MOSFETs' source-drain resistance

[BlackMelon]

Hello,

First of all, my analog multimeter can turn an LED on by putting a - lead to the anode and a + lead to the cathode, and can't do so if I swap them. As a consequence, I just remember that the + lead and - lead are actual + and - only when they measure a voltage of a load, which receives power from another voltage source. On the contrary, in powering an LED on, the analog meter is acting like a voltage source, so I the - lead is, in fact, the + of a voltage source and must be connected to the anode.

My first question is that is my understanding correct?

Second, I put my - lead to the source and + lead to the drain of a newly bought MOSFET without any uses before. This reads 5 ohms. Subsequently, I swap those leads and get 200k reading.

My second question is that why the resistance in the first measurement is so low, even I've shorted gate-source together once before.

Thank you
BlackMelon

 

[hevans1944]

There is a body diode "built in" to the MOSFET. You are reading the forward-biased conduction of this diode when the drain is negative with respect to the source for N-channel MOSFET. The opposite polarity is true for P-channel MOSFET. The actual source-to-drain resistance is quite large because enhancement mode MOSFET requires a substantial gate-to-source voltage to turn the conducting channel on. The body diode is in parallel with this channel, so whether the gate is connected to the source or not does not affect the channel conduction... unless you deliberately charge up the gate-to-source capacitance with an external voltage source.

You are correct about ohms function polarity being reversed on some meters. I always use another meter's voltage function to check ohms polarity before using a new multimeter, although those (usually digital) meters that have a diode test function will normally use the red lead for the anode and the black lead for the cathode, measuring and displaying the forward voltage drop across the diode. Handy for sorting silicon versus germanium diodes and transistors... well it would have been handy fifty years ago if available then. Don't often see germanium devices anymore.

 

[BlackMelon]

Oh! That was very clear! Thank you very much!

About the body diode, is it for countering an inductor kickback?

In H-bridge configuration, there are drain-to-source diodes at 4 MOSFETs, and also one across an inductive load. Totally, there are 5 diodes. Can I minimize parts by just having a diode across an inductive load?

Thank you

 

[hevans1944]

Oh! That was very clear! Thank you very much!

About the body diode, is it for countering an inductor kickback?

No. The MOSFET body diode is intrinsic to its construction.

... In H-bridge configuration, there are drain-to-source diodes at 4 MOSFETs, and also one across an inductive load. Totally, there are 5 diodes. Can I minimize parts by just having a diode across an inductive load?

Thank you

Some MOSFETs rate their body diode to counter inductor back emf when the MOSFET turns off, otherwise you would have to provide it separately. In an H-bridge configuration current flows both directions through the inductive load (typically a motor winding), so a diode across the inductive load would not be effective. If you think this not to be true, please upload a schematic of an H-bridge MOSFET circuit with a diode across an inductive load. Please provide the source of the schematic.

 

[dorke]

Oh! That was very clear! Thank you very much!

About the body diode, is it for countering an inductor kickback?

In H-bridge configuration, there are drain-to-source diodes at 4 MOSFETs, and also one across an inductive load. Totally, there are 5 diodes. Can I minimize parts by just having a diode across an inductive load?

Thank you

NO,
What you need is the 4 diodes.
It may be the case that they are the MOSFET internal ones...