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Topic: Scrap what I've got - lets get the right stuff. (Read 2 times) previous topic - next topic

DaveO

In another post, I was looking to use some C106D1G or 2N2222A's for controlling larger V and A to other appliances that the output pins could not handle.

Why ? well, simply because I already had these components.

But thinking about it, I think it would be wiser to use better components instead of trying to save a few cents.

So looking at other posts, I get the impression that I should be using a MOSFET, activated by the output pin ( with Resistor in series and a pull down resistor ) to control power to a 12V relay.

Please tell me if you see any flaws in this ( I need to go thru this learning curve ) :

I want to drive a small relay which, in turn, will activate a gates' magnetic lock or similar item (requires 12V 1A).
The relay spec is : N4078 2c 12v 0.2(w).  1A/125VAC.  2A/30VDC.

So my logic is :
1.  Relay can easily carry the current required by the end device.
2.  0.2W / 12V = 16mA required to drive the relay coil.

So I need to find a MOSFET that I can drive with the output pin, and can easily handle 12V 16mA. Correct ?

Here's where the difficulty starts :
Our local supplier has stock of IRLZ44NPBF MOSFETs.

Unfortunately, I don't know what I am looking for on the datasheet in relation to :
1. what V is required to fully activate the MOSFET.
2. what A the MOSFET will draw from the output pin ( need to make sure it is within the specs for the board ).
3. is there a minimum current that the MOSFET will pass from S to D ?
4. what is the maximum that can pass thru the MOSFET ?

Once I know these values, I can highlight them on my datasheet (http://docs-europe.electrocomponents.com/webdocs/0791/0900766b807913d7.pdf) and learn the terminology used. Then if you don't think this is the correct component, I can go searching for something better ( and available ).

pwillard

#1
Apr 10, 2011, 12:55 am Last Edit: Apr 10, 2011, 01:15 am by pwillard Reason: 1
I'm trying to understand why you think you can't drive your small relay (N4078) with a 2N2222A.

retrolefty

#2
Apr 10, 2011, 12:57 am Last Edit: Apr 10, 2011, 01:00 am by retrolefty Reason: 1
Here's where the difficulty starts :
Our local supplier has stock of IRLZ44NPBF MOSFETs.

That is a fine LOGIC level Mosfet and it will do the job. However it's overkill to an extream for your use to pull in a 12v 16ma relay coil. That mosfet can handle up to 47amps (with proper heat sinking), and is most suited for switching large DC motors. A simple small signal npn bipolar transistor should be more then enough to handle the relay coil and should cost less then the mosfet. I would think it would be hard to find a npn transistor that can't switch that relay coil on and off.

Unfortunately, I don't know what I am looking for on the datasheet in relation to :
1. what V is required to fully activate the MOSFET.

That is a little tricky to find on a datasheet. The Vgs(th) is called the Gate Threshold voltage and it tells you at what gate voltage the mosfet just starts to conduct. The Rds(on) spec tells you how hard the device will turn on at various gate voltages, in your example a +5vdc on the gate will result in a device resistance of just .025 ohms which is full on for all pratical purposes. There are also graphs in the datasheet showing drain current Vs different gate voltages. Again a logic level mosfet is perfect for interfacing to an arduino digital output pin.

2. what A the MOSFET will draw from the output pin ( need to make sure it is within the specs for the board ).

The suprizing answer is that the gate will draw no current at all once it's fully off or on. The gate only draws current during the brief time it's transiting from on to off or off to on. A 300-500 ohm series resistor from the output pin to the gate will protect the output pin from the momentary current spikes during switching of the mosfet.

3. is there a minimum current that the MOSFET will pass from S to D ?

No minimum current rating, it can assume full off to full on from 0-47amps for that device.

4. what is the maximum that can pass thru the MOSFET ?

Yes, every mosfet will have it's own specified maximum current limit, your datasheet show Is (continous source current) of 47 amps, with proper heatsinking of course.

Once I know these values, I can highlight them on my datasheet (http://docs-europe.electrocomponents.com/webdocs/0791/0900766b807913d7.pdf) and learn the terminology used. Then if you don't think this is the correct component, I can go searching for something better ( and available ).

I still think to power a simple 12vdc 16ma relay coil, a simple small npn transistor is more in line. See if they have a 2N2222A or 2N3904, or really any small npn transistor. Just wire a 1k ohm resistor from the output pin to the base of the npn, wire the emitter to ground and the collector goes to one lead of the relay coil and the other relay coil terminal goes to +12vdc. Be sure to jumper a wire from the +12vdc supply negitive terminal to a arduino ground pin, and you should be good to go.

Lefty

Graynomad

Quote
I want to drive a small relay which, in turn, will activate a gates' magnetic lock or similar item (requires 12V 1A).

You can of course forget the small relay to directly control the mag lock (or whatever) with the FET.

The only reason I can see for having an intermediate relay is to make it a generic voltage-free output that you can hook pretty much anything to without worrying about voltage spikes etc.

Doing it this way keeps the FET connections short to a local relay, and the actual control wires (which may be long and prone to being abused) are connected to a relay that is very robust.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

DaveO

@pwillard -- thank you so much for such a detailed drawing. really appreciated.

@Graynomad -- Yes, I am running long 12V wires ( about 40 meters ) so wanting to keep that 12V circuit separated from the Arduino by the relay.

@retrolefty -- to you too, my sincere thanks for the time and effort you put into your reply.

I have looked at the 2N2222A and 2N3904.

The 2N2222A is almost the same price as the IRLZ44NPBF MOSFET ( R4.28 in my currency -- $0.54 ) whereas the 2N3904 is only 0.31c ($0.03).

My calc tells me the relay needs 5V 0.2W = 40mA. The 2N3904 can manage 200mA continuous and needs between 0.85V and 6V to saturate the gate, so doesn't require a resistor between the pin and gate, unless I want to drop the output-to-gate to about 2V using a 150ohms resistor.

The only thing which I don't seem to be able to find, is the A that the 2N3904 base will draw from the output pin ?  Is it so small that is isn't even mentioned on the datasheet, or am I just not seeing it ?



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