PID/SSR question

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mkirkland

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So im in the process of getting some materials together to build a RIMS tube. My question is will the SSR power the heating element directly? The PID will send the SSR signal(3V - 32V) to the SSR relay, the SSR relay will in turn send the 120V line to the heating element.. I see some people use contactors after the SSR so i was wanting clarification.

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iijakii

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The contactor/rated switch is to have a mechanical cut-off. Supposedly SSRs can fail with the circuit closed, so you could have power shooting through to your element even when the PID isn't sending the signal. It's also nice to not switch it on until you have it submerged so you don't dry-fire it. Up to you if you want these.
 
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mkirkland

mkirkland

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The contactor/rated switch is to have a mechanical cut-off. Supposedly SSRs can fail with the circuit closed, so you could have power shooting through to your element even when the PID isn't sending the signal. It's also nice to not switch it on until you have it submerged so you don't dry-fire it. Up to you if you want these.
So then the contactor is a backup failsafe for SSR failure?
 

augiedoggy

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So then the contactor is a backup failsafe for SSR failure?
yes and also they are easily controled with an on or off rotory switch which makes it easy to wire the system to prevent both elements from being on at the same time if you only have service for one element.
 

BadNewsBrewery

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Simply put, an SSR is not a perfect or 100% reliable device. They leak a small amount of current, even when off. They can fail closed (power flowing). The PID can turn them on at any moment, even if the PV is well above SV. All of these things can result in A: dry firing of the element; B: a shock to you if you're working on the system / cleaning / whatever.

A properly rated switch or a contactor serves as a mechanical break in the line, physically disconnecting the power supply / SSR / element chain. When your switch / contactor is turned off, there is ZERO connectivity between your power source and your element, regardless of what the PID is telling the SSR to do. This eliminates the possible hazards listed above (so long as you actually use it - there is no way to eliminate human error).

I would not recommend anyone build an SSR driven element without a mechanical switching means in line.
-Kevin
 
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mkirkland

mkirkland

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Simply put, an SSR is not a perfect or 100% reliable device. They leak a small amount of current, even when off. They can fail closed (power flowing). The PID can turn them on at any moment, even if the PV is well above SV. All of these things can result in A: dry firing of the element; B: a shock to you if you're working on the system / cleaning / whatever.

A properly rated switch or a contactor serves as a mechanical break in the line, physically disconnecting the power supply / SSR / element chain. When your switch / contactor is turned off, there is ZERO connectivity between your power source and your element, regardless of what the PID is telling the SSR to do. This eliminates the possible hazards listed above (so long as you actually use it - there is no way to eliminate human error).

I would not recommend anyone build an SSR driven element without a mechanical switching means in line.
-Kevin
So can i not use an SSR at all? Can i just use the contactor?
 

MrNatural

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So can i not use an SSR at all? Can i just use the contactor?
Contactors don't have the response time of SSRs. While they don't work fast enough, they would wear out super fast.............
 

atoughram

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What everyone I think is trying to say... It's OK to use an SSR, but just make sure you have a set of mechanical contacts somewhere up-stream of it to shut it down in case of a failed closed situation. And I would agree with that on a 240V system where both wires are hot. We usually use only one SSR and control one leg of the 240V with it, leaving the other hot leg switched by a mechanical set of contacts.

So yes, an SSR is ok to use. Just make sure you have disconnecting mechanical contacts in front of it.
 

BadNewsBrewery

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So can i not use an SSR at all? Can i just use the contactor?
MrNatural nailed it - they do two different things. The contactor / the high-amp rated switch serves as the safety link, physically separating the electrical connection.

The SSR is a Solid State Relay. The PID may cycle the SSR many times a minute, meaning that it will constantly be opening and closing. The SSR is designed to handle this constant switching. A physical contactor may not survive long in such a situation as it physically moves a contact block when you switch it, and multiple cycles in a minute can wear it out. The other issue you may run into is that many (not all) PIDs provide output at a low DC voltage, and many (not all) contactors require higher AC voltage. Sure, you can find something that meets your requirements from a power side, but you're not going to find a physical contactor / relay that can cycle as many times or as quickly as a SSR.

-Kevin
 
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mkirkland

mkirkland

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OK thanks for all the info guys

What i am looking to do is i am building a RIMS tube with a 5500W 240V element wired with only 120V so i should get about 1375V out of the element. I want to have this controlled with an auber PID and k type thermocouple. Also have 2 push buttons for turning on 2 separate march pumps. Looking for diagrams that are already made. Just a simple control box
 

atoughram

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RIMS tube with a 5500W 240V element wired with only 120V so i should get about 1375V out of the element.
1375 Watts = about 11.5 amps at 120V - so any simple $1 light switch will work for a mechanical disconnect.
 

Bobby_M

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Think of the SSR and PID combo to be software control of your elements. You need it because it's slick, fast and adaptive which is a requirement for RIMS. You just don't want it being the only means of ON/OFF control of your element. Say you stick your flow out of the MLT, the recirc pump loses prime, a hose disconnects off the RIMS and it empties, etc.. When you are not in active RIMS mode, you just want a simple switch that turns off power to the element where OFF means "REALLY F-ING OFF". At best, when an SSR is told to be off.. it's "probably off".
 

MrNatural

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The other issue you may run into is that many (not all) PIDs provide output at a low DC voltage, and many (not all) contactors require higher AC voltage. Sure, you can find something that meets your requirements from a power side, but you're not going to find a physical contactor / relay that can cycle as many times or as quickly as a SSR.

-Kevin
Thanks, another key attribute I neglected to mention.

Think of the SSR and PID combo to be software control of your elements. You need it because it's slick, fast and adaptive which is a requirement for RIMS. You just don't want it being the only means of ON/OFF control of your element. Say you stick your flow out of the MLT, the recirc pump loses prime, a hose disconnects off the RIMS and it empties, etc.. When you are not in active RIMS mode, you just want a simple switch that turns off power to the element where OFF means "REALLY F-ING OFF". At best, when an SSR is told to be off.. it's "probably off".
Well stated.
 
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