# Can I guess the Rds(on) at 3.3 V from this MOSFET datasheet?

I ordered some MOSFET switch modules. Because they were advertised as "for Arduino" and "Voltage: 3.3V, 5V", I assumed they had chosen a decent MOSFET for the job (like the IRLU7843 or at least the IRLZ34N). There is no additional circuitry on the modules.

Now I see that they use the IRF520, which struck me as odd, as I remember them to be a bad fit for a microcontroller project.

Here's a datasheet. Is there a way to guess the RDS(on) at VGS = 3.3 V or 4.5 V from it?

• (1) You're right when you say that IRF520 doesn't make a great logic level MOSFET. (2) When you say "$R_{ds(on)}$ at 4.5 V", do you mean $V_{DS} = V_{GS} =$ 4.5V ? – Nick Alexeev Jun 20 '18 at 7:35
• Guessing is not required as the information is in the graphs. It also tells you, you need at least 4V GS so forget about 3V3. – Oldfart Jun 20 '18 at 7:41
• @NickAlexeev Yes, at least $V_{GS}$. $V_{DS}$ usually doesn't matter that much, does it? – AndreKR Jun 20 '18 at 7:41
• @Oldfart Oh, Fig. 3, I completely missed that. You want to add an answer? – AndreKR Jun 20 '18 at 7:44
• I ordered some MOSFET switch modules. Because they were advertised as "for Arduino" - It seems like every other thingy on Ebay/DX is advertised as "for Arduino", even if it's something like a bare LED (no series resistor). The "for Arduino" qualification is bordering on useless on those sites, at least. – marcelm Jun 20 '18 at 10:35

## 2 Answers

Figure 3 show that ID starts at 0 at VGS of 4Volts. The specifications lists the threshold VGS(th) as 2.0...4.0V so I assume the graph is for the 4V worst case. No operation at 3V3!

Assuming you want to use it as a switch, figure 1 shows ID is 1 Amp at VGS and VDS around 4.5V so that would mean a RD(on) of about 4.5 Ohms.

If you look at graph 1 you get the drain current per drain-source voltage for each gate-source voltage. The fact that Vgs 4.5V already limits the Id to 1A, means the Rdson is high.
You will need a gate driver.