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I'm using a DAC to control the voltage at the source pin of BS170 N-Channel MOSFET. The purpose is to control the current (and therefore brightness) of a LED while the MOSFET has the gate and drain connected to act as a resistor.

MOSFET has the gate and drain connected to act as a resistor

The DAC operates as expected while the voltage is high (highest DAC voltage for the chip is 3.29V). However, the issue I'm facing is that the DAC is not driving the voltage at the transistor source below approximately 1.4V. Additional information: the board being used is the Nucleo-L432KC; the DAC is buffered so its driven by an output op-amp; max sink current for the DAC is 20mA (chip datasheet).

I'm using this circuit as an experiment, and I understand that the LED current can be controlled by connecting the DAC to the transistor gate, or even using PWM to control the LED brightness. I'm curious to know what might be causing the DAC to be unable to drive this source voltage below 1.4V.

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    \$\begingroup\$ "while the MOSFET has the gate and drain connected to act as a resistor". This will make your DAC sink current. This might fry your DAC, as DACs are supposed to output/give current, not sink it! \$\endgroup\$
    – Christianidis Vasilis
    Commented Mar 31, 2022 at 8:03
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    \$\begingroup\$ Also, your DAC is 3.3V, while you will try to sink a maximum of 5V into it. Again, a issue for the DAC, it will probably fry it. \$\endgroup\$
    – Christianidis Vasilis
    Commented Mar 31, 2022 at 10:08

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You have connected the MOSFET as a "diode-connected MOSFET" with a diode voltage equal to the MOSFET's threshold voltage. More detail here.

As a result, pulling the source lower than the combined drops of D1 and Q1 below VDD1 will sink a lot of current.

As the sinking capability of the DAC is rather limited, you see a lower limit in the source voltage.

I guess, what you actually want to do is connect the DAC to the gate and connect the source to GND. This will allow you to control D1 current over a (very, potentially harmfully) large range, but won't be very stable as it lacks feedback control.

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  • \$\begingroup\$ Thanks for the explanation, I did not consider the diode component of the MOSFET. I know the best method is to use the DAC on the transistor gate to control the current, but using a feedback control would be a very good idea. What would be a good place to start for that? \$\endgroup\$
    – calvinjarrod
    Commented Mar 31, 2022 at 17:21

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