Hi, I'm using a digital output to controll a transistor like a switch. With the transistor I power the coil of a 5V relay, but the 5V comes from the arduino 5v pin because I don't have any other source of 5v in my project. Unfornately sometimes when the relay triggers the arduino resets (I imagine that because of the noise in the power produced by the relay coil). I find a solution adding a cap of 4700uF directly in the 5V and GND of my arduino. This seems to work (the leds in the other outputs dim a little bit) but now the arduino don't restart.
The other alternatve I have is to connect the transistor to another line that I have of 6~6.4 volts that I have to power a servo.
Which solution do you think is better? I think the 6.4 might burn the relay coil, but I don't feel confortable having a 4700 uF directly to the Arduino board, there is a problem or risk with that? If there is ok, probably the cap is the better solution.
I know that there are 6v relays but my local electronic store isn't open this weekend and I need to fix this for monday =(
I would first like to see a datasheet for the specific relay, but assuming the arduino's 5v pin has sufficient current to drive the coil you need first to have a reversed connected diode wired right across the coil terminals to suppress the transient spikes created when turning off the current flow to the coil, and then perhaps a big cap in addition, say a couple of hundred uF, not thousands.
travis12:
The other alternatve I have is to connect the transistor to another line that I have of 6~6.4 volts that I have to power a servo.
Which solution do you think is better?
Yes, that's more like it; depending on the coil resistance, 1V or so over is OK. If that makes anyone feel squeamish, add a low-value resistance in series to pad it down that much.
Do you have a diode connected in parallel with the relay coil? See retrolefty's post.
If you are worried about overheating the 5V relay coil when you feed it with 6V, connect a resistor in series with it, to lose the extra 1V. The value of the resistor should be about one fifth of the resistance of the relay coil (which you can measure with a multimeter). Or, if you don't have a suitable resistor, connect one or two silicon diodes in series with the coil instead. You will probably be dropping about 0.3V in the transistor anyway.