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2 relays with a shared common

Discussion in 'General Electronics Discussion' started by john2k, Sep 19, 2019.

  1. john2k

    john2k

    120
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    Jun 13, 2012
    I've got the following relay design where I've got two relays sharing the same common directly from +12v of a car battery. The relay coil is powered by two different circuits. In most cases only Power Source 1 or Power Source 2 will be on. However, there will be occasions where power source 2 may be on for about a minuate max when power source 1 is also on. Questsion is, will that be ok? I can't see why not because technically if both are on at the same time, then all that's happening is the same +12v power is getting merged. As a precaution I could put a diode but i'm thinking is it necessary?

    Any advice will be appreciated.

    Thanks

    [​IMG]
     
  2. JMW

    JMW

    90
    3
    Jan 30, 2012
    I see no problem. Should work just fine. No diode is needed. What did you use to draw the schematic?
     
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  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Perfectly o.k.
     
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  4. john2k

    john2k

    120
    1
    Jun 13, 2012
    Thanks

    MS Paint :eek: lol
     
  5. john2k

    john2k

    120
    1
    Jun 13, 2012
    Power source 2 provides powers to a 5W bulb but it fades in and fades out. I believe it does this by adjusting the voltage that eventually rises to 12V and then fading out it goes down from 12V to off position. Question is, will a regular relay work under these voltage fades? is there a particular kind of relay I need? If it will work, then I am assuming the relay will only activate when voltage reaches 12V. If so is there anything I can do to make the relay be on from any voltage from 5V to 12V?
     
  6. BobK

    BobK

    7,633
    1,657
    Jan 5, 2010
    The relay will activate at some voltage below 12V. It is guaranteed to be on at 12V, but the minimum voltage is not normally specified. 5V will probably not do it.

    But I question your assumption. If a bulb is being varied by voltage, putting the relay coil in parallel to it will affect the output voltage, and might even damage the circuit that is reducing current to the bulb, since it will increase the current drawn at any voltage.

    I would use a transistor switch, which you can make operate easily at 5V to 12V.

    Also, are you sure the voltage is being varied? It might be PWM.

    Bob
     
  7. john2k

    john2k

    120
    1
    Jun 13, 2012
    The reason I say its varied is because when I measured voltage with a multimeter it climbed up to 12V and then climbed down when turning off. This made me believe it is being varied.

    Any guidance on how I can do this in my above relay diagram scenario?

    Thank you.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
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    Nov 17, 2011
    A multimeter measures voltage rather slowly. The effect of a pwm modulated source will be averaged and not be visible other than by a slowly varying display.

    This will work only if you have another, stable 12 V power source. Then you can use a transistor to sense power source 2 and control the relay accordingly. But if you had a stable 12 V supply, what is the kick in using power source 2?

    Please explain what you want to achieve and why you think this is the sensible way to do it. Understanding what you want to do, not how you want do it usually helps a lot in finding a manageable solution aka circuit.
     
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  9. john2k

    john2k

    120
    1
    Jun 13, 2012
    OK so you are right, Power source 2 is from a PWM circuit. Basically let me explain what i'm trying to achive:

    This is to power a LED unit on a car. The Power Source 1 is +12v ACC Ignition which when it's live it lets the +12V feed turn on the LED. I also have a puddle light or approach lights whatever it's called which basically lights up as you approach the car or unlock the car. So what I want is that when the puddle lights are on for my LED unit to get on too. The puddle light circuit is completely different and controled by a electronic module in the car and hence why I have two different relays because Power Source 1 and two do not have the same ground.

    So it seems that the puddle light source is PWM. Can I put a big capacitor on it to smooth out the pulses to even it out?
     
  10. john2k

    john2k

    120
    1
    Jun 13, 2012
    What about adding a signal diode and a 4700uF capacitor in line across the PWM positive and negative terminals as pictured below?

    Or are there any types of relays that can handle PWM? So that I can possibly just put a different relay for the second relay? i read something about solid state relays.

    Also, this PWM circuit is never on for more than a maximum of 1 to 2mins

    [​IMG]
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You're dealing with a few different issues here:
    1. The pwm source may not be designed to handle inductive loads (such as the relay). The diode is a good start but may not be sufficient.
    2. The pwm source is definitely not designed to drive a huge capacitive load.
    3. Smoothing out the pwm will make moot the whole point of using pwm in the first place.
    You need to decouple the pwm from the relay, for example like this:
    upload_2019-9-20_7-57-19.png
    R1, C1 will smooth out the pwm.
    R3 will limit the base curent into Q1.
    R2 will discharge C1 in case pwm id off, thus turning Q1 securely off.
    D1 protects Q1 from teh flyback voltage when current through L1 is interrupted.
     
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  12. john2k

    john2k

    120
    1
    Jun 13, 2012
    Thank you @Harald Kapp I've tried to visually understand it by putting your decoupling example into my diagram below. But i'm little lost at the transistor part. I believe a transistor has 3 pins, which pins will connect where and how does it connect to the relay?

    Also, is R1 and R3 a 475ohm resistor? and R2 is a 100k ohm resistor? any particular type?

    Thank you

    [​IMG]
     
  13. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
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    Nov 17, 2011
    It is all shown in my diagram, post #11.
    The relay pins 85 and 86 are where L1 is in my schematic (lacking a relay symbol)..
    Yes, they are.
    No, Use e.g. 1/4 W types.

    Note that the 2N2222 is good for max. 800 mA. Your relay coil must operate on less current, otherwise the transistor will blow.
     
  14. john2k

    john2k

    120
    1
    Jun 13, 2012
    Still a little confused after looking at your diagram with the final connections to the relay. Is it something like my diagram below? i think something is wrong :eek:

    [​IMG]
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You think right:
    • the diode has to be parallel to the relay coil
    • the other end of the relay coil needs to go to the positive supply of the pwm source
     
  16. john2k

    john2k

    120
    1
    Jun 13, 2012
    I've tried again below, but i still think something wrong :eek: the relay coil seems like it has two positive connections on the 85 and 86 o_O

    [​IMG]
     
  17. john2k

    john2k

    120
    1
    Jun 13, 2012
    I found a electronics supplier who have suggested a High Capacity Extended Optical Logic board. They said the logic board can accept ground or positive switching. The inputs to the logic of the board are 'optically isolated'. The board will kick on and off when the level raises above and drops below around 2 volts on the PWM. Only problem is that the board will require its own power. Which means I will need to provide a permenant power feed to the board. Does this sound like a good solution between the PWM and my first relay?
     
  18. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    No. The connection from pin 85 of the relay to the pwm output negates the meaning of the whole rest of the circuit. You need to connect pion 85 to the positive supply of the pwm source.

    May be viable, but probably costly. And no guarantee that this board will smooth out the pwm.
    Here's my solution. A bit more complex than your original idea, but requires only one relay:
    upload_2019-9-21_19-14-13.png

    U1/R1 and U2/R2 are optically isolated control inputs from your two "power sources". When either of U1 or U2 is active, C1 is charged from teh 12 V car battery through R7 or R6, thus smoothing out noise or pwm on any control input.
    The voltage on C1 is used to turn Q1 on via R5.
    R8 is there to definitely turn off Q1 if no control signal is active.
    The current through Q1 turns on the relay (L1) which in turn activates the output via the relay contact (indicated by the dotted line).
    Power for the circuit is drawn from the car battery only when the circuit is active.
     
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  19. john2k

    john2k

    120
    1
    Jun 13, 2012
    Thanks. Out of curiousity, are there bad quality relays out there? Prices seem to vary hugely and some chinese ones are much more reasonably priced but wondering if these cheaper relays are bad quality? They are rated at 40A 12V and some are just the 4pin Normally Open and some are 5 pin with normally closed too.
     
  20. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Of course there are.
    Price can be an indicator, but is not necessarily a measure for quality. Some Chinese manufaturers offer astonishingly good quality for a low budget, others don't. If you're looking for parts for a one off poject it does pay, imho, to soend a bit more on quality parts. If you were to start series production, a few cents saved on a single component can sum up to a lot og money with rising numbers of produced parts, so there you'd have a different incentive to look for low prices.
     
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