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Need A Relay Design.

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  • Boss .

    Could you use an opto coupler instead? The drive from the 120V supply would then be a lot lower.

    Do you have a separate supply for the relay and alarm?

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  • Richard Caldwell

    The detection circuit is powered by the generator and the alarm has its own power source this is why I need a relay to isolate the alarm from the detector. Here is a block diagram of what I need. .  

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  • Richard Caldwell

    May I can find a relay that will energize at say 6VDC and yet handle the generator's 120VDC.

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  • Brad Levy

    What sort of load is the alarm, in terms of DC or AC, max voltage, minimum and maximum current?
    This determines the necessary contact rating on the relay. (Or substitute for the relay.)
    And why a 6V relay?

    Optocouplers can provide voltage isolation, but usually have relatively low output current capability. Some have open collector transistor outputs, capable of only switching DC of a known polarity. Some have triac or scr outputs, only capable of switching AC.

    Solid state relays (SSRs) are similar to optocouplers but frequently with added features like input signal conditioning or higher output currents. Some have back-to-back FET outputs capable of switching AC or DC.

    Optocoupler inputs are usually low voltage and low current compared to mechanical relays. (Essentially, they are an LED on the input side.)

    If the alarm you triggering/driving is just a contact closure input to a smart alarm system, an optocoupler could be just fine for the job. Likewise, some optocouplers could easily handle the current need of a Sonalert or similar piezo alarm module.

    If the alarm is a DC car horn on a 12 V battery, you will need more than an optocoupler.

    There are relays with 120 VDC coils, but they will typically pull 1-3 watts, which may be more than you want to give up of your generator's total 20 watt output.  Example: Phoenix Contact 2966281

    There are other questions that would influence the choice.
    A one-time, single-installation project may go for a more expensive per copy off the shelf solution, while the design for a product to be manufactured in quantity can justify engineering circuitry more tailored to the problem.


     

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  • Brad Levy

    "May I can find a relay that will energize at say 6VDC and yet handle the generator's 120VDC."

    That brings up another question:
    What is the minimum output of the generator when it is working?
    Most relays have a minimum operating voltage threshold that is typically 60-80% of their nominal coil voltage. Voltages above their rated coil voltage will cause them to overheat, especially if the ambient temperature is high to start with.
    Generators vary in their output characteristics. Some might have some temporary but nasty voltage spikes on power up or power down. Some may have relatively steady output voltage during normal operation. Others may have output voltage that varies significantly with variations in the mechanical or solar input source. You need to decide what voltage should represent a power good/no alarm level. And you may need to add protection circuitry to your relay or optocoupler to prevent damage from generator spikes.

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  • Richard Caldwell

    I really do not know the spec of the generator. My customer does not know all the specs and does not know electronics, so I am shooting in the dark.

     

    Anyway, I came up with this circuit. This is my first time using a 5 Watt Zener diode. I welcome any thoughts you might have.

     

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  • Boss .

    Hi Richard,

    I kept my original reply brief to try get more info from you.
    Based on your circuit above the issues requiring to resolve are:

    Relays as Brad says have very wide ranges of operation which are not ideal for voltage detection
    Below I extracted some typical data for a 12V relay (to match your diode rating) and chose a low power 150mW coil. 
    12V coil pull in 9.6V
    Release voltage 0.60V
    Coil resistance 960Ω±10%
    Coil power 150mW
    As you will see the alarm will sound over this complete voltage range >9.6V to trigger and then down 0.6V to turn off.

    I suggest you require some electronics with hysteresis around the alarm threshold voltage. However, you still have to power this circuit and with your resistor and Zener diode which will regulate over about 15mA the resistor will be dissipating approx 1.7W.
    Unless you have found a lower power relay coil mine (TE. D2n Relay V23105) would require approx 25mA. To which you need to add the electronics current consumption and allow for the Zener regulating current so you may need 30mA.....
    This would dissipate over 6W in the resistor.

    So where do you go from here?
    1. Check/test the alarm to see what it requires to switch, this will determine if a relay is required and what switching rating is required.
    2. If by chance the current ratings are low enough you may find a higher coil resistance relay, or even an opto may be suited.
    3. Check if you can add any electronics at the alarm, to reduce the triggering requirements. Get the part number and spec.
    Is the alarm battery-powered or a low voltage device, this could help keep things simple.

    There are probably lots more considerations but hope the above allows you to think through a solution.

    The above were just quick calculations, so check if you use them! But the key issue is the pull-in voltage and drop-out voltage of the relay.

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  • Brad Levy

    To implement a solution correctly, you also need to know more details about what conditions should signal alarm, and how quickly it needs to be signaled.

    You've said what the output of the generator is at maximum turning speed, but not at what the minimum acceptable turning speed. That is an important consideration for an alarm output. If the generator is being turned by a motor or engine with a fairly constant normal operating speed or fixed speed range, the desired alarm threshold might be close to the output at full turning speed, allowing rapid detection of loss for switchover to an alternate source before the generator output disappears entirely.

    In other cases (for example, a wind-powered generator keeping a battery bank charged), quite a bit of variance in the generator output may be a normal, no-cause-for-alarm condition. Here, the maximum turning speed might easily be ten times the nominal speed encountered day-to-day. Depending on the characteristics of the system, this might mean that an average power output of two watts (vs twenty watts at max turning speed), might be a normal, non-alarm condition. In this use case, rapid alarm output might not be necessary - alarming within thirty seconds of loss of generator output might be perfectly acceptable.

    I bring these up because they will affect your choice of relay (minimum guaranteed pull-in voltage, etc.), as well as other parts of your circuit. You have a filter capacitor in you zener circuit that will affect the response speed. That, combined with the coil resistance, gives an RC time constant long enough it could be seconds to tens of seconds before the relay drops out and signals the alarm.

    Another point is that the amount of energy needed to actuate the coil is usually relatively constant for a given model of relay, so coils specified for higher operating voltage are usually higher resistance and require less current than a low coil voltage variant of the same relay model. So if you need to turn the relay on when the voltage is above 50 volts, choosing a 48 volt coil version of a relay instead of a 12 volt coil version of the same relay family may reduce the power consumption of your alarm switch circuit by a factor of 4, leaving more of the generator output available for useful work instead of being wasted as heat in a power dropping resistor. The following specs from the relay Boss mentioned illustrate this difference in coil resistance and currents:

    Note that that particular relay is available in several different coil operating power (sensitivity) ratings in addition to different coil voltage ratings, and the minimum/maximum operating voltages and release voltage are different for the different sensitivity (coil power) ratings of the same coil voltage ratings. The variables in choosing a proper relay are frequently under-appreciated!

    There are other approaches as well, both relay-based and opto-isolator-based. These vary in cost, complexity, and flexibility. Making a good choice depends on fully identifying the requirements of the application.

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  • Richard Caldwell

    Sorry for the delayed response/reply.

    I have been waiting for them to respond. I have been demanding for more but they aren't  electronic inclined so the do not know the spec of the generator. They said the generator is a bicycle or pebble type unit. The alarm is a tens unit. If the person peddling the generator voltage drops to 0 the power to be the tens unit will turn on.

     

    That all info I have.

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  • Boss .

    Hmmm, well from how you describe your customers' knowledge I would first question the 120VDC output from the generator, are you sure it's not 12.0VDC?

    0V is both precise and vague, in the sense are they interested in "near zero"? 
    Assuming a likely 12V generator, I would be tempted to suggest you just require a 12V relay such as Brads suggested type 003 which would disable the alarm once they are above 9.6V and enable if they fall below 0.6V (which is near zero). The new problem is, do you want the Tens unit triggered when they first start peddling i.e. are at below 9.6V required to activate the relay.

    If it is actually 120VDC output, find a relay of suitable operation for both turning on and dropping out.
    Anything more complicated will require a circuit designed to meet the precise requirements.

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  • Richard Caldwell

    I just got the web site they bought the generated from

    https://www.k-tor.com/shop/generators/power-box/ 

     

    I have sent a request for more tech info from their support team and now waiting for a response from them.

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  • Boss .

    Well, that confirms it is 120V DC....

    I suggest you examine relays in the 110V to 120V operating range.
    Non-Latching Relays | RS Components (rs-online.com) select some options on coil voltage then sort by price.

    This may be suitable RT314110 | TE Connectivity, 110V dc Coil Non-Latching Relay SPDT, 16A Switching Current PCB Mount Single Pole | RS Components (rs-online.com)  RS pt no 680-3745

    110VDC coil 77.0V DC Operating voltage 11.0V DC release voltage  28800 ohms coil resistance.

    From the graph of the recommended operating voltage 120V is approx 1.1 x V recommended so this should be OK.

     

    As mentioned previously this will trigger the Tens unit until up to speed, so you may wish to add a switch in series to disable the relay until the output voltage is greater than 77V which will also provide a means of turning this off when finished.

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  • Richard Caldwell

    That sounds very siimple.

    Is this is what you are suggesting.

     

                                

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  • Boss .

    Yes, but do consider the start up and ending situations, you may require a disable switch?

    and if not built in a diode across the coil. You could also add the bridge rectifier into the circuit so not needing to worry about the supply polarity.
    One word of warning check the coil voltage ratings, I found the RS spec summary said DC, but when you examined the manufacturer's data using their part number it was actually an AC rated coil for some parts.

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  • Richard Caldwell

    As for the switch I do not understand what your talking. Can you show me where to place it.

     

    Yes I will include a diode to handle the inductive kick back, I just have not drawn it in yet. Also yes I will be including a bridge rectifier to ensure the proper polarity to the relay. I am also thinking about a large uF cap to filter out voltage spikes.

     

    Thank you our the man ( ͡° ͜ʖ ͡°)

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  • Boss .

    The switch is in my edited diagram of yours posted above. Just a switch in series with the Tens power unit.

    If you do not have a switch the Tens will be powered all of the time when not generating enough voltage.

    The Tens would also be powered when you start up and have not peddled fast enough to generate enough voltage to enable the relay.

    You need to check with your customer if they are happy with the switch or how they wish to handle the above start-up and finishing conditions.

    I'm not sure that you need a capacitor, relays are fairly slow and immune to spikes considering their activate and deactivate voltages, but that's your choice.

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  • Richard Caldwell

    (blushing big time) I did not noticed the switch in your mark up. :((

     

    Yes there is going to be several revsions to get it right and there happy with the design.

     

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  • Richard Caldwell

    Boss:

    I finished the schematic, would like any comment or suggestions you have about it.

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  • Boss .

    SW1 is wrong, when not connecting to the relay it will power the Tens unit continuously, just remove the link between the two poles or use the single pole switch I drew earlier.
    Unless my sums scribbled down are wrong the 480R will only dissipate about 0.02W and it will make the pull in and drop out voltage about 2% higher.

    What are you trying to achieve by adding this resistor?

     

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  • Richard Caldwell

    The resistor was to limit the current to 250 mA, I will remove it.

    Yes, I will be using  3 positions DPDT as shown giving the user a choice of constant on or off or controlled by the relay.

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