Electronics > Projects, Designs, and Technical Stuff
How to use the same RELAY on 12V & 24V vehicles
JDW:
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.
Richard Crowley:
Why not drive the relay coil with an appropriate CURRENT to pull it in?
So that a constant-current circuit would automatically adjust for the supply voltage.
Dundarave:
How about using a 5V relay powered by a little 12/24V-to-5V buck circuit board?
Then no matter what the vehicle supply is, it would still drive the relay at 5V.
Just a thought.
JDW:
--- Quote from: Richard Crowley on August 08, 2019, 01:45:49 am ---Why not drive the relay coil with an appropriate CURRENT to pull it in?
So that a constant-current circuit would automatically adjust for the supply voltage.
--- End quote ---
Thank you very much for the idea. Would you be able to provide a link to a circuit similar to what you are pondering? I'm curious about the implementation of a Current Source in light of the fact my Vin will be from a vehicle that uses either 12V or 24V, and there is noise and even Load Dump voltage spikes to consider.
--- Quote from: Dundarave on August 08, 2019, 01:47:20 am ---How about using a 5V relay powered by a little 12/24V-to-5V buck circuit board?
Then no matter what the vehicle supply is, it would still drive the relay at 5V.
--- End quote ---
Thank you for the idea. But for reasons of cost, board space and keeping noise to a minimum (my current power supply uses an LDO), I'd like to avoid adding a small switcher to power only the relay.
NiHaoMike:
Use 12V relays with a 12V LDO.
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