problem to control a compressor

BOBCACHELOT

hi , i need to control a compressor 12v dc 8 amp with my propeller, I searched for a relay but i dont know a lot about this, did someone have an idea for me ?

Mike Green

Here's one example of a relay that you can use. It's rated in excess of the current you need and it'll run off 12V which you have available. You'll need a switching transistor to handle the current and voltage of the relay. The Nuts and Volts Column #6 shows how to do this for a Stamp, but the same idea applies to a Propeller. RadioShack has several appropriate transistors (this too). You'll need a smaller base resistor. Anything from 150 to 220 Ohms should work fine.

You might look for a relay with a higher current rating given that you want to use this with a compressor which would have a high starting current. A relay with contacts rated at 12A or 16A would be preferable. You want a 12VDC coil voltage so you can run it off the same 12V supply you're using for the compressor. Either of the transistors mentioned should work. Remember that you need a diode across the relay coil as shown in the Nuts and Volts Column. A 1N4001 diode will do fine.

bill190

A relay is just a switch which is turned on by electricity!

With a relay, you can use a "small amount of electricity" to turn on a "large amount of electricity".

For example look at the very BIG wires going to a starter in a car. Imagine trying to run those big wires to the key ignition switch on the steering column! Not possible with the limited space. So instead they use small wires to the ignition switch and use a relay (solenoid) to electrically switch on the power to the starter.

sylvie369

bill190 wrote: »

A relay is just a switch which is turned on by electricity!

With a relay, you can use a "small amount of electricity" to turn on a "large amount of electricity".

For example look at the very BIG wires going to a starter in a car. Imagine trying to run those big wires to the key ignition switch on the steering column! Not possible with the limited space. So instead they use small wires to the ignition switch and use a relay (solenoid) to electrically switch on the power to the starter.

In the case of the car starter, that's essentially right, but there's more to it than just physical space when you're talking about controlling a 12V 8A compressor from a microprocessor pin. As that Nuts and Volts column explains, if you were to simply try connecting the pin to the relay, you'd quickly learn the hard way that you need some additional help/protection for the microprocessor - specifically, the transistor, resistor, and the diode.

BOBCACHELOT

thank you guys, I asked a technicien and he said almost all the same thing than yours... I will use a transitor because i need more current to use the relay, then after i use my relay, that now I understand how it work!!!

bill190

Don't forget that "backwards diode" on the relay coil (mentioned above). When power is removed from the relay, it will send back a high voltage which will damage a transistor. The diode (also called flyback diode) will short out that high voltage. This needs to be installed "backwards" to the normal + / -.

Diode...
http://www.allaboutcircuits.com/vol_3/chpt_3/1.html

Flyback Diode (See page 2)...
http://www.physics.unlv.edu/~bill/PHYS483/relay.pdf

Capt. Quirk

Depending on what the pump is used for, I suggest you confirm
the amperage reading. You might need a 20 amp meter to be safe.

Bill M.

bill190

So far as multimeters with amperage readings, that is a pet peeve of mine. Especially in automotive stores, they only sell meters with a 10 amp maximum. That may have worked fine for cars made in the 1960's, but these days cars have 30 amp and higher circuits. Alternators can be 100 amps or more.

Some things always stay the same...