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clueless about relays

Discussion in 'Electronic Basics' started by Jim Elbrecht, Apr 13, 2005.

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  1. Jim Elbrecht

    Jim Elbrecht Guest

    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

    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 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-

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

  2. John Fields

    John Fields Guest

    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
    when the kiln temp is lower than the setting on the controller and
    then drop to zero when that temperature is reached, or does it do
    something else? Also, if the current is small, (that is, used for
    signal purposes only) then the relay will need to be driven by
    something else and there may need to be another power supply added for
    the relay.
  3. Chris

    Chris Guest

    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

    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
  4. Jim Elbrecht

    Jim Elbrecht Guest

    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 ]

    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

    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]

    [if anyone is a glutten for punishment- the 100 page manual is
    the 8 page overview is ]
  5. Chris

    Chris Guest

    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

    Feel free to post again if you need more help.

  6. John Fields

    John Fields Guest

    Yes, those are "EV"ents 1, 2, and 3. SPST NO relay contacts rated for
    250VAC or 30VDC at 5A into a resistive load. It doesn't look like
    there's an internal supply that you can use to activate your external
    relay, so I'd recommend that you get a relay with 120VAC 20 amp
    contacts and a 120VAC coil and run the relay from the mains in series
    with the controller's relay contacts, like this:

    +---------+ |
    | | |
    | | <--O------------+
    | | | |
    | O----O---------+ |
    | | | O--> |
    +---------+ [COIL]- - -|
    | |
    | [HEATER]
    | |
    | |

    A good relay to use would be a P&B (Tyco) T92P7A22-120.
    (Digi-Key, $8.83) Sealed, 30A 120/277VAC DPST NO contacts, 120VAC
    coil, chassis mount with faston terminals. With the extra set of
    contacts as spares, when the first set burned out all you'd have to do
    to get things working again would be to move the terminals over to the
    other set, as long as the first set hadn't welded closed. Or, if you
    wanted to switch both mains hot and neutral you could do that.

    I think that might be a time-proportional (or ON-OFF) SPDT relay
    output, but I'm not sure. Take a look at page 3-6 of the manual and
    also the top of page 8-6.
    It looks to me like 8-9 is your sensor input and 10-11-12 is the SPDT
    relay output.

  7. Chris

    Chris Guest

    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.

  8. Jim Elbrecht

    Jim Elbrecht Guest

    Thanks to both of you-- since I started this reply both Chris and John
    have posted some great info that will take me a couple days to absorb.
    I'll be back.

    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.]

    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
    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 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
    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.

  9. Rich Grise

    Rich Grise Guest

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

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