# Does the MOSFET Rds(on) value really matter?

I am using MOSFETs in the ubiquitous RAMPS 1.4 Arduino shield for 3D printing use.

The MOSFETs switch the voltage to the heated bed and the hotend (where the filament comes out).

I need to replace one of the original MOSFETs, which is a STMicroelectronics STP55NF06L (datasheet). I chose to replace it with a TI CSD18532KCS (datasheet). Same TO-220 package, but I don't know if it will work properly.

So here are the specs for the STP55NF06L:

And here are the specs for the CSD18532KCS:

It appears that for most values, the two MOSFETs are very similar. But for RDS(on) (highlighted in red), the test characteristics are different, and the results are different. It's not a big difference (RDS(on) equals about 20 milliohms for the ST chip and about 4.5 milliohms for the TI chip).

I understand that RDS(on) is the resistance of the MOSFET when the gate-source voltage is at the sample VGS level. Because the difference between the two MOSFETS in RDS(on) is so minor, this shouldn't be an issue, right? The circuit isn't precise to more than 2 or 3 degrees of C for the heater cartridges anyway, so I believe it won't make a difference.

Are these MOSFETS interchangeable?

• The difference between 20 milliohms and 4.5 milliohms is more than a factor of 4. Don't dismiss it as "not a big difference". – Pete Becker Mar 2 '15 at 14:39

$R_{ds(ON)}$ would matter in case of huge currents (like 50A). Otherwise it should not matter in your case.