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    How to use the same RELAY on 12V & 24V vehicles

    I designed a circuit that operates on both 12V or 24V vehicles (my 150mA-max output voltage regulator steps 12V or 24V down to 3.3V for an MCU and other digital circuitry). I also need a relay on the same PCB, and the MCU will use a transistor to switch the relay ON/OFF via the relay coil's GND side. Normally, this is very easy when dealing with a single voltage like 12V only or 24V only. I would just choose a 12V or 24V relay and pull the coil's high voltage from Vin before my voltage regulator. But in this application, I need to use the same device on both 12V cars and 24V trucks. I cannot use a 24V relay coil on 12V because it just won't switch. And using a 12V coil on a 24V circuit would fry the coil because I need to keep the relay power for long periods of time (during the entire duration the vehicle is being driven).

    I could use jumpers to add a resistor in line with the relay coil, but that relies on the human element. If this device is ever mass produced and professionally installed, installers would without a doubt forget to set the relay coil jumper, and such would result in blow coils and complaints. So I want to avoid jumpers as a solution to this problem. I am wondering if there is a solution that is fully automatic and doesn't relay on the human element to adjust the relay coil voltage.

    Thank you.

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    Re: How to use the same RELAY on 12V & 24V vehicles

    Why not use a regulator, or even a zener, to drive the relay?

    Or, a little more complicated and a little less reliable, have the MCU monitor the input voltage and select one of two transistors to turn on the relay.

    Or, use a 3.3 V relay.
    Last edited by barry; 8th August 2019 at 03:09.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    First, I question that you "need to use the same device on
    both 12V cars and 24V trucks". Want to, maybe. Good
    idea if practical, sure. But manufacturers have model
    numbers because one size does not fit all. How much
    BOM cost are you going to add, to provide a feature
    that nobody cares about (besides you)?



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    Re: How to use the same RELAY on 12V & 24V vehicles

    Quote Originally Posted by barry View Post
    Why not use a regulator, or even a zener, to drive the relay?

    Or, a little more complicated and a little less reliable, have the MCU monitor the input voltage and select one of two transistors to turn on the relay.

    Or, use a 3.3 V relay.
    Shunting all that current (24V down to 12V) through a Zener is perhaps not the best approach. But you are correct about 3.3V relays being available. It's just that they typically only carry 250mA, and with a 70Ω coil resistance being driven at 3.3V, that means 47mA of my 150mA-max LDO's current output would be required for the relay, which is a tad more than I would like. It is something for me to ponder further, however.

    - - - Updated - - -

    Quote Originally Posted by dick_freebird View Post
    First, I question that you "need to use the same device on
    both 12V cars and 24V trucks". Want to, maybe. Good
    idea if practical, sure. But manufacturers have model
    numbers because one size does not fit all. How much
    BOM cost are you going to add, to provide a feature
    that nobody cares about (besides you)?

    Rethinking the circuit is still on the table. Thank you for the suggestion.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    SSRs don’t draw anywhere near 47mA. You make no mention of what your load current is, which will dictate the coil current. Reed relays draw about 10 mA.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    Quote Originally Posted by barry View Post
    SSRs don’t draw anywhere near 47mA. You make no mention of what your load current is, which will dictate the coil current. Reed relays draw about 10 mA.
    Thank you for the tip.

    I actually posted this same question in another forum today so I could get a lot of ideas at once. A suggestion made there was that I should consider using PWM on the relay coil, at 20kHz or higher, with at least a 50% duty cycle, keeping it at 100% during the first 500mA to ensure the relay switches. That's something I've never tried before and seems to be a generally good idea, assuming there are no any serious caveats. My 3.3V MCU would drive an NPN transistor to switch the GND side of the relay coil, and I could program that output to be PWM. I would have a single 1N4004 or equivalent across the coil for spike suppression.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    Hi,

    A discrete solution could be a "constant current" circuit.
    2 resistors
    1 zener
    1 bjt

    Or a 9V regulator and a 9V relay

    Or a 12V LDO and a 12V relay

    For sure the PWM solution may work...
    --> low power dissipation
    --> high effort

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    PWM is waaaaaaay overkill. You can accomplish the same function with a parallel resistor and cap driving the relay.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    Agree with the regulator suggestion. 12V regulator or 9V regulator. If 12V is fed to a 12V regulator it should just saturate at its dropout voltage (~11.5V for example) and will work fine.


    Also as you menttion when losses are a concern you can reduce the voltage to the relay after its switched:
    -Two drivers, one low current that stays on, one higher current that's AC coupled to provide an initial kick
    -Reduce the regulator voltage after the relay is switched
    -Make a current source with a decaying current.


    If the MCU knows whether its 12 or 24V pwming is a clever option, just maybe too clever.



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    Re: How to use the same RELAY on 12V & 24V vehicles

    what load does the relay handle? V? I? etc

    Kyotto makes a series of solid state relays with a control voltage range of 4 to 32 V

    load voltage ranges 24 to 280 VAC and currents 10, 15, 25, 40, 50 75, 90 and 100 A

    about 44mm by 57mm by 25mm



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    Re: How to use the same RELAY on 12V & 24V vehicles

    I intend to use a small dual-switch signal relay to break two signal wires coming from the PUSH start button in modern cars. The current relay I have handles a max of 2A, but even that’s overkill because there are just digital signals being passed on these two wires.

    Solid state relays are not even a consideration due to cost.

    Using a cheap LDO regulator with a maximum Vin of 30V to power the relay isn’t an option because of Load Dump voltage spike considerations. Since this will be used in 12v and 24v vehicles, I would need a TVS with Vc=50V (or so) and a voltage regulator with Vin max higher than the TVS Vc. I am pondering use of the LM9076 for my MCU to obtain its 3.3V rail, but I’d rather avoid using a second LM9076 for only the relay due to cost and is why I posted here to get ideas.



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