reed relay - panel mount

Discussion in 'General Electronics Chat' started by crush, Jan 24, 2011.

  1. crush

    Thread Starter New Member

    Jan 24, 2011
    2
    0
    Hello,

    I see this became a long post, so here's the short version: I hope you can help me find a long-life nc relay that can be panel mounted. From what I've read, I could use a p-channel depletion mode mosfet, but I can't find any of these. I could use a reed relay, but I've not found any that are panel mounted. The application is built on the backplane of an enclosure - terminal blocks, ring terminals and screwed power relays, and 8-pin relays with sockets, so PCBs are not really on the cards...

    The control and signal power to the nc relay will be no more than 12V, and current typical to control a 240v 40A SSR, so presumably a few 10's mA. The switch may be operating once or twice a second for many hours, which I understand rules out mechanical relays, since they will wear out quickly. Switch speed is not all that critical, say <100ms.

    Can someone suggest a relay that fits this profile?

    Here's some more background into that might help paint the landscape I'm in (though I guess you might prefer to hear about the beer I'm making. :D)

    I'm building a home brewery, which has two 5.5kW heating elements in two kettles (one element in each kettle), and a 240v 30A control panel. Each element is controlled via separate a temperature sensor PID, which activates an SSR to provide power to the element.

    Since the panel is only 30A, it's not possible to run both elements at the same time. In the original design, (http://www.theelectricbrewery.com/control-panel-part-2), each element has it's own 30A power relay connected in series. A 3-position (2 no) switch is used to select between OFF, element 1 and element 2 by directing current to the coil of one of two power relays, allowing power to the connected element (if the PID has also closed the SSR.) something like this:

    240V/30A max ---> SSR load --> 30A power relay (load) --> element
    PID -> SSR (control) ---^ switch (pos1) --^ (coil)

    (and same again for the 2nd element, activated with switch in pos2.) When the switch is in pos 0, neither element is active.

    Most of the time when brewing, we do not need the full power of the elements. For example, once the water has reached the boil, much less power is needed to maintain the boil. And brewing requires only one of the 2 heated kettles at any time, the first kettle is used during the first half of the brew, and the second kettle during the second half - so it is possible to do "back to back" brews, where the second batch is started before the first has been completed, using the other unused kettle.

    Since it's not possible to actually have both elements on at the same time, I was thinking of using a master/slave relationship, where one element (the slave) is unconditionally off if the other element (the master) is on. To achieve this, could use a nc relay - the relay disconnects the slave SSR control signal when the master control signal is active, ensuring the slave is off. The control of this master-slave relay would be fed from the master element SSR control (from the master PID). The load of the relay is the slave PID SSR control signal, which passes through the relay to the slave SSR control. With this arrangement, the slave SSR control signal is unconditionally off when the master is on, but passes through unchanged when the master is off.

    At a push (a big one!), I might be able to build a small PCB myself to implement this with a DIP reed switch. But I'm not so comfortable with that, and this is a design that I hope will be used by others who also may not be familiar with PCB work. A panel mounted solution would be ideal.

    I hope someone can help. I have the feeling I'm missing something obvious, like a panel mounted DIL socket or something!

    Thanks for reading, and for any insight!
    crush

    PS: Just to put minds at rest, the panel is protected with a 30A ground fault and overcurrent device (RCBO), so if the relay fails in the open state and both elements are inadvertently powered, the overcurrent device will trip.
     
  2. soda

    Active Member

    Dec 7, 2008
    174
    13
    Hi

    Why don't you use a auto 12v relay(relays that's been use in cars). It's contact ratings usually can handle any amperage's up to 40Amp.

    You can buy this type of relay in any electronic store.
     
  3. crush

    Thread Starter New Member

    Jan 24, 2011
    2
    0
    Thanks for the tip soda.

    I'm using the usual ice-cube relays elsewhere in the panel. My understanding is that reed relays will last a lot longer than other types of mechanical relay. In this application, it will be switching on/off every 1-2 seconds or so.

    With mechanical relays rated at 100,000 operations, it seems like that might wear out pretty quickly?
     
  4. soda

    Active Member

    Dec 7, 2008
    174
    13
    Normal relay's last for quiet some time as far as i know.The newer relays one gets today are much better than the older types. The one thing about a read relay is that it can only switch currents up to 1Amp max.
     
  5. KMoffett

    AAC Fanatic!

    Dec 19, 2007
    2,574
    230
    Switching at 1-2 second intervals will be 43200 to 86400 times a day. You need to go the a solid state solution.

    Ken
     
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