1
\$\begingroup\$

The task is to power 3 servos each require 500mA at 5V.

I'm just trying to learn about electronics. I think I made a mistake and purchased a IRF630.

When I posted over on the Arduino Forum they asked me if it was a logic level mosfet. I think there is some math to do to figure that out I'm not sure how.

I came to this forum and read that I should be using FQP30N06L mosfet.

Also with the first mosfet its an N channel and I think that switches Neg side Would I not want a P channel to switch the pos side ?

Thanks

\$\endgroup\$
8
  • 2
    \$\begingroup\$ Commas help. Try them. \$\endgroup\$
    – user76844
    Commented Jul 21, 2018 at 15:23
  • 2
    \$\begingroup\$ @LongPham actually you usually do need to do math, as data sheets don't tend to explicitly state what gate voltage is needed to switch a 5v circuit with little loss. You're left to read it off a drain current vs. applied voltage graph with curves for various gate drive. Typically Vth is the gate drive to pass micro-amps, to pass a couple of amps with low loss in practice you often need something like 2-3 times the specified Vth applied to the gate. Something like DMN2041L (NFET) or DMP2035U (PFET) could be applicable but they are surface mount. Some use USB switch chips for servo power. \$\endgroup\$ Commented Jul 21, 2018 at 16:35
  • 1
    \$\begingroup\$ Possibly, the figure 3 makes it look like you're under 40 millihoms with 5v gate drive. However, you still have some awkwardness in using a low side switch vs. a ground referenced signal. It may be workable but you'd probably need to make sure the PWM command signals are high or floating when the FET is off, and a series resistor on the PWM might be a good idea. Also make sure that your servo power supply about matches the Arduino's 5v, but do not power the servo from the Arduino. \$\endgroup\$ Commented Jul 21, 2018 at 17:35
  • 1
    \$\begingroup\$ Again, Vgs is a condition of measurement - ie, a voltage between gate and source. It is not a specification. A specification would be the Vgs needed to achieve some performance or in the maximum limit. \$\endgroup\$ Commented Jul 21, 2018 at 21:05
  • 1
    \$\begingroup\$ what is the purpose of the mosfet? what servo? many just want to see a logic signal. \$\endgroup\$ Commented Jul 21, 2018 at 21:13

1 Answer 1

1
\$\begingroup\$

The relevant question is: with the V_DS you'll be using and the V_GS that your Arduino will supply, will the MOSFET you have conduct enough current for the device you want it to power?

Fig 1 on the datasheet of the IRF630 is the most relevant part of it. Now, if you look at the Arduino web site, it'll tell you that the output voltage on the GPIO pins when set to high is 5V. Don't believe a word of it ... the actual figure you should use is the one that comes from the ATmega328's datasheet (p365), and that's 4.2V. You'll probably get a bit higher than that, because the current you'll be using is lower than the current the chip is tested for, but there's little in the way of guarantee. Assuming there's a little space to play with, I'll assume you actually get 4.5V of output.

Fig 1 of the MOSFET datasheet shows curves for different gate voltages. The one at the bottom is 4.5 (indicating that 4.5V is just barely enough to cause the MOSFET to switch on). For any V_DS > 1V the output is essentially flat, and is roughly 700mA.

So, the answer to your primary question is, no, this MOSFET cannot be used to power 3 x 500mA servos. You could use one per servo, and just barely get away with it, but you'd be much better off using a MOSFET that switches at low voltage.

Answering your other question, what constitutes a "logic level" MOSFET: the term is a bit vague, but generally it refers to one where the output current curve is close to the highest it can be for V_GS figures that are substantially lower than minimum output voltages from a logic gate. That depends on what kind of logic gates you're using, and what voltage you're running them on, but it would generally be somewhere in the region of 2.5V-3V.

You also suggest an alternative of a FQP30N06L. Looking at the equivalent diagram for that MOSFET, note that the bottom line on the chart is for 3V... it doesn't work particularly well for 3V (although it still manages to handle 10A, but that's substantially lower than it can manage for just slightly higher gate voltages), but for 3.5V it's doing reasonably well, and by 4.5V it's managing to handle approximately 14A for V_DS > 1.2V. This will work fine for your application.

\$\endgroup\$
4
  • \$\begingroup\$ @ChrisStratton - good point. :) \$\endgroup\$
    – Jules
    Commented Jul 21, 2018 at 21:24
  • \$\begingroup\$ Thanks Jules that's a good explanation. I will say to a newbie those graphs aren't the easiest to interpret but I think I know what to look for. I have two follow up questions one about the spec in the data sheet its the Vgs spec. Chris said "A specification would be the Vgs needed to achieve some performance" With the FQP30N06L it says 20. Im still not clear on the (plus or Minus 20v) to me that would mean the mosfet would need 20v at the gate to run at optimum performance. \$\endgroup\$ Commented Jul 22, 2018 at 15:34
  • \$\begingroup\$ Two when someone needs to find a mosfet for a project how does that person go about looking for the prober one. Is there a web site somewhere with mosfets and there specs \$\endgroup\$ Commented Jul 22, 2018 at 15:34
  • \$\begingroup\$ Sorry I lied one more question with this mosfet does it matter if the load is feed on the source side of the mosfet or the drain side \$\endgroup\$ Commented Jul 22, 2018 at 15:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.