Help Regarding SPDT Solid State Relay or Equivalent

Discussion in 'The Projects Forum' started by grizzle, Mar 5, 2010.

  1. grizzle

    Thread Starter New Member

    Mar 5, 2010
    3
    0
    I need an electronic component that will act as a SPDT solid state relay in the following configuration:

    http://img694.imageshack.us/img694/3342/infractionl.gif

    Please take note that there are three contacts in the relay, the common (output), a normally-closed contact and normally-open contact.
    This is the thing that has made asking this question an all-day effort on many boards:
    A battery or battery pack is connected to the normally-open and normally-closed contacts. Therefore there is no complete circuit, nor is there, at any time, a flow of current. Think on this before replying for a minute, and please understand that it has been explained to me a dozen times or more that this makes no sense with regard to accepted electronics theory. There is no point in debating this. I can tell you that the device is very useful when the output is connected to it's destination. I am simply asking for your advice and am very appreciative of your time. If I can't get an answer anywhere, I'm sure I can hack it out myself, with enough time, but it sure would be nice to have some help. So if we could just ignore the "doesn't make sense" aspect for a minute, I would appreciate it. Thanks in advance.

    These are the criteria for the relay and oscillator:

    1. Very low power consumption, as it will run continuous duty for from 8 hours to several days at a time, and one version is worn on the person.

    2. Sufficient isolation between the normally-open and normally-closed contacts to prevent a dead short at the currents and voltages involved.

    3. Whether between the NO contact and the common, or between the NC contact and the common, whichever is connected at the moment, there must be less than 1 ohm DC resistance, preferably less than 0.5 ohms.
    (The solid state relays I have looked into have a very high DC resistance [25 ohms or more] which will not work)

    4. The oscillator should be capable of alternating the relay from one cycle per minute to 1MHz, however one cycle per 10 seconds to 50 KHz is acceptable.

    5. Low number of parts (simplicity)

    6. Economy as to parts ($)

    This is not a commercial venture. I simply wish to come up with a more simple version of the machine I have been using (electromechanical) to increase it's alternation frerquency range and so it can be made by anyone with electronic hobbyist skills.
    I look forward to your replies, and again, thanks very much for your time.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Mechanical relay is out of the question? I guess so.

    The result of the relay is to reverse the polarity to the common pin at a user settable frequency via the variable oscillator. I suppose.

    If this is the case, a current reversing circuit with a 555 and transistor should do you fine.

    How much current are you needing to work with?
     
    Last edited: Mar 5, 2010
  3. grizzle

    Thread Starter New Member

    Mar 5, 2010
    3
    0
    The function of the device does not fit into standard electronics theory. It is something that coincides with the presence of a battery, and the output device. Something not characterized by known science.

    They keys to success are:

    1. Low DC resistance between the NC or NO contact (whichever is connected) and the common. All the solid state relays I have looked into have a high DC resistance (25 ohms or more).
    2. Isolation between the NO and NC contacts so as to avoid a dead short.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Stumped....
     
    Last edited: Mar 5, 2010
  5. grizzle

    Thread Starter New Member

    Mar 5, 2010
    3
    0
    I apologize, I know it's an unusual question.
    However, it really shouldn't be any harder than an alternating LED flasher.
    The only things are that the solid state relays and optocouplers I have looked into seem to be designed to switch AC signals, and therefore no care has been given to the idea of lowering DC resistance. I have been told that FETs wired to act as a SPDT relay have a DC resistance in the milliohms. So that is a possibility. I have even considered using solar cells with an alternating LED flasher in pohotoconductive mode. But I simply don't understand enough about electronics to put this together. I have been building the electromechanical equivalent of the machine using clock motors to trip an analog SPDT switch, but it takes too much time and effort for peoplke to become interested in making their own. That's my goal, to spread the idea around.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I could give you a thousand options.. but I cant think of one that can switch back and forth at 10MHz or even close. You can use a 555 flip flop. A member here Bill_marsden posted on in his circuits blog. It can flip between two different nodes (or leds) pretty quickly.. but I do not think 10mhz. maybe 50kHz.

    And if you want to use logic circuits, Ill need to know the voltage and current you want to pass through.
     
  7. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    We can easily use a 555 to switch two transistors on and off alternately.

    The resistance will be very low. and speed very high. But If you want to put 300vDC through it or even 12v and 5A, we will have problems.

    If it is a few volts and few milliamps. It can be done quickly and easily.

    By wanting a SSR in the beginning of the thread, It seemed you were needing to handle higher voltages and currents.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Well, I always like new, on the edge stuff. And if its a good idea, Ill help spread it.

    I can help get your switching done. Even if we had to open a relay and add copper/silver/gold plate or wire to lower the resistance. But transistors ARE switches. and if you only have 2 options and they go to the same place, so it is an easy solve. As long as the proper rating is followed on the components.

    As for your led with solar cells idea.. its been done. They are called optocouplers. They use an led that shines on a light sensitive cell to transmit power with out contact. But you need to transmit NEGATIVE power for half of the cycle, so we will need one NPN and one PNP transistor.
     
Loading...