I have a logic circuit I need to switch a power mosfet.
It is not working at the moment as the logic only produces about 2.5-3V at its output.
The mosfet needs to be big enough to handle about 400mA (many LEDs). I have some IRF520s around which would be used, ideally.
The logic is running off of a 3.3V voltage regulator, from a 12V supply which is also going to power the LEDs.
What is the simplest solution? Mosfet driver? Special logic-level power mosfet?
Many thanks for any replies.
How fast does it need to switch ?
You can make your own 'high side driver' btw. Use a 555 or whatever to boost your supply and configure a couple of transistors as the driver.
Graham
Switching is slow - 30Hz.
I don't follow your 555 idea. Isn't 555 a timer? The switching is synched to a pulse controlling the logic. I probably don't understand what you're getting at.
Thanks!
I'm trying to find a mosfet that will do this. I'd rather not use an additional component. Should I consider signal fets as well??
snipped-for-privacy@yahoo.com wrote:
The recommendation is to use the 555 to produce a higher voltage which then is used to drive the mosfet gate.
Are you locked into a mosfet? You could use a bipolar.
Ed
If you MUST use a MOSFET, try the NDS331N (good for > 1 A, SOT-23 package) or any other logic level gate threshold MOSFET. If you insist on using a high threshold MOSFETsuch as the IRF520, then you have to use simple logic level shifters which can implemented with one or two transistors (bipolar or MOSFET). This will transform the 0-3 V signal to a 0-12 V signal which can easily drive the gate.
Mario C.
snipped-for-privacy@yahoo.com wrote:
Mario,
I do not necessarily need to use a MOSFET. What would you recommend instead? Darlington pair?
Thanks a lot!
A Darlington pait would be a good choice for this application to replace a logic-level MOSFET: it requires roughly 1.5 V to turn on and requires very little input current to saturate at 400 mA. Use an adequate base current limiting resistor. The only disadvantage over the MOSFET is that it will saturate at 0.7 V (Vce) whereas the MOSFET will exhibit a lower-than-Ohm resistance (for ex. 0.2 Ohms * 400 mA = 0.08 Vds).
Mario C.
snipped-for-privacy@yahoo.com wrote:
I ended up finding another solution! I put a pull up resistor to the
12V source at the output of the logic and now i have the full 0-12V swing!Thanks everyone
View in a fixed-width font such as Courier.
. . . +12V . | . .--------+------------- . | | . | | . [1K] | . | |< . +------| PNP . | |\\ ----->TO LEDS . [5.6K] | | . | | | . |/ | |- . >--[10K]--| NPN +----| . |> | |- . | [1K] | . | | | . '--------+------+-------.. . | . --- . /// .
This will probably work better and saves a resistor: View in a fixed-width font such as Courier.
. . . +12V . | . .--------+------------- . | | . | | . [1K] | . | |< . +------| PNP . | |\\ ----->TO LEDS . | | | . | | | . |/ | |- . >---| NPN +----| . |> | |- . | | | . [1.2K] [1K] | . | | | . '--------+------+-------.. . | . --- . ///
On Wed, 31 May 2006 12:28:46 -0700, sansampbass top-posted:
Is there some special reason you're constrained to MOSFET? Why not just a darlington?
Good Luck! Rich
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