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Old 08-21-2012, 11:31 PM   #21
DavidinTexas
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Back to the original question: Mechanical Relays versus SSRs

Note that SSR stands for Solid State Relay. If you switch the mechanical relays just as fast as the SSR you should get the same level of control (holding of desired temperature). The question is: how long will a mechanical relay last switching as often as every two seconds? As I remember that is the fastest cycle time of most of these small PID controllers.

If you are happy with the life of that relay the only other issue might be the small radio pulses - like tiny lightning static every two seconds.... which may. or may not, be noticeable.

I would be very curious how long they last in that service - a quality relay will do many thousands of cycles - so it may be a few years.

 
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Old 08-22-2012, 12:16 AM   #22
AggieBrewer01
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I think Geoffey will be fine if he uses relays of the appropriate amp rating and adjusts his cycle times so they are not switching frequently. In fact, that is what I am going to go with, since I can adjust the Johnson controller so it doesn't 'short cycle'. (Disclaimer: I have no experience with brewing PID controllers and have no advice as to how to switch the frequency, but it sounds like he has that part handled.)

I explained to an electrician friend of mine what I wanted to do, and he happened to have a uesd relay laying around that he gave me. In light of the 'MTBF' and life expectancy issue, I hope it's not too old, but I guess I'll see how long it lasts. I think he said it came out of an industrial water heater. It is good for 40 amps, and is actually meant for 3 phase, so it has 3 poles (or sets of contacts; I'm unsure of the terminology). Anyway, I'll use two of the poles to switch the hot legs going to the heating element. I might use the third pole to run an LED or something, depending on how crazy of a control panel I want to build. Geoffy, let us know what you decide to go with and how it performs. There is a possibility that my used relay will burn out sooner than later and I might need a replacement

 
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Old 08-22-2012, 12:47 AM   #23
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I agree - With a 30 amp rated relay, the only thing you have to worry about is mean time between failure and stratification (In my estimation)

I could see an issue, unless you are recirculating, where if the element is on for 30 seconds and off for 90, that the heat may not evenly distribute.

But I have nothing to back that up.

Carry on and let us know how it works out!
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What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!

 
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Old 08-24-2012, 01:04 AM   #24
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@SweetSounds Thanks for your input on this thread. You did a great job of explaining things.

I've just purchases some relays and SSRs this week for my rig. The SSRs will be for control and the mechanical relays will be for main power shutoff. I actually found the 40 amp SSRs to be cheaper than the 30 amp DPDT mechanical relays. My system will be a little different because my relays use a 12 volt DC coil instead of 120 volt AC, but those are even cheaper than the 120 volt coil relays.

Mechanical Relay model # JQX-30F 2Z about $7.50
Solid State Relay model # SSR-40DA 40A /250V 3-32VDC about $5.50

These prices are on eBay with free shipping. You just have to wait a few weeks to clear customs.

 
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Old 08-24-2012, 01:51 AM   #25
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Quote:
Originally Posted by bendiy View Post
@SweetSounds Thanks for your input on this thread. You did a great job of explaining things.

I've just purchases some relays and SSRs this week for my rig. The SSRs will be for control and the mechanical relays will be for main power shutoff. I actually found the 40 amp SSRs to be cheaper than the 30 amp DPDT mechanical relays. My system will be a little different because my relays use a 12 volt DC coil instead of 120 volt AC, but those are even cheaper than the 120 volt coil relays.

Mechanical Relay model # JQX-30F 2Z about $7.50
Solid State Relay model # SSR-40DA 40A /250V 3-32VDC about $5.50

These prices are on eBay with free shipping. You just have to wait a few weeks to clear customs.
Glad to help!
And sorry if it came off a little harsh (After re-reading some of the posts)
But I think you're on the right track!

Keep the progress updates coming
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Originally Posted by Ecnerwal View Post
What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!

 
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Old 02-02-2013, 05:29 PM   #26
geoffey
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Just a quick update on this old thread I started:

I completed the build in September and have brewed 5 batches so far. I ended up changing my relays out to a 30 amp latching relay which was rated to handle the requires load (ended up installing a Wattstopper relay panel, for those interested). A quick thanks once again to Sweetsounds for helping me to fully understand the implications of the system I was proposing

I ended up having more trouble with the PID controller than anything else. Auber model SYL-2362 is what I purchased because it has both a relay output AND SSR output. I expected this would give me some future flexibility if I ever decided to go SSR control. The problem I didn't realize is that this model doesn't have fuzzy logic. I'm not 100% on what fuzzy logic is, but I'm now convinced its important on a system like this.

When I auto-tuned the PIDs the system became extremely sluggish. Trying to change the temp by just 10 degrees would take way too long as the relays continually cycled, even when the target temp was not close yet. I eventually manually tuned the P, I, and D variables until the system was faster to respond. But I consistently overshot my temps by 3-5 degrees.

I decided that was unacceptable for me in the long run and just pulled the trigger on model SYL-2342, which has a relay output and fuzzy logic. After one auto tune I am able to hit my temps on the dot and hold them.

So my suggestion to anyone looking to build a relay based control rig would be to make sure you use the Auber model with Fuzzy logic.

 
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Old 02-14-2013, 06:13 PM   #27
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Let's do the math!

Heat capacity of water is ~4 Joules per mL, per degree C.

So, for 20L of beer, that's 20,000mL of beer. So it will take 80,000J to heat it up 1 degree.

5500W is 5500 Joules / second. That means it will take 80,000/5500 = 14 seconds for a 5500W element to heat up 20L of beer 1 degree C.

 
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Old 02-14-2013, 08:24 PM   #28
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OK, so a 10 second cycle would probably be OK on a HLT especially if it was really full. Maybe you could get away with a mechanical relay for that.

But you'd want finer control over mash temperature.

 
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Old 02-14-2013, 09:56 PM   #29
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Quote:
Originally Posted by geoffey View Post
I plan on using lighting relays instead of SSR's. There are a couple important reasons I'm going in that direction, which it isn't really important to me to discuss at this point.
1) You're a tightwad and you have access to free relays.
or
2) You really like the sound of clacking mechanical relays.

It's okay to fess up to either of those. Plenty of tightwads on this board. You're among friends.

 
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Old 02-14-2013, 10:11 PM   #30
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I am using relays for controlling my 240v heating elements. I decided to go this route because 1) they were free, 2) it switches both legs of power with one relay, 3) no bleed through like you get with SSR's. I kind of like the clicking sound because I can hear that it is working.

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