Hello, i wanna activate a 12 v automotive relay using arduino uno.. i was searching in the forum but i dont find how can i energize this relay by ground… i need do it with ground (i read that is low side? Of relay)…
So i have 12v from car ignition in one side of relay (i cant change it) and i have to “send” ground with arduino to energize relay coil…any idea?
Thanks
Hello fernandof22
Post the current connection and wiring diagram of the project.
This is the way I would do it. The transistor choice is not critical, another common one is 2N3904, which will also work. Don’t forget the diode either. Some automotive relays have a built in diode. It should be on the diagram on the relay usually.
Others can post a similar diagram utilizing an FET. I made the sketch using components I have on hand and have used before.
Very important to connect the vehicle chassis ground to the Arduino “GND” pin.
I would use a different transistor or a MOSFET. The 2N2222 is rated about 30 V 600 mA. Your base drive is also week, the transistor has a gain of about 100. Automotive relays draw a lot of power, the more poles the more power they require.
I actually use the 2n2222A with the TO-18 case. Hfe minimum of 100, Vce 50V, Ic 800 mA.
You are correct though I should know the coil current before making a recommendation. I was picturing a “standard” auto relay
Coil current about 170 mA.
I realize an FET would be more suitable in todays world but didn’t feel comfortable making a recommendation I haven’t tried.
thanks for your answers...
here is a example diagram what i need to do...
i was calculating relay coil current and it is about 200mA and 75ohms resistance...
so... which transitor i need to use and how i have to connect it?
Which is irrelevant for switching.
A bjt transistor behaves differently when saturated (collector voltage lower than base voltage).
The 5-10% rule is commonly used (10% for larger transistors).
Use a 2N2222 for your 200mA relay, with a 220 ohm base resistor. Diagram from post#3.
Leo..
I will agree with that. Lowering the base resistor value will do no harm. I apologize for the roughness of my sketch. My hands aren’t as steady as they used to be. I am looking for a good free schematic CAD program. I wouldn’t mind seeing a suggestion for the circuit using an FET though.
Up to about 400mA a 2N2222 is fine, if you have the required base current available.
Above that a logic level mosfet is better choice.
See this page.
Leo..
I have neuropathy and my drawings are mud maps, I use;
You will have to sign up for free, but it is a non-topheavy CAD and produces nice clean schematics and you can make your own components.
A typical example;
Tom.. 
PS. I do a lot of reverse engineering for troubleshooting and it is simple and quick to use.
Thanks, I will take a look.
This will work:
Make sure that the Arduino GND. this circuit ground and the automotive ground are all conneted.
Not for a 200mA relay.
The 2N3904 is a small signal transistor, that shouldn't be used over ~100mA.
A 2N2222 (or BC337) has an 800mA abslotute max rating, so is better suited for this task.
330 ohm is still ok for a 2N2222 and a 200mA load.
Leo..
Thanks a lot for yours answers… i have another query..if i use a mosfet instead of transitor, i need use diode between 12v and ground of relay coil? Because im using original relays and it’s difficult to put diode between coil terminals..
Is the relay shown in your diagram the exact one that you rate using? I have looked at several sites selling that model and none show a built in diode. So you will need one. If you can pop the cover off of the relay you can probably solder one on internally.
You are correct.
I don't recommend using a MOSFET
This will work:
I use KiCad, it goes from schematic capture through PCB gerber files you need to get BCBs made. It is free for the downloading although they ask for a donation. I have using it for about 5 years now. There is a lot of support on line for it.
If you have some latitude on cost vs. time, you can throw money at this problem and buy solid state relays off the shelf, e.g.
There are lots of varieties on input signals (the pin right from your GPIO) and voltage/current on what you're switching.
The guts of them are probably very similar to what's described above; they just include the relay, too, which removes an electromechanical component that's known to not age well. The relay probably becomes a big MOSFET (or parallels) in most cases. No crazy spikes from the coil collapsing, which Jim is trying to protect against with the Zener in his designs.
There's more than one right answer...
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