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### Topic: Measuring MOSFET Transistor Resistance (RDS) (Read 2419 times)previous topic - next topic

#### rasmasyean

##### Nov 08, 2017, 09:38 amLast Edit: Nov 08, 2017, 09:46 am by rasmasyean
So I tried to use a popular FQP30N06L to turn on a 6V motor.

When I make VGS = 5V:
I = 0.09 A         (reading from power supply)
VDS = 0.003 V  (reading from multimeter)
RDS = 4 ohm    (reading from multimeter)

Now this look like it's working fine.  In fact if you calculate RDS,
VDS / I = 0.033   (~ 0.035 ohm advertised in the spec sheet).

BUT...taking an actual resistance measurement from the multimeter says RDS = 4 ohm!

This similar phenomenon is observed in 2 different MOSFET's I tried as well.

What is this "RDS" that they advertise in the specsheet?  Is this an actual resistance that can be measured directly?  Or is it some sort of "virtual resistance" that would be equivalent to how your power system would behave if you substitute the MOSFET with an actual resistor for that particular scenario?

Thanks!

#### jackrae

#1
##### Nov 08, 2017, 10:09 am
It's the resistance to the current flow during actual operating conditions.  Measure DS voltage and current flow to derive the RDS value.

#### TomGeorge

#2
##### Nov 08, 2017, 11:05 am
Hi,
Quote
BUT...taking an actual resistance measurement from the multimeter says RDS = 4 ohm!
And that is why its different.
RDSon varies with Vds  and Ids.

Your DMM is applying a different Vds voltage to the voltage from your circuit.

How did you use you DMM?
Can you post a diagram of the connections to the MOSFET.
Did you disconnect the power supply from drain and source?

Your in-circuit V and I measurements are the most accurate, because they are made under proper circuit conditions.

Tom...

Everything runs on smoke, let the smoke out, it stops running....

#### MarkT

#3
##### Nov 08, 2017, 11:56 amLast Edit: Nov 08, 2017, 11:56 am by MarkT
Rds(on) is basically constant and independent of Vds and Ids.  Assuming the device is fully switched
on.  This is the "ohmic" region of the device.  So if you have 5V between gate and source, the resistance
from drain to source should be about 0.035 ohms.  Its very constant if you stay well within the device's
maximum ratings.  (This means Vds is much less than 1V typically)

From the readings in the OP, Vds = 3mV, Ids = 90mA, so Rds(on) = 0.03 ohm, not 4.  That multimeter
is not working on low ohms (most don't).

To measure low resistances you simply measure current and voltage separately and divide.  This is called
a Kelvin measurement (or 4-wire), and takes 2 multmeters or one that you move around between V and I
measurements.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### jackrae

#4
##### Nov 09, 2017, 12:11 am
From the readings in the OP, Vds = 3mV, Ids = 90mA, so Rds(on) = 0.03 ohm, not 4.  That multimeter
is not working on low ohms (most don't).

The OP has inadvertently inserted the ohmic reading into the 'operational' results list.  So his 4 ohms measurement simply relates to what his DVM showed when connected D to S under inert conditions.  And hopefully he now understands why he cannot measure RDS in this manner

#### rasmasyean

#5
##### Nov 15, 2017, 06:01 amLast Edit: Nov 15, 2017, 06:06 am by rasmasyean
In the first post:
I connected the motor between 6V and D.
S was tied to ground.
G was varied from 0-5V by the power supply.
And I took measurements when the motor was ON.

As a new experiment:
I took the motor off and tried to just measure RDS without anything connected to it.
At VGS = 5V ---> RDS = 0.18 ohm

So it isn't exactly close to 0.035 ohm either.  Yet closer than before.

I also tried an IRF510 (0.54 ohm advertised) and I got:
At VGS = 5V ---> RDS = 0.58 ohm

So that's pretty close.

Is this what you mean by most Multimeter's can't measure low R?  As R decreases, the accuracy decreases?

Also...is the reason why my "Motor-ON RDS reading" was so much higher, because the motor circuit used up most of the charge carriers in the field effected channel, thus leaving only a small amount available for the Multimeter to push through while it pulses?  Thus, perhaps it's interpreting this result as a "higher resistance" than normal?

#### MarkT

#6
##### Nov 15, 2017, 02:01 pm
In the first post:
I connected the motor between 6V and D.
S was tied to ground.
G was varied from 0-5V by the power supply.
And I took measurements when the motor was ON.

As a new experiment:
I took the motor off and tried to just measure RDS without anything connected to it.
At VGS = 5V ---> RDS = 0.18 ohm

So it isn't exactly close to 0.035 ohm either.  Yet closer than before.
I'm betting that's mainly the resistance of your multimeter leads - you need to use a 4-terminal
measurement (Kelvin connection) for low resistance values - that takes 2 multimeters.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### rasmasyean

#7
##### Nov 29, 2017, 02:03 am
I'm betting that's mainly the resistance of your multimeter leads - you need to use a 4-terminal
measurement (Kelvin connection) for low resistance values - that takes 2 multimeters.
Yes, you are right.  It looks like when I connect the sensing terminals together ( each via 54" alligator clip wire + 2" solid breadboard wire) I get around 0.15 ohms.  So adding the advertised 0.035 ohms would give the reading of ~0.18 ohms.

#### rasmasyean

#8
##### Dec 06, 2017, 03:02 am
FYI, I did the experiment with the 2 multimeters:
One connected between a power supply and D (the force)
One connected across D and S (the sense).

I hope I did it right.

---

Setting the power supply to deliver:
IDS = 0.092 A

VDS = 0.0033 V

Thus calculating gives:
RDS = 0.036 ohm

---

Setting the power supply to deliver:
IDS = 0.195 A

VDS = 0.0070 V

Thus calculating gives:
RDS = 0.036 ohm

Is this how you normally do a Kelvin Measurement?

#### Southpark

#9
##### Dec 06, 2017, 07:14 am
Is this how you normally do a Kelvin Measurement?
One quick way to find out is 'google'. Eg. google : 'four wire resistance measurement'.

#### larryd

#10
##### Dec 06, 2017, 07:57 am
One quick way to find out is 'google'. Eg. google : 'four wire resistance measurement'.

No technical PMs.