clueless about relays

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Hi, Jim. Relays are rated by their contacts (that's the switching part) and coil (that's the part that creates the magnetic force that pulls the switch.

You need a relay whose contacts are rated at least 15 amps at 120VAC resistive.

The coil may be a bit more of a problem. 22.5VDC is an oddball voltage, but it's almost 24VDC. DC relays are usually guaranteed to pull in at 80% or 85% of rated coil voltage. So, nearly all 24VDC relays will actually pull in at 20.4 volts DC and up. I'd be a little concerned, though, about whether your source that will drive the relay would bog down with the load of the relay coil.

Before we do anything else, you should take a look at the docs on your controller, and see what kind of output you have, as well as how it's wired. Many temperature controllers are made to provide a voltage output for a solid state relay (which would be a very good solution for you, except for that "fail-off" business). That may mean it's only capable of pushing 20mA or so into a SSR input. Feel free to contact the manufacturer of the temp controller -- they'll probably be happy to help you find out what you've got from the controller model number.

But assuming you really do need a 24VDC coil relay, let's see if we can give you a hand with your immediate problem here. The Tyco/Potter&Brumfield T92S11D22-24 is a great relay for industrial control applications. It's DPDT (double pole, double throw) contacts are rated for 30 amps at 120VAC resistive, and has a 24VDC coil that's

350 ohms. It's also available at Allied as Stk.# 886-0149. I like it because the contacts aren't open to the air (nothing messes up relay contacts faster than ceramics dust -- it prevents them from closing fully and leads to a miserably short life), and it has faston connectors. Bolt it down with two screws, plug in the faston connectors, and you're done. I like that kind of solution.

You can download the P&B data sheet from the website. This will help you wire it up, as well as giving you a bit of an intro to how relays are spec'd.

By the way, see if you can get a 330 ohm, 3 watt or more resistor before you order your relay. Put it across your 22.5VDC source and see if it bogs down below 21VDC. If the voltage holds up there, you're good to go. If it does bog down, it's probably a SSR driving output. In that case, you might want to consider a smaller relay with a higher ohm coil or look to some other means of driving the relay coil (possibly a transistor).

Also, you should put a 1N4002 diode across the relay coil to absorb the inductive kick of the relay coil when it turns off, so whatever's doing the controlling won't have to.

Feel free to post again with any additional questions, especially if you have a SSR output on your controller. There are ways to get around that so you can use a relay instead.

Good luck Chris

Reply to
Chris
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--
You don't say what the 22.5 VDC output from the controller does, or
how much current it can supply.  For instance, does it go to 22.5V
 Click to see the full signature
Reply to
John Fields

I'm trying to set up a digital temperature controller for a kiln.

I have the controller & it outputs 22.5 volts DC. I want to run a

15amp 120VAC heating element. [a 20A relay would be nice as it would leave me some leeway if I change elements-- but 15 will work]

So I need a relay, but I've been looking at them at a dozen different vendor's sites and don't see any rhyme or reason to how they are listed.

I would prefer an electro-mechanical relay [because I've been told they usually fail 'off' which I prefer to one that fails 'on'.

The latest website I've searched is alliedelec.com. I searched AC relays for 15A & 20A and though I get some hits on relays that will handle the amps, I don't see where they mention the voltage needed to trip them-- or whethter they are e-m or solid state.

Here's the results-

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Can someone try to educate me on how to search for the relay I need? Or will one of these work?

Thanks, Jim

Reply to
Jim Elbrecht

haven't

10-11

to

for

22.5VDC

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Hi, Jim. The correct term is "glutton", and I guess I qualify. Mmmmm, temperature controller manuals...

It wasn't necessary to look at the full manual, though -- the info was on the 8-page blurb. Honeywell is usually good about that kind of thing. You have what's called the Model C21 6D, which has time-proportional voltage PID output. It specifies 22.5V in series with a 1.1K resistor -- in other words, it's a SSR driving output. If you put the 350 ohm load resistor on the output, you would see it bog down to around 5.4 volts. Not good for driving the T92 relay.

In addition, the output is time-proportional, which means our relay would probably bang itself to an early grave if you could use it. Let's say the controller has a 2 second time constant (I'm not downloading the entire manual to check), and it's calling for 50% power. That would mean the output is on 50% of the time, and your relay would turn on every 2 seconds, remaining on for 1.0 seconds each time. If your relay is good for a two million mechanical operations, your relay would be good for a month and a half of continuous use near the set point. Sorry, no go on the relay.

I would guess the SSR (Solid State Relay) is the way to go for this controller. They're not as bad as all that, if you make sure to heat sink them well (you'll need to dissipate about 20 watts) and use some kind of external safety. Of course, you should do that with a relay, too (relay contacts _do_ stick sometimes). What to do with this depends on your setup. One possible is to use a thermal fuse which has been made to fuse at a temperature past that which you expect to use in the kiln. Another setup is to use a second thermocouple and temp controller as a safety. Unfortunately, most of this stuff is very dependent on your local electrical codes as far as safety requirements. But if you've already got an overtemp safety, you can just tie into that, and not worry about the SSR at all.

So here's the drill: install a 25 amp SSR that has DC input to 32V (don't get the low voltage 3-8VDC input -- it might be high, depending on the circuit in the SSR.) Make sure the output is AC, can switch your AC voltage, and is _not_ "zero-crossing". That degrades resolution on time-proportional outputs that have a small time constant. Connect your + (pin 10) to the + input of the SSR, and the - (pin 11) to the - input of the SSR. Connect the SSR output in series with the heaters. Use a heat sink made for SSRs, and be sure to use a very thin coat of thermal grease (also called transistor heat sink compound) between the alcohol-cleaned SSR back surface and the alcohol-cleaned heat sink front surface. Make sure the heat sink can breathe, too. Heat related failure is by far the number one cause of SSR death. As far as safety considerations for the dreaded "fail shorted" problem, you'd have to let us know a little more about your setup.

Feel free to post again if you need more help.

Chris

Reply to
Chris

-snip-

I *think* that's how it works--- but to tell the truth I still haven't been able to figure out how to program the thing to test it. The wiring diagram on the side shows those contacts with a normal closed switch on them. [here's the diagram

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]

As I have understood the manual. [100 pages of mostly 'greek to me' stuff] - I will hook the relay to EV1, EV2, or EV3. But now as I look at that diagram for the 100th time, I wonder what terminals 10-11 are doing. .. vpulse sounds promising.

I searched the manual [its a pdf file- ] and found no mention of vpulse -- and I looked through the chapter where I thought it explained all those terminals, but it skipped those. I'll go back to reading.

I'm not sure what 8-9 or 10-11 do. The others are power in- [1-2-3] and the remote switch and computer connection options.

Also, if the current is small, (that is, used for

I think the current will just trip the relay-- The current is 22.5VDC

+/- 15%, internal resistance 1.1 K ohms. [page 20 of 100- second diagram as mine is the 6D]

Thanks- Jim [if anyone is a glutten for punishment- the 100 page manual is

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the 8 page overview is
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]

Reply to
Jim Elbrecht

do

the

10-11

to

for

22.5VDC

If I could add something, Mr. Fields. The EV outputs are SPST relays, and they can be set to turn on or off at a set temperature (with a programmable hysteresis in this controller). They're made for a number of ancillary purposes, like overheat alarm, turning on an internal oven convection fan, and other things. You could use one of the event relays as the main control output. However, this turns a self-tuning PID controller into a bang-bang ON-OFF controller. Not sure that's what the OP needs, even though it's tempting as the simplest solution. In my experience with kilns, anyone who's trying to make ware make money needs any advantage he can get.

(Actually, the thermal mass of the stack will even out the temperature on the inside, but there will be overheating on the outside, and on corners. They respond very quickly to the slightly higher peak temperatures from bang-bang controller oscillation.)

Might be wrong, but the way I see the full manual, I believe the output that has self-tuning PID control is at pins 10 and 11, and is made to drive a SSR.

Thanks Chris

Reply to
Chris

--
OK.
Reply to
John Fields

-snip-

Thanks. [and if anyone was reading that & rolled their eyes thinking he was being too elementary--- I read it three times before I really 'got' it.]

-snip-

I think the 'Events' will drive the relay as they say +/- 15% which brings them right into the ranges you mention. But now I also think you're correct to think I'm trying to get power to the wrong terminals and that the EVents are there for 'accessories'- not necessarily the element.

-snip- I'm re-reading that manual this weekend. Each time I read it I have another 'Eureka' moment.

Hmm-- I found 'Tyco Relays and Circuit Breaker - Databook' under free literature & they'll ship it with my order.

& I just found this page
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that should keep me reading until the next millenium. Something is bound to sink in.

I'll conduct that little experiment just as soon as I can get some power to that terminal. I'm still struggling with the programming.

Will do-- I recall the manual mentions a Zener diode being an option someplace--- might this have been the spot? [nope-- I just went back to the manual. That's what 7-8-9 are for. It does still look optional- 'resistance thermometer'? Is that the same as a thermocouple?]

I'm sure I'll be posting again. You & John have given me a lot to digest & clarifie da few things so the manual makes a bit more sense. Thanks for the translations.

Jim

Reply to
Jim Elbrecht

This is sci.electronics.basics. We don't roll our eyes at newbies. Sometimes at each other, but never at newbies. ;-D

Cheers! Rich

Reply to
Rich Grise

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