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I measure in common source connected mosfet (19N10L) Vds about 2.4V in on state. The current is 2A.

In datasheet is defined Rds 0.1ohm. Why is Vds in this mosfet so high (2.4V)? Shouldnt be Vds = 0.1 x 2A = 0.2V?

Vcc is 18V.

enter image description here

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  • \$\begingroup\$ What is the value of Vgs when you are making your measurement? The RDSon is specified at Vgs=10V. Vth is as high as 4V, so if you are not providing the full 10V gate drive your RDSon could be much higher. \$\endgroup\$ – John D Jan 5 at 23:25
  • \$\begingroup\$ Vgs is 18V. Gate is connected to ground with 12K resistor. Supply voltage is also 18V. \$\endgroup\$ – Michal Podmanický Jan 5 at 23:28
  • \$\begingroup\$ Maybe add a schematic of your exact test circuit. It's possible you just have a damaged FET or the wrong FET too. \$\endgroup\$ – John D Jan 5 at 23:37
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Rds = 100mOhm @ Vgs = 10V

https://www.fairchildsemi.com/datasheets/FQ/FQD19N10L.pdf

Are you driving the gate voltage high enough? If Vgs is not high enough Rds will be higher.

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The FQD19N10L is a logic-gate MOSFET, and will turn on completely with only a few volts on the gate. Rated current is nearly 16 amps, so 2 amps is not even close to the device limit.

Maximum gate voltage is 20 volts, so 18 volts is not an issue.

There is one possibility I can think of. Are you sure you have not connected your circuit like

schematic

simulate this circuit – Schematic created using CircuitLab

Then 2.4 volts is a reasonable number, being about the gate voltage a logic-level device needs.

If so, you need to change your circuit to

schematic

simulate this circuit

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  • \$\begingroup\$ I am sure, the load is in Drain. \$\endgroup\$ – Michal Podmanický Jan 6 at 0:07
  • \$\begingroup\$ @MichalPodmanický - then you have a bad FET. \$\endgroup\$ – WhatRoughBeast Jan 6 at 0:07
  • \$\begingroup\$ Its possible, it is not a new one, its desoldred from LCD monitor made in about 2004 year. I am thinking, if mosfets in older ages shouldnt be worst in parameters, so the 2.4V was ok. \$\endgroup\$ – Michal Podmanický Jan 6 at 0:11

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