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Old 02-14-2013, 09:34 PM   #31
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Getting back to the mtbf of a mechanical relay, a little quick math tells me that assuming you are controlling it with a PID, and you had a cycle rate of 4 seconds, that works out to be about 900 cycles per hour,

Of course, in an actual application, your controller is not going to cycle at that rate continuously.

The point is, that mechanical relay is rack up a huge number of cycles over the life of your system and will most likely be the first thing to fail from simply wearing itself out.

IMHO just another reason to consider an SSR for your current control device.



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Old 02-14-2013, 09:43 PM   #32
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Quote:
Originally Posted by processhead View Post
Getting back to the mtbf of a mechanical relay, a little quick math tells me that assuming you are controlling it with a PID, and you had a cycle rate of 4 seconds, that works out to be about 900 cycles per hour,

Of course, in an actual application, your controller is not going to cycle at that rate continuously.

The point is, that mechanical relay is rack up a huge number of cycles over the life of your system and will most likely be the first thing to fail from simply wearing itself out.

IMHO just another reason to consider an SSR for your current control device.
I have 3 SSR's and 2 mechanical relays that have been in my brewery for over 5 years and the only one that I've had to replace was a SSR. All relays are subject to failure so it is tough to say which will go out first.


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Old 02-15-2013, 12:26 AM   #33
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Originally Posted by LandoLincoln

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.
Nope, neither of those were the reasons I chose to use relays. I believe I went on to explain myself later in this thread. But when building a system with an overall cost of over $1,000 I didn't consider $40 for an SSR and heat sink a breaking point.
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Old 02-15-2013, 04:19 AM   #34
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Quote:
Originally Posted by processhead
Getting back to the mtbf of a mechanical relay, a little quick math tells me that assuming you are controlling it with a PID, and you had a cycle rate of 4 seconds, that works out to be about 900 cycles per hour,

Of course, in an actual application, your controller is not going to cycle at that rate continuously.

The point is, that mechanical relay is rack up a huge number of cycles over the life of your system and will most likely be the first thing to fail from simply wearing itself out.

IMHO just another reason to consider an SSR for your current control device.
My PIDs are set for a 20 or 40 second cycle, I wouldn't set them to any less. If they wear out its a quick replacement fix for me at no cost since I can source them for free. But the main thing is that at that cycle rate I can consistently hold temps within one degree, which is what's important to me.
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Old 02-15-2013, 11:46 AM   #35
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For what it's worth, I think the best arrangement is to have a power on switch governing a DPDT mechanical relay and the PID controlling the element via an SSR. That way you get the best of both worlds - the fast switch rate of the SSR and the fail safe mechanism of the mechanical relay.

Not either relay or SSR but both.



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