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Constant current source for LED driver with MOSFET control
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pigtwo:
Hello all,

I'm working on a little circuit where I need to drive an LED from a variable voltage source.  I also need to turn the LED off with a MOSFET.  The input voltage can range between 18V and 36V.  I want to drive the LED with 7mA.  Then I will have a 5 volt signal driving the MOSFET to turn the source on or off. 

I came up with the attached circuit but I'm not great with analog circuits so I might be missing something.  The MOSFET is DMN5L06K-7 and can't be changed.  The zener diode(4.7v) and NPN transistor are generic and can be changed.  Is there anything inherently wrong with this?  I've simulated it and it seems to work but I'm worried I'm overlooking something.

Thank you!
Neilm:
I would have thought that the 5V signal on the FET will not work. The FET is up at 4.7V so you don't have any headroom to turn it on unless your reference is with respect to the 4.7V zener.

I would look at a small 3 terminal regulator that could be configured as a constant current source and use the FET to connect it to 0V when you want it turned on.
hamster_nz:
If precise current isn't important, consider using two ordinary diodes in series place of the zener, and replacing the 480 resistor with one around 70 ohms.

It will also lower the votage required to turn the MOSFET fully on my about 3.4 V or so.

0xfede:

--- Quote from: Neilm on April 27, 2018, 09:20:54 am ---I would have thought that the 5V signal on the FET will not work. The FET is up at 4.7V so you don't have any headroom to turn it on unless your reference is with respect to the 4.7V zener.

I would look at a small 3 terminal regulator that could be configured as a constant current source and use the FET to connect it to 0V when you want it turned on.

--- End quote ---

The circuit will work as intended. It should be considered as a constant current generator circuit enabled by the mosfet. I've attached the simulation screenshot (since I'm lazy for the NMOS I used the NDC7002N that is similar to the one chosen by the OP) and the LTSPICE files.

Best,
0xfede
Zero999:

--- Quote from: 0xfede on April 27, 2018, 09:35:51 am ---
--- Quote from: Neilm on April 27, 2018, 09:20:54 am ---I would have thought that the 5V signal on the FET will not work. The FET is up at 4.7V so you don't have any headroom to turn it on unless your reference is with respect to the 4.7V zener.

I would look at a small 3 terminal regulator that could be configured as a constant current source and use the FET to connect it to 0V when you want it turned on.

--- End quote ---

The circuit will work as intended. It should be considered as a constant current generator circuit enabled by the mosfet. I've attached the simulation screenshot (since I'm lazy for the NMOS I used the NDC7002N that is similar to the one chosen by the OP) and the LTSPICE files.

Best,
0xfede

--- End quote ---
The zener diode isn't doing anything. Look at the voltage across it and current flowing through it. It's way below the zener turn on/threshold voltage.

If the 5V logic supply is regulated, then remove the MOSFET and zener and it will work perfectly. Otherwise, use a lower voltage zener, with a suitable resistor, powered from the 5V control signal.

By the way, just upload the .asc file. No need for the other files in a zip.
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