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Author Topic: MOFET selection to run 12v  (Read 1491 times)
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I am trying to control some LED strip lights (12V, 2 AMP) with the arduino and am trying to select the correct MOSFET to handle the load. I thought this one (http://www.vishay.com/docs/91325/sihlz14.pdf) might work but it does not. When connected with all the pull down and limiting resistors, the lights are dim. Then I noticed when I connected the Drain to Source without the gate power was getting through and dimly lighting the lights...I have researched several sites and similar projects but they do not have the same conditions.
 Any thoughts? Thanks community!
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MOSFET - is what I meant to type..
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Valencia, Spain
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It helps if you tell us how you connected it up.

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Here is the link to the schematic:


http://img19.imageshack.us/img19/8075/12vschematic.jpg

Thanks for the help!
I am trying to figure out how to get an image on the forum..

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You don't need limiting resistors for MOSFETs. Try it without them.

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I tried it without and no go.  I diconnected the gate and kept the source and drain connected to the projects grounds and power was getting through and I wasn't sure why..
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what's the voltage difference between source and gate when it's turned on?

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I'll have to check and post tonight.. Thanks for the input
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You don't need limiting resistors for MOSFETs. Try it without them.



What?  You shouldn't recommend this - a power MOSFET gate can be in range 1nF to 20nF, that's like a short-circuit load to an Arduino output pin on short timescales - they can only take 40mA abs max so a gate resistor of 150 ohms or so is recommended.  The datasheet seems to lack max load capacitance figures, but typical values are around 100pF or so for logic outputs in general, well below nanofarads.
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I am trying to control some LED strip lights (12V, 2 AMP) with the arduino and am trying to select the correct MOSFET to handle the load. I thought this one (http://www.vishay.com/docs/91325/sihlz14.pdf) might work but it does not. When connected with all the pull down and limiting resistors, the lights are dim. Then I noticed when I connected the Drain to Source without the gate power was getting through and dimly lighting the lights...I have researched several sites and similar projects but they do not have the same conditions.
 Any thoughts? Thanks community!

That MOSFET has a rather high Rds(on) of 0.2 ohms, so it will dissipate 800mW for a 2A load - it will run very hot - small heatsink highly recommended.  Its perfectly upto the job though as it is logic level and will only lose you 0.4V in voltage drop - clearly you have it wired up wrongly.

If you connect drain to source then you completely bypass the device, and if in that state the LEDs aren't lighting up properly its clearly nothing to do the the MOSFET!
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Quote
You shouldn't recommend this

It all depends.

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they can only take 40mA abs max

They are specified to 40ma max but can deliver a lot more.

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so a gate resistor of 150 ohms or so is recommended. 

Again, it depends. But in general, for digital switchign applications, anything more than 47ohm here needs to be seriously evaluated, because of potential high switching loss it can cause.

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The datasheet seems to lack max load capacitance figures,

Fig 5. And the gate charge, a far more useful figure than gate capacitance, is specified right on the 1st page.

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but typical values are around 100pF

Typical gate capacitance for a to220/to3p mosfet is 2-3nf.
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No, I mean the max capacitance load for the Arduino !!!!   And MOSFET gate capacitance varies with transconductance, I've one with 25nF gate capacitance for instance.  300A rating too smiley-wink
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I mean the max capacitance load for the Arduino !!!!   

I have never paid any attention to that but most digital devices are specified with a 7 - 15pf Cl. Load capacitance isn't as big an issue with an output pin with substantial output resistance. It does matter significantly in a read-modify-write operation.

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I've one with 25nF gate capacitance for instance.

Driving something like that would be a challenge, for sure.
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That's not the best MOSFET around [might get a little hot], but all in all, the ckt should work
perfectly fine as per the schematic. For low-frequency on-off switching, the resistor values
will be ok. You want the 10K pulldown to keep the load turned off during initial bootup
by the Arduino.

IOW, everything looks ok per the schematic, so 95% likelihood you have it wired wrong.

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If you connect drain to source then you completely bypass the device, and if in that state the LEDs aren't lighting up properly its clearly nothing to do the the MOSFET!

I was getting to that...first we have to get them to light up "dimly" ;-)

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