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Old 06-16-2012, 06:23 PM   #1
geoffey
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Default Relays vs. SSR's

Guys, (and gals too) I'm looking for some feedback on an electric rig I'm planning on setting up. It is inspired by Kal's system at theelectricbrewery, but differs in one important respect: 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. Suffice it to say I plan on using lighting relays to control my 240v heating elements, relays which I am very certain can handle the switching cycle of my brewing system for at least 5 years.

In order to use relays in a system with PIDs I would use a PID with a relay output control. The one I'm looking at is Auber SYL-2342, which is the exact model Kal's system uses with the exception that it has a relay contact output instead of a 12vdc SSR output. The hitch here is that the timing cycle needs to be set at a minimum of 20 seconds since relays are not designed to switch as quickly or as often as SSRs. (that's the whole reason he designed his system with them). This is an important issue to point out: the timing cycle in an SSR PID set up is 2 seconds by default. This means if the PID requires the system to fire the heating element 10% of the time it will turn it on for .2 seconds and off for 1.8 seconds. If the timing cycle were set at 100 seconds then under the same scenario it would turn on for 10 seconds and off for 90 seconds. In my setup the least I could set it for would be 20seconds. This would mean my relay would cycle on/off a maximum of 3 times per minute. And let me repeat: I have chosen relays which can handle this switching cycle.

My dilemma is that I will lose precision of temperature control under my planned scenario. I just don't know how much precision I will lose. 2-3 degrees I could tolerate. 10-12 degrees I could not, for reasons I would think are fairly clear. So I'm wondering if anyone else out there has tried this before and could give any feedback. OR if anyone with an existing PID SSR setup would be willing to set up a simple test: change your default timing cycle from 2 seconds to 20, 40 or 60 seconds and see if you can hold 5 gallons of water at 150 degrees for 60 minutes. If not, how much temperature drift did you get? This will give me a very good idea of the performance I can expect from my planned system.

Please note: I am not asking anyone to mess with their default PID settings unless you are confident you understand what you are doing and know how to make this change and then return it back to your previous setting.

Also, please consider the fact that I am not currently looking for advice on whether or not to use relays in place of SSRs. "Just use SSRs" isn't really the feedback I'm looking for. I have good reasons to be exploring the option of building my system this way, but I'm hoping to get a little help/feedback on the possible temperature drift before I start putting money into it.

Thanks in advance for any help you can give!



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Old 06-16-2012, 07:02 PM   #2
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If you are set on not using ssr's then you have already decided to trade the simple feedback fast response system that ssr's give you with a slower responding control loop. You might be able to use some feed forwarding controls to keep the temperature closer to your set points, but you have to really know what you're doing and it complicates things a bit. Everyone's setup is a little different, so you'll just have to tune your loops until you find some level of control you are happy with.



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Old 06-16-2012, 08:18 PM   #3
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I dont think your accuracy is going to suffer greatly. With the fairly large volumes of water and wort we are working with 1-2 degrees in an hour in an insulated cooler style MLT is normal. The largest issue you might encounter would be over shoot and waiting for the temp to settle back to mash temp before proceeding. If you are stirring you pots your overshoot should be minimal if your PID's are even close. Over time any stainless or Alli pot is gonna lose some temp, but if using a insulated Mash tun that will be completely minimized to a much more tolerable level quite easilly. Just some out loud thinkin'
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Old 06-16-2012, 08:32 PM   #4
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Originally Posted by geoffey View Post
Also, please consider the fact that I am not currently looking for advice on whether or not to use relays in place of SSRs. "Just use SSRs" isn't really the feedback I'm looking for.
Yup. That would have been me. That's the feedback I originally gave Geoff when he emailed me... he wants to stick with the relays so since I don't have any experience with them I suggested he post here for ideas.

Good luck!

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Old 06-17-2012, 04:25 AM   #5
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I would really like to know the reason behind this, not to say one is better or you NEED to do this or that, just for my own sanity, when you get time please post your reasoning behind using relays instead of ssr's.

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Old 06-17-2012, 10:32 AM   #6
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Originally Posted by Rbeckett View Post
I dont think your accuracy is going to suffer greatly. With the fairly large volumes of water and wort we are working with 1-2 degrees in an hour in an insulated cooler style MLT is normal. The largest issue you might encounter would be over shoot and waiting for the temp to settle back to mash temp before proceeding. If you are stirring you pots your overshoot should be minimal if your PID's are even close. Over time any stainless or Alli pot is gonna lose some temp, but if using a insulated Mash tun that will be completely minimized to a much more tolerable level quite easilly. Just some out loud thinkin'
Wheelchair Bob
I'd tend to agree. Really there are two major differences between SSRs and Relays that are relevant.

1) SSRs can cycle more often than Relays
2) SSRs don't actually shut off, where Relays do.

So... the way I see it if your ok with a little loss of temperature control a Relay would work fine and may actually provide some theoretical safety addition. As far as the accuracy/precision factor goes... given the volumes I also would tend to agree that the relay would be just fine. When I mash in my keggle I only turn my gas burner on once ever 30 minutes or so. A relay could easily handle that. Plus the worst thing that happens is you decide later you want the SSR you could always swap parts later. Or you could get a SYL-2362 and then use a separate relay or SSR (the SYL-2362 has both outputs).
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Old 06-17-2012, 12:50 PM   #7
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Efaden, thanks for the heads up on that Auber model. I didn't know they made one with both output types. That is a great solution for me. One of my fears was dropping $100 on the PIDs for relay control and having the system not work to my satisfaction and having to repurchase PIDs for SSRs.

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Old 06-17-2012, 12:59 PM   #8
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Efaden, thanks for the heads up on that Auber model. I didn't know they made one with both output types. That is a great solution for me. One of my fears was dropping $100 on the PIDs for relay control and having the system not work to my satisfaction and having to repurchase PIDs for SSRs.
No problem. Seems like a good way to go...
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Old 06-17-2012, 01:34 PM   #9
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I use a relay to switch the elements in my HLT via PID. It has worked fine for a few years now. An auto tune and a little tweaking of the delay time and I don't get any overshoot. The only real difference is the noise from the contactor.

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Old 06-19-2012, 03:51 AM   #10
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Thanks for the great feedback so far everyone. I'm fairly encouraged I'm going to be satisfied with my results. May take a few months to get there, but I'm excited to get underway soon.

To satisfy curiosity, there are a few reasons I'm interested in using relays in place of SSRs. First of all, I work in the commercial lighting control industry and work with these types of relays on a daily basis. I'm extremely familiar with them and very comfortable with wiring them. I have put together a plan using parts and pieces from my industry which I can source at little to no cost, including the touchscreen front end which I can provide the custom programming for myself. Incorporating SSRs into this system wouldn't necessarily be difficult. But it's out of my comfort zone and a bit of another unknown to me.

My control panel will be simply the touchscreen and two PIDs. I'm planning on having everything else installed above the ceiling, etc. I'm not that interested in portability.

More on this later if anyone is interested. Thanks for the help so far. I'd love to hear more from others!



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