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I wish to drive a MOSFET using a 555. I dont have a driver handy, will it be fine to use a transistor instead? So the output of a 555, could activate a transistor(as swtich), which allows a source to drive the FET?

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    \$\begingroup\$ Can you not drive the MOSFET directly with the 555? What MOSFET? How fast do you need to drive it? At what voltage? Your question boils down to basically, "can I use transistors to drive things?", and the answer is "yes", but without more detail, there's not much more of an answer than that. \$\endgroup\$ – Phil Frost Jan 17 '14 at 20:19
  • \$\begingroup\$ Why not drive the MOSFET gate directly from the 555 output? What is the background to this question, what is the transistor intended to help with? (presumably you mean a BJT since a MOSFET is also a transistor) \$\endgroup\$ – Anindo Ghosh Jan 17 '14 at 20:20
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    \$\begingroup\$ A standard 555 can source/sink 200 mA. Are you sure you need more than that? \$\endgroup\$ – Dave Tweed Jan 17 '14 at 21:08
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    \$\begingroup\$ Please, tell us more info about your circuit. Switching frequency, voltage you need to switch, is a low-side MOSFET satisfactory? \$\endgroup\$ – Dzarda Jan 17 '14 at 21:42
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    \$\begingroup\$ A MOSFET like the IRLML2502 will comfortably handle 2-3 Amperes, and is easily switchable directly from the 555 output, and with voltages as low as 3 Volts or lower. No additional drive BJT required. \$\endgroup\$ – Anindo Ghosh Jan 18 '14 at 7:17
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The 555 output can Source or Sink 200 mA so unless you want to drive the mosfet in high frequencies it will be more than enough. A single transistor is probably going to drive the mosfet worse than the 555 itself

enter image description here

If you are need to use a driver between the mosfet and 555 then you are better off with a push-pull (totem pole) driver configuration that can sink/source current equally strong in order to turn on/off the mosfet quickly (again only if needed)

enter image description here

or even better with a dedicated mosfet driver like TC4426/27/28

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  • \$\begingroup\$ Thank you for these suggestions! What would you define high frequencies? \$\endgroup\$ – Pete Oct 22 at 20:53
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You could do it this way:

https://arduinodiy.files.wordpress.com/2012/05/fet2.gif

Just hook up the outgoing end of R2 (where it says "To Arduino") to your 555 output. Note that this circuit is inverted - the load will be ON when the 555 input is LOW. If that is a problem, just put a inverter (74HC04 or similar) in between your 555 output and R2.

If you have what's known as a "logic level MOSFET", you don't need the NPN transistor because logic level MOSFETS can be switched by a lower voltage such as the one provided by the 555. Usually these will have the letter "L" in the component name (for example IRL540). If it's a logic level MOSFET, it should clearly say that in the datasheet.

If it's not a logic level MOSFET, then you need to use another component to switch a higher voltage (12V in this case). NPN transistors work just fine for this purpose. Regular MOSFETs typically need close to 10-12V to switch on (notice that MOSFETs are voltage-driven and not current-driven).

If you'd like to read more about driving MOSFETs, I'd recomming the following reading:

  1. http://arduinodiy.wordpress.com/2012/05/02/using-mosfets-with-ttl-levels/
  2. http://blog.oscarliang.net/how-to-use-mosfet-beginner-tutorial/
  3. http://www.electronics-tutorials.ws/transistor/tran_7.html

EDIT:

Check out Switching a current with an NPN transistor and a P-MOSFET .

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