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Low voltage power switching

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.
 
P

Pooh Bear

Jan 1, 1970
0
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?

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
 
Pooh said:
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??
 
E

ehsjr

Jan 1, 1970
0
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!

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
 
M

Mario Chenier

Jan 1, 1970
0
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.
 
Mario said:
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,

I do not necessarily need to use a MOSFET. What would you recommend
instead? Darlington pair?

Thanks a lot!
 
M

Mario Chenier

Jan 1, 1970
0
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.
 
Mario said:
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).


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
 
F

Fred Bloggs

Jan 1, 1970
0
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.
 
F

Fred Bloggs

Jan 1, 1970
0
This will probably work better and saves a resistor:
View in a fixed-width font such as Courier.

..
 
R

Rich Grise

Jan 1, 1970
0
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??

Is there some special reason you're constrained to MOSFET? Why not just
a darlington?

Good Luck!
Rich
 
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