Confused About Axon Pins

[Mastermime]

Hello everyone,

I'm a little confused about how much current the Axon pins can supply.  My question is; Can I activate my relay coil (draws about 150ma) with my Axon?  The supplied voltage to the Axon is 12v.  I understand that input/ output pins can only supply >20ma, but since the row of (battery headers) are interconnected, that shouldn't apply here, correct?

[lrmall01]

I don't think so - you'll need some type of driver between the micro and the relay coil.

[Webbot]

It ain't coming from the battery its the signal pin so its being sourced/drained by the chip! Check the datasheet for ATMega640 - its about 20mA max.
The supplied voltage to the regulator may be 20V - but the supplied voltage to the Axon is 5v (assuming you are talking about an Axon or Axon II).
You 100% need external circuitry to activate the relay

[Mastermime]

Ok a transistor circuit like this should work, right?
(See image below)

Note: I forgot to put 1k resistors between gate and drain in the schematic

[lrmall01]

Your example will work if your relay coil is grounded on the other side.  It is more common to sink the current through the low side of the relay though, also nice to throw in a diode for current to bleed off through once the switch is opened (probably not critical though for a hobby project - this helps with electrical noise caused by the stored energy in the coil).

http://www.pcbheaven.com/wikipages/Transistor_Circuits/

[Mastermime]

It is more common to sink the current through the low side of the relay though

I'm not sure what you mean here.  Could you elaborate?

[Razor Concepts]

It is more common to sink the current through the low side of the relay though

I'm not sure what you mean here.  Could you elaborate?

Your circuit is "sourcing" current - you are providing a positive voltage, allowing current to flow from the positive to ground.

"sinking" current means providing a connection to the negative voltage, allowing current to flow from the positive (which is already connected to the relay) to the negative voltage (ground)

[Mastermime]

Ok I think I understand.  I should be applying the concept of electron flow rather than conventional flow.  Could you verify the image to make sure I'm correct

[Mastermime]

Can somebody confirm that I will be wiring this correctly based on the wiring the image above?

[lrmall01]

Yes - that looks correct.

Here is another reference for you.

http://www.free-circuit.com/2011/05/16/relay-control-with-mosfet-irf511-circuit/

[Mastermime]

Thanks a lot for your help

[Soeren]

Hi,

A few things to note...
- A TO-220 MOSFET is way overkill. A BC337 is ample for such.
- Remember a fast diode over the relay coil, or the active element will die screaming.
- The schematic you were pointed to (one of the late Tony VanRoon circuits, stolen from his site BTW), does have a (slow) diode in place, but the resistors at the gate are way too large (always scrutinize his circuits and correct accordingly, he was a really great guy, but unfortunately, several of his circuits are flawed and he didn't get enough time on earth to correct them all).