I'm using a mosfet to switch 4 leds powerd by a 12v power supply and aarduino to switch it but no matter which combo i try i cant get it to switch its either on or off what am i doing wrong?
mosfet to-92 http://www.st.com/stonline/books/pdf/docs/10202.pdf

 

Hello,

Can you post a schematic over here?
That way we can see where the trouble lies.

Bertus

 

 

Hello,

The main problem lies in the fact that you are using "normal" mosfets.
Those will conduct fully when the gat voltage is 10 Volts.
The arduino will give 5 Volts at max at its output.

You can solve the problem by using "logic gate" mosfets.
Those will conduct fully when the gate voltage is 5 Volts.

Bertus

 

ah that sucks ok i'll have to make a trip to radio shack, do you think this one will do?http://www.radioshack.com/product/index.jsp?productId=2062615
i need it for up to 240mw switching

 

Hello,

The transistor you show is a bipolair junction transistor (BJT).

If you want to stick with mosfets take a look at the IRL510.
(perhaps a bit overkill, but it will give you some headroom).

Both datasheets are attached.

Bertus

 

I couldn't find thw one's you suggested @ radio shack. Unfortunatly i need it today so i have to get it from radio shacks limited supply do you think any of these from there store will work
http://www.radioshack.com/product/index.jsp?productId=2062615
http://www.radioshack.com/product/index.jsp?productId=2062609
http://www.radioshack.com/product/index.jsp?productId=2062608
http://www.radioshack.com/product/index.jsp?productId=2062612
http://www.radioshack.com/product/index.jsp?productId=2062616

 

You COULD add a comparator (eg. LM339 available at the Shack) in between the 5v. signal and the gate of the FET. This would allow you to pull the gate up to 12v (full on) when the comparator is open ("on", input "+" voltage exceeds input "-" voltage) and down to ground when the comparator closes "off" (opposite of above), with the comparator on/off depending on the input voltage vs., say a 1v. reference.

Of the transistors you listed, I'd pick the MPS2222A.

 

Everyone, please look at the datasheet for Rds-on.

Rds-on = 1.8 ohms typical, 5 ohms max at Vgs = 10V, ID = 0.5A
Rds-on = 2 ohms typical, 5.3 ohms max at Vgs = 4.5V, ID = 0.5A

5V is entirely sufficient to get good conduction.
Your probably lies in something else, specifically it looks like you're trying to use the NMOS on the high-side.
This won't work with 5V, you need to switch it to low-side.

Refer to this thread:
http://forum.allaboutcircuits.com/showthread.php?t=42605

Specifically post #7 and #11

 

5V is entirely sufficient to get good conduction.
Your probably lies in something else, specifically it looks like you're trying to use the NMOS on the high-side.
This won't work with 5V, you need to switch it to low-side.

Agreed, the circuit should work at 5V, if not efficiently. The circuit supplied doesn't make it clear (a red flag) if it is wired high or low. And the problem may be even simpler: On that MOSFET device, the pins are G-S-D. As drawn, the gate is not connected to the input, hence the on-or-off behavior without switching.

 

As drawn there's no common ground either.