# logic level mosfet resistance

Discussion in 'Electronic Design' started by John O'Flaherty, Jul 31, 2006.

1. ### John O'FlahertyGuest

For a logic level mosfet with RDSon specified at 0.09 ohms at about
10a, with VGS at 5V, will it act as a .09 ohm resistor even for low
currents, like 10 to 100mA, giving a drop of 1 to 10 mV, or is there a
voltage burden? The data sheets that have graphs don't show this area
very well.

2. ### colinGuest

yes, the data sheets usualy show the V/I curve is equal to rdson below some
current and it does go all the way down to zero.

Colin =^.^=

3. ### Rich GriseGuest

I just wonder why you're using a 10A MOSFET at 10 to 100 mA.

Thanks,
Rich

4. ### Andrew HolmeGuest

No. Rds is non-linear.

See http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm

This link is from an article about using a JFET as a voltage-variable
resistor in an audio compressor; however, what it says about Rds linearity
could be applied to MOSFETs.

5. ### Gary ReichlingerGuest

I would expect it to act like a resistor at currents below
maximum and a fixed VGS. However, if the answer was important to your
circuit, I would get a voltmeter and a few selected resistors and
actually test it. Just put a resistor in series with the mosfet to
limit the current to your desired level and measure the voltage across
the mosfet.

6. ### John O'FlahertyGuest

I want the low RDSon because I'd like to measure a single ground
referenced voltage and be able to calculate the resistance of a load
and the power delivered to it. With this arrangement...

+5V Regulated
+
|
|
.-.
| |
'-'
|
|------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

the load current is the measured voltage divided by 10 ohms, and the
load voltage is 5V-the measured voltage. The measurements will be taken
on the fly.

If there's a low power unit that still has low RDSon, that would work
too. Any suggestions?

7. ### John O'FlahertyGuest

Thanks, Colin.

8. ### Tim WescottGuest

it you can just get any old logic-level FET. If you want to read the
voltage at the load you'll have to tap that separately, of course.

+5V Regulated
+
|
|
.-.
| |
'-'
|
|
||-+
Duty Cycle ||<-
-------||-+
Control |
o------------
| measure V
.-.
| |
| |10 ohm
'-'
|
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

9. ### JoergGuest

Hello John,
Can't you use a mux and also measure the voltage at the drain? Then a
subtraction would yield the voltage across the 10ohm resistor.

10. ### John O'FlahertyGuest

I did want to get by with a single measurement to get all the load
information- I,V, Power. Plus, with the FET ground referenced, I'll be
better off by about a volt or so on the drive voltage (for the higher
end of the current range). I found the IRF3711 for \$1.11 that has 5.9
mOhm (typical) RDSon at 4.5V gate drive, so that will only be 0.059% of
the 10 ohms.

11. ### John O'FlahertyGuest

Well, if everything goes according to plan, I'll be able to use a
single measurement, saving the trouble of multiplexing and the time to
make the second measurement. It should work ok if I can rely on <10
mOhm resistance even at lower current levels.

12. ### Rich GriseGuest

If you're saturating the thing, why not put the current sense resistor
in the source lead and get a single-ended measurement?

Good Luck!
Rich

13. ### colinGuest

You can get current sense fets if all you want to do is measure current -
they have a fourth terminal wich is conected to a current sense resistor
through wich a small part of the current flows.

However the on resistance of a normal FET rises with voltage/current,
therefore it is lowest at zero volts, some SMPS controller ICs actualy use
the VDS to monitor the current.

How accurate do you want to be ?

Colin =^.^=

14. ### John O'FlahertyGuest

My hope was to get V, I, R and P of the load with a single measurement.
I should be able to do that within the needed precision with the
circuit I had, and have 1 volt or so of extra drive to VGS, as long as
the RDS is in the area of 10mOhms, and if it remains negligible over a
range of currents from, say, 5mA to 100mA. The RDS could even be higher
and be calibrated out, unless it's non-linear and/or temperature
sensitive. I may not even need to worry about those factors if I start
wiith the lowest RDS possible. I found a fet with <10mOhm for a little
over \$1, so I'm happy with the way I'm going; I just wasn't sure if the
simple resistance remained valid at low current levels. The consensus
seems to be that it does.

15. ### John O'FlahertyGuest

Not just current . I want load current, voltage, power and
resistance with a single voltage measurement, since my circuit will be
lazy as sin.
I'll be digitizing with 10 bits, so anything better than 0.1% would be
a waste. With a power fet running a maximum of 100mA, with 10mV RDS and
at about 500Hz, endogenous heating won't be a problem! I hadn't heard
about current-sensing FETS; thanks for mentioning them.