I have built the following to dim the brightness of the LED backlight of a display: I am using a BS170 MOSFET (the diode is no extra part but included in the MOSFET, according to the datasheet). The BL pin is the supply of the LED backlight. (I have left out all other pins.)
Unfortunately, it does not work. Even if I set the PWM pin to 255, the backlight does not light (actually it's completely off). If I short BL and Vcc, it lights up as it should.
Also, the voltage readings confuse me: I measure 3.3 V on the gate and drain against ground, 1.5 V on source against ground (= the voltage across the backlight) but only 1 V if I measure drain against source.
So my questions are:
- Shouldn't this add up to 3.3 V?
- More importantly, why do I get only 1.5 V at BL?
- Most importantly, what do I need to change? Different MOSFET? No MOSFET?
The primary aim of controlling the backlight is to be able to switch in off completely, thus reducing power consumption. (Being able to dim it is only a plus.)
I should have added details about the display. It's one of those:
I haven't found a datasheet / specs for exactly this model. Most datasheets I found refer to a different version that is not 5 V compatible. A very similar model is described here.
Concerning my question I haven't found information on how much current the backlight would draw. Therefore I didn't want to drive it directly from the PWM pin. (Actually I could just measure it...)