car relays to use in projects

Question

Hi all

i have quite a few car relays and was wondering if i can use these in some projects

problem is i have been told that they need 12 volts to turn on

these relays say 12volts and 30Amps

i tried using one in a project a while ago and it didn't work the Arduino voltage wasn't enough

when i tried to power the relay with a 9v battery i heard it click

so i was thinking about stepping up the voltage to power the relay but dont know how to do this

i have seen boost converters on eBay but there about $15 and don't want to spent that much each and every

time i do a new project.

thing is i can get car relays for free and if i can use them without spending lots of money it would be good

i do realize that i can get relays pretty cheap but if i can use the ones i have with out too much difficulty

and spending lots of money i will be happy.

these are 4 pin relays 12V , 30A

bobcroft · Answer

Hi Timmy,

The car relays you have will have 12 volt DC coils and really you need to find out what the current (amps) is needed to drive them, this will determine the interface to the Arduino. The 12V 30A will be the contact rating. these are not untypical of relays used on cars to drive lights etc. Therefore you will need a 12 volt power supply to use them. You could use this same voltage, suitably reduced, to power the Arduino, although personally I would use a separate supply for the Arduino.

In order to drive these relays from a 5 volt or 3.3 volt arduino you need a transistor or MOSFET driver and most importantly a 'flyback' diode across the relay coil. This diode is connected with its cathode to the + volts coil terminal and its anode to the - volts coil terminal. This prevents the reverse voltage from the coil damaging the driver transistor. Peter Oakes wrote a very good article on E14 a few weeks ago on using MOSFETS and I suggest you read that. Alternatively Google Arduino relay drivers. It is important to protect the Arduino outputs from the electrical noise and transient voltages generated by relay coils.

Hope that helps

Bob