say 25% (.25 of a second on and .75 second off).


 
This turned into a great informative thread for me, as well as others I'm sure. I know with an analog card you can use a plc to control PID. Is there other ways ? How was this done before PLC's ?


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## dronai (Apr 11, 2011)

mxslick;78594
Correct said:


> Only one leg of the 240V power is being switched in the relay. The other leg went directly into that double throw two pole breaker you see in the panel. These relays are thin compared to a direct purpose mechanical type I'm used to seeing.


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## guest (Feb 21, 2009)

dronai said:


> Only one leg of the 240V power is being switched in the relay. The other leg went directly into that double throw two pole breaker you see in the panel. These relays are thin compared to a direct purpose mechanical type I'm used to seeing.


Yeah but remember that in a 240 v circuit ALL of the load current still passes through that relay (and breaker). :thumbsup:


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## dronai (Apr 11, 2011)

mxslick said:


> Yeah but remember that in a 240 v circuit ALL of the load current still passes through that relay (and breaker). :thumbsup:


You mean 30A one phase  I think the heaters are 6K 240V.

These heaters are infrared technology. The heater only transmits heat to objects, but doesn't heat the air. Good for areas wjhere heated air would be hard to contain


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## Jlarson (Jun 28, 2009)

mxslick said:


> You can get the compound at Radio Shack or Fry's.


I've gotten the one pole SSR's from Fry's electronics in the past too in a pinch.

I usually get SSR's from these guys http://www.power-io.com/ 

Great selection and they'll send stuff out pre mounted to heat sinks with fans and stuff if needed.


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