logic level mosfet resistance

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 =^.^=

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

Thanks, Rich

No. Rds is non-linear.

See

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.

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.

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...

Thanks, Colin.

-- john

The following will be insensitive to RDSon. Instead of worrying about 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.

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.

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.

-- john

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

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

taken

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 =^.^=

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

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.

-- john