Using MOSFETS with TTL levels (5 Volt and 3.3 Volt)

FETVarious Arduino projects that need to switch a high DC load are using MOSFET’s to do this, according to the circuit at the right (R1 is optional and may be necessary to switch off the FET if the pin goes low.

Popular MOSFET’s that are used are the IRF510 and IRF 520

irf510 irf520
IRF510 IRF520

Looking at those graphs one can see that at a gate to source level of 5V (Arduino levels) the IRF510 is only capable of delivering 1 Amp, whereas it is specified for 5,6 Amps continuous current. The 520 is somewhat better: at 5 V it delivers 3 Amps from its max of 9.2. This is because these FET’s are designed to pass the max current at gate voltages of around 10 Volts and that is beyond what most microcontrollers can deliver.

For the IRF522 it is even worse.

irf522

Looking at the curve, at a gate to source voltage of 5V the IRF522 is hardly turned on. You are limited to a current of about 200mA. Much better to use a cheap Darlington transistor then.

The IRF530 is a better choice:
irf530

At 5V volt on the gate, the IRF530 will pass something around 4.5Amps.

If you are shopping for a MOSFET for the Arduino consider the IRL540 The L shows that is a logic level mosfet. A logic level mosfet means that it is designed to turn on fully from the logic level of a microprocessor. The standard mosfet (IRF series etc) is designed to run from 10V.

Here is the curve for the IRL540:

irl540

Now at 5V you are out of the linear region and the MOSFET can already deliver its specified 28 Amps continuous current.

You may also consider the IRLZ44.

The IRLZ44 data-sheet says that with a 3V TTL-level drive the FET drops less than 0.15V at 4A (at 25C, Rds(on) is about 0.04 ohms), and under 0.25V at 175C (Rds(on) < 0.063 ohms).
So we know the FET’s I^2-R ohmic dissipation will be under 1 watt, and that’s good. If we use Vgs = 4V, specified for AVR chip outputs, the dissipation should be about 0.4W at 25C (0.8W for Tj = 175C).

Wether a MOSFET is a standard MOSFET or a ‘logic’FET becomes clear from the Datasheet. If for instance you look at the Datasheet  of the IRFZ44N at the Rds(on), This lists the ‘on-resistance’ under the condition that Vgs=10V (and Id=25A). If there is no rating for Rds(on) when Vgs=5V (or 4.5V), then it is not a logic-level MOSFET. A logic level MOSFET will have Rds(on) specified for Vgs=5V or 4.5V.  If its only specified for Vgs=10V, its not logic-level.

Another thing to beware of in datasheets is Vthresh (threshhold voltage).  This is not the gate voltage to turn the device on, its the gate voltage at which it switches fully off (less than a few uA of current, typically).  If Vthresh is given as 2..4V range, it cannot be a logic level MOSFET (Vthresh is usually 0.5 to 1V for logic-level MOSFETs).

When designing with MOSFETs be aware that instead of having a Vsat like a bipolar transistor, a fully-saturated MOSFET acts as a low-value linear resistor.  If for instance you want to switch 5A in a 12V circuit and you only want to waste 0.5V across the MOSFET, then its on-resistance (Vds(on)) should be <= 0.1 ohms (0.5V / 5A)

The dissipation then is 5x5x0.1=2.5 Watt. But suppose the FET you choose has 0.05ohm Vds(on) and carries 10A then it will dissipate I^2R watts, ie 10x10x0.05 = 5W.  This will need a good heatsink if the load is on for more than a second or two, but it is no issue if it gets millisecond pulses every few seconds.  ‘ON-resistance’ of 0.2 to 0.001 ohms are available (though less than 0.005ohms gets expensive).

The relatively cheap BUZ11 is also an option. Although it is no Logic level MOSFET, it will go into saturation with a 5 Volt gate voltage  at around 7 amps and a VDS of about 0.5 to 1 Volt. But  it’s RDS(on) will be far from ideal and you will lose 3.5-7 Watts in the FET:
BUZ11

Should you however be stuck with a FET like the IRF522 that really needs a high voltage to switch efficiently then use the following circuit:
MOSFET

Realise though that it is an inverting circuit. A HIGH on the Output of the Arduino will switch the Load Off. Also the 520 and 510 will be more efficient with this circuit.

MOSFETwith low voltage input
Remember to use a heatsink for the MOSFET if you are using any serious loads
MOSFET Print

If you are using this circuit to switch any serious loads, then it is wise to solder some thick wire over the tracks coming from the MOSFET. You will find the print design here. This is for direct transfer so it is already mirrorred the right way.

If you have a non logic FET (like the IRF series and need some galvanic separation from your  microcontroller circuit, the following circuit comes in handy:
fet520

3.3 Volt levels

For a long time ‘TTL” meant 5 volt. Nowadays more and more 3.3 Volt boards are available as well in the Arduino series  as in the popular ESP8266. On these boards the STN4NF03L can be a good choice. Not an ideal choice, but a good one. Check section 2.1 figure 4 in the datasheet.

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