Simple circuiting question, using Arduino

Question

I want to control a 24v DC circuit with a my arduino, what would be the proper component and roughly how would I use it. Basically when I set a pin to high, I want it to switch the 24V on, and turn it off when set to low. From what I can gather, I need a MOSFET, but I have minimal knowledge on what to look for in one. Would someone be able to assist me?

(p.s. This is my first discussion, sorry if I am doing this incorrectly)

johnbeetem · Accepted Answer

Mason Adkins wrote:

Im actually not using a motor, its a electric water valve from home depot.
http://www.homedepot.com/p/Orbit-Yard-Watering-Valve-27987/203019976#.UdDyLvlvOn0

From the research I've done, it takes 24v+ to open and 24v- to close. I was hoping it would be realtively easy to time it with an Arduino. As a first practical project, it seemed like a good starting point, due to the simplicity of the device.

I guess my understanding of a transitor is still flawed. From some reason I cannot wrap my head around it.

From what you describe, you need to run current one way through the relay to open and the other way to close. This is like driving a DC motor where you want to run it both forward and reverse. Even though your valves is not a motor, you're driving it the same way and have to consider motor issues like flyback diodes. Speakers are like motors too -- anything with an electromagnet has motor issues.

If this is the case, the circuit to drive it is called an H bridge. It's fairly complicated internally, so you probably want to use an H bridge controller chip such as the ST L298. I've never used that chip (or any other H bridge chip) myself and there may be others better suited to your application.

Here are a couple of recent element14 discussions talking about H bridges:

http://www.element14.com/community/message/65356#65356/l/the-magpi-magazine--aimed-at-learners--printed-edition-kickstarter

http://www.element14.com/community/message/72951#72951/l/help-with-wiring-piface-to-an-l298-h-bridge

You may want to get a low-current 24V supply and play with the relay off-line to see how it behaves. If you don't have flyback diodes, expect to see sparks which could damage your supply.

ntewinkel · Answer

Hi Mason, You should probably at least have a resistor between the Arduino pin and the mosfet. Oomlout has a circuit example using a smaller transistor: http://www.oomlout.com/oom.php/products/ardx/circ-03 Not sure if in your case you might also want an optocoupler ( http://en.wikipedia.org/wiki/Opto-isolator ) because of the difference in voltage. Cheers, -Nico

johnbeetem · Answer

Here's a short Raspberry Pi discussion about driving a 5V relay from a RasPi GPIO. The second comment shows a simple NPN transistor circuit. I believe you can use the same circuit with your 24V external device provided that you select a transistor with a high voltage rating. [This opinion provided without warranty.] Note that the circuit has a 1K current limiting resistor between the GPIO and transistor base. You don't want to drive an NPN transistor base directly from a GPIO with the transistor's emitter grounded, because the voltage drop across the transistor base-emitter junction should not be more than 0.6 volts and if the GPIO pin tries to pull it up to 3.3V you're likely to damage something. The 1K resistor limits the current to about 2.7mA assuming a 3.3V GPIO, or about 4.4mA with a 5V GPIO. The transistor multiplies current by a factor of "beta", with is typically 10-20. That is, the transistor can amplify a 2.7mA current through its base-emitter junction to 27 mA or more through its collector-emitter junction depending on what's hooked up. If this is not enough current, you can use two NPN transistors in the form of a Darlington Pair , which gives you a current multiplication of "beta" squared. Before using your external 24V device, be sure to test using an LED so you can tell if you've got things hooked up correctly. You typically want to limit LED current to 2-5mA, which means you need a 5-10K resistor between the LED and 24V. The voltage drop across a working LED is 2.4V or so, depending on color. If you put more than that across an LED, it will emit a very bright light once before it turns into a DED (dark-emitting diode). Edit: a high-voltage transistor with at least 40V reverse breakdown voltage should be all the protection you need. However, since 24V will easily kill a micro-controller, if you're worried you can use a 5V relay or opto-isolator to control the 24V circuit.

Simple circuiting question, using Arduino

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