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Replacing a 9Vdc relay with a transistor switch

Discussion in 'General Electronics Discussion' started by tmetford, Jul 14, 2015.

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  1. tmetford

    tmetford

    16
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    Apr 14, 2011
    The relay in my timer circuit consumes too much power and drains the battery very quickly.

    I want to replace this relay with a transistorised switch in an attempt to make the circuit more power efficient and sustain the life of the battery as much as poss, but I can’t work out the circuit and necessary components.

    This double pole relay needs to carry only a small current (approx 10mA) through one of its poles and even less on the other one which simply completes a circuit for a sensor.

    The complete circuit is shown below with the DPDT relay circled in red. I have successfully assembled this circuit on veroboard twice before and it works very well given that it has a mains adaptor to supply the power! Some of the values shown have been tweaked to make it work on my PC programme (Circuit Wizard).
    upload_2015-7-14_11-44-52.png upload_2015-7-14_11-45-4.png
    The bit I want to focus on is replacing the relay and, ideally, I need a circuit diagram of the components and their values that could replace the function of the DPDT relay.
    I hope this makes sense and can you help please?
    Thanks in anticipation.
    Tim
     
  2. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    Hey Tim!

    Sure, anything that can be done with a relay can be done with semiconductors.....but sometimes it requires a great deal of effort to "make the switch" (all puns intended ;-) )

    First thing to know is the voltages and currents being switched.....one would think the "sensor" might be low voltage/low power likely DC, but there is no way to tell from your schematic what you might be driving with the other pole of the relay.....If you are switching a 3kW motor the answer is very different than if you are driving a 15mW LED....

    As a precursory example the 4053 ic is essentially 3 SPDT relays in the form of bilateral switches and is available in voltages ranging from 5Vdc to +/-18Vdc but, of course, it comes with a few caveats.....as all semiconductor solutions will....So before you get all excited and order some, you had best fill in the blanks about what you are trying to achieve.

    Good Luck!

    Fish
     
  3. Minder

    Minder

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    Apr 24, 2015
    Look at darlington arrays such as ULN2803 and 2n7000 for single switching.
    M.
     
  4. Alec_t

    Alec_t

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    Jul 7, 2015
    More details (especially a schematic) of the sensor would help. Is one of its two terminals being shorted tied to either power supply rail, for example?
     
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Do you want complete isolation for the sensor?
     
  6. tmetford

    tmetford

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    Apr 14, 2011
    Yes I do, and my apologies for not including enough information - newbie status!
    Furthermore, I have to admit general numptiness here as well. I had tried to simplify what I asked for to help focus on the problem but now, I realise, this was not such a good idea after all, Please bear with as I explain.
    I am looking to resolve a problem with our central heating system which, after much consideration, could be achieved by installing a manual override to the thermostat (this thermostat is the Sensor shown in the circuit above). This override will cause the boiler to keep running for approx an hour after the thermostat had switched the heating system off (the timer circuit shown above does this bit of course).
    Now I know this scheme might seem only a little bit "off the wall" but we've tried adjusting the thermostat and the thermostatic radiator valves to create the desired consistency of temperature through our house. We can set these devices, after much careful tweaking, such that all works fine for most of the time. But, every now and then, the temperature drops because the thermostat has switched off prematurely. Rather than tweaking the thermostat (and ruining its setting that was working fine most of the time), I'd like to be able to occasionally bypass the thermostat for about hour, after which the timer circuit would return to it usual inactive state until the next time we want to override it.
    I'd understand if you've now switched off from this like my thermostat! :)
    At this late stage I've finally got my voltmeter out and the voltage across the sensor (thermostat) is 90vAC - hence isolation for the sensor (thermostat) is necessary.
    The other pole switch in the relay is carrying about 10 to 20 mA at approx 9vDC.

    Maybe I should get out more, but if you're still willing to help I'm really keen to find out how this could be done using a semiconductor solution!
     
  7. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    You can get a CMOS relay @1mA
     
  8. tmetford

    tmetford

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    Apr 14, 2011
    Thanks for that but how would this CMOS relay be wired into the circuit? Would it replace the electro-mechanical relay directly or would it need to be isolated from the 90Vac supply from the boiler?
    Thanks
     
  9. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Just connect in place of the original relay. If you can only get SPDT, use 2 x CMOS relays.
     
  10. tmetford

    tmetford

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    Apr 14, 2011
    Okay. i may have the wrong end of the stick here but I've searched for details of some CMOS relays but most of the circuits, that I've looked at, show them used for operating an electromechanical relay which is what I'm trying to replace. Would you give me the details of a CMOS relay so I can find a circuit in which it is being used please? Thanks
    Ah! I've just looked at the datasheet of the Tempatron CMOS relay (http://www.tempatron.co.uk/resources/product/datasheet_86.pdf) which does have those very low current requirements e.g. 240uA at 5volts.
    Is the advantage of the CMOS relay its low power consumption and the fact that it provides electrical isolation?
     
    Last edited: Jul 15, 2015
  11. Alec_t

    Alec_t

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    Jul 7, 2015
    Perhaps use an opto-triac (e.g. a MOC3023) driving a triac to switch the thermostat contacts? Just a few mA would be needed for the opto input.
    An equivalent of this assembly is available off-the-shelf as a SSR (solid state relay).
     
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    You're correct in assuming that this is not what you want. I think the SSR that Alec mentioned is the rout you should be taking.

    Chris
     
  13. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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  14. Minder

    Minder

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    Apr 24, 2015
    If you want compactness and using relatively low current, there is the Opto22 modules that can either be used with different boards to populate or individual modules can be soldered into PCT board,
    If higher density is needed you can get the IDC5Q series etc, they come in different I/O voltages and 5v or 24v input.
    Plentiful on Ebay.
    M.
     
  15. TedA

    TedA

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    Sep 26, 2011
    Tim,

    It might help us if we had a clearer version of your schematic. You might try posting a version with better resolution. I think that there is some limit to images that appear right in the post, but other threads include examples that seem much clearer. Another approach is to put the image on some image sharing site, and post a link on this forum.

    Do you have the make and model of your thermostat? How about the control at the boiler end? Even a photo might help. Often information can be found online.

    Have you considered connecting your timer on the boiler side of the thermostat? No fiddly sensors to worry about, and plenty of mains power close at hand.

    If you give a location for your installation, some forum member will know how heating controls are done there, and can help fill us on local customs or standards. In the USA, the boiler to thermostat wiring is almost always 24VAC, so it would be convenient to deal with. Different countries have different ways of doing this, though.

    Ted
     
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