Question About PID

[Maxkling]

From looking at a lot of the builds with the Auber PID's Ive got a quick question.

Is there a reason why people don't use the relay output of the 2342 to pull in the contactor for the heating elements?

I see a lot of people using the SSR's to pull in a contactor for an element. Output of PID to SSR to Contactor. I may be looking at it wrong but if you have a 120 relay output off the PID why would you just go straight to the coil to pull in the contactor?

 

[ihomebrewing]

I am interested in this question, but I don't know exactly what you mean by pulling in a contactor.

I use an Auber relay with a 12V DC output, and have a SSR with that for input. The output side connects to my element, and is rated for 30A.

If the model you are speaking of has a built in relay, then what is it rated for?

If it has a built in relay for 10A, for example, then one should be able to hook up an element rated for 1200W of load directly to the PID relay output.

 

[CodeRage]

The PID will cycle the heating element relatively quickly. A mechanical relay is going to be cylcling its brains out which will cause it to wear much quicker. The solid state output going to the SSR allows for rapid cycling with no noise or wear. The SSR is the relay that is driving the element. Rapidly cycling a large contactor to drive the element would be very noisy and rough on the contacts of the contactor.

Most people use a contactor to break power feeding the element SSR. This way the element can not fire unless the contactor is pulled in. Its a safety thing and it wouldnt be turned off and on more than a couple of times a brew session.

 

[Maxkling]

Yea it makes sense to a certain level.

Ive never messed with anything on this kinda level. So I have no clue how often the PID would fire the element for the MLT. I'm guessing a lot from your previous post...

 

[CodeRage]

Yeah, depends what you have the minimum on time set to. My RIMS controller is 0.5 seconds so it could, and does at times, cycle 120 times a minute.

 

[RukusDM]

There might be another reason. The SSR may be able to handle the inductive kick from the Relay coil better than the relay contacts.

When a coil is de-energized, the magnetic field of the coil collapses and kicks back a spike as the contacts are opening. The larger the coils, the larger the kickback. This causes flashing or burning of the contacts with repeated use.

You can also install a MOV or Snubber across the coil to help dump the Coil kickback current.

I don't have a manual to review the specs. But that is one possible reason.

Edit: Also to protect from In-Rush current spikes when the coil is energized.

 

[CodeRage]

You're close Rukus. High current loads on the contacts can cause damage to the contacts. Larger relays like motor starters slam open and closed very very hard and fast to minimize the damage caused by an arc. Repeated use is also an issue here.

The inductive kick back from the coil is as you described but it tends to affect things on the logic level. The dV/dT can be so significant and can cause an elevation in the supply voltage and ground. It can cause physical damage to components or cause hiccups, like resetting the micro processor, in it's operation.

The thing to remember is the coil and ontacts are isolated in a mechanical relay.

SSRs can be switched much faster and generate virtually no noise on the coil side.

 

[Maxkling]

Wow I had no clue the PID would cycle that often, which makes me really happy I asked this question. I was planning on using some AB 30A small contactors that I have laying around. Im still going to use them but I deffanitly need to use the SSR's. I was planning on using the varistors on the coil of the contactors because I knew they would cycle a good bit but no where near what you just stated.