Only 3.5V output in MOSFET logic gate with 9V supply?!

[AxelanDerGrosse]

Hello there,

recently I've been building an logic XOR-gate with a few N-channel MOSFETS, a few resistors and two push buttons. Everything was working fine when I put a LED on the output; It only lighted up when one button was pushed. But when I tested the Voltage across the resistor and LED with a multimeter, I saw that there were only 3.5 Volts.

The voltage seems to drop across R1.

Can anyone tell me why?
Thanks, AxelanDerGrosse

 

[Akanimo]

Ideally, voltage across an LED is fixed. I guess you measured it across the LED. A blue LED can have a forward voltage drop of 3.5 V.

 

[KlausST]

Hi,

some issues:
* never leave MOSFET´s GATEs floating with respect to it´s SOURCE
* always drive MOSFET´s GATEs with respect to it´s SOURCE. The datasheet specification is V_GS, which means: Voltage between GATE and SOURCE

The absolute voltage (w.r.t. GND) is NOT of interest when drivng MOSFETs.



Klaus

 

[FvM]

The voltage seems to drop across R1.

Can anyone tell me why?

Just expectable. Maximal unloaded 'H' output voltage of this very simple logic gate is about 7 V according to gate threshold voltage, above 2 mA output current, voltage will further drop.

You didn't tell about load resistance.