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Use MOSFET to switch on LED's

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mickelsen

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led mosfet

I want to use a single MOSFET to switch a large number (30-40) of LED's on and off. I plan to group three LED's in series with a load resistor and put 10-12 groups in parallel with a single MOSFET to switch them on and off. I will then have several similar bunches of LED's to control. Then I would like to use a logic gate, a NAND or AND function, to control each MOSFET. I have no idea what MOSFET to use or what logic family to use to drive it, such as CMOS, TTL or other. The inputs of the logic gates will be driven by the output pins of a PIC microcontroller.
I know just barely enough to ask the questions but not enough to answer them. If anyone could help me select the parts I would greatly appreciate it.
Thanks.
 

mosfet led

Voltages in your circuit(s) will be rather low and as MOSFETS are rated for at least 50V (+) this will not be of any concern ..
There are two parameters which you will need to look at:
- MOSFET's current rating
- Rds(ON) ..
Choosing a MOSFET that is rated for 10, 20 or 50A is not a problem ..
The important thing will be its open-channel resistance - the Rds(ON) - the lower the better, the less heat will be generated inside its structure during the ON state ..
For example, if you choose the IRF540 ( see its data sheet at: http://www.irf.com/product-info/datasheets/data/irf540n.pdf ) you will learn that its Rds(ON) is roughly 44mΩ (0.044Ω); that is not bad at all ..
Anything below 0.1Ω will be suitable for your application ..
Another examples of popular MOSFETs:
BUZ71
**broken link removed**
MTP3055V
**broken link removed**

Because you will not be switching these LED at high voltages and high frequencies, as drivers you can consider Schmitt trigger gates (4093) or inverters (74HC14) or just simlle buffers 4050 (non-inverting) or 4049 (inverting) ..

Regards,
IanP
 

mosfet switch

Thanks very much for your reply and recommendations. It will be of great help to me. Could you also recommend a smaller MOSFET that I could use for experimentation on the workbench, one that came in a smaller package such as a TO-92 and would only have to switch one or two LED's?
Thanks again.
 

mosfet switch led

Hi, Since you LED current is no more than 10mA per LED and if you use 12 LED's parallel you are talking about some 120mA max. Than you can use the BSS138, cheap and simple. Can be controlled directly from microcontrollers or other logic chips.

Paul.
 

popular mosfet

Thanks. The only problem is that it comes in a surface mount package and that can be a little hard to deal with when you're just trying to kludge something together to try it. I couldn't find a BSS138 in anything except an SOT-23 package.
 

mosfet as switch

Try 2N7000 or BS170 .. both in T092 packages ..
**broken link removed**
They also can be driven directly of microcontroller pins, but I would add a 10-100Ω resistor between pins and gates ..

Regards,
IanP
 

    mickelsen

    Points: 2
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mosfet led switch

Thanks, this helps a lot.
 

using mosfet led

One more question if everybody doesn't mind: is it alright to drive the MOSFET's using 74HCT gates or should I stick with 74HC or 4000 series gates?
 

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