Transistor and MOSFET switch calculator

How the calculator decides

A transistor used as a switch should be fully off or fully on. Fully on means saturated for a bipolar transistor and at its low RDS(on) for a MOSFET: the drop across it is small, and so is the heat. The calculator checks that your pin can drive it there using only the figures the datasheet guarantees, then works out the heat and the junction temperature.

Bipolar transistors: base current and base resistor

The hFE in the datasheet is measured with several volts across the transistor, not in saturation. For the saturation voltage VCE(sat) the datasheets use a much lower gain, usually IC = 10 × IB (20 × for the BC547, 250 × for the TIP120 Darlington). The calculator uses that same forced gain, so the saturation voltage in the result is guaranteed.

IB = IC ÷ β(forced)
RB = (VGPIO − VBE(sat)) ÷ IB
P = VCE(sat) × IC + VBE(sat) × IB

Example: an ESP32 switching a 12 V relay coil of 70 mA with a 2N2222A. IB = 70 ÷ 10 = 7 mA and RB = (3.3 − 1.2) ÷ 0.007 = 300 Ω, rounded down to 270 Ω so the base gets a little more. The transistor drops at most 0.3 V: about 30 mW, nothing to worry about. At 300 mA the base would need 30 mA, more than the 20 mA an ESP32 pin should give: that is where a MOSFET takes over.

MOSFETs: the gate voltage that is actually guaranteed

The threshold voltage VGS(th) is not where a MOSFET turns on: it is where it barely starts to conduct, at 250 µA or 1 mA. What matters is the RDS(on) the datasheet guarantees at a given gate voltage. The AO3400A guarantees it at 2.5 V and switches fully from a 3.3 V pin. The IRLZ44N guarantees it from 4 V: fine from 5 V, not guaranteed from 3.3 V. The IRF520 and IRF540N guarantee it only at 10 V. Between the threshold and the lowest guaranteed point the datasheet promises nothing, and the calculator says so instead of guessing.

P = I² × RDS(on)(Tj) + Pswitching
Tj = Ta + P × θJA

RDS(on) rises with temperature, by about 1.5 times at 125 °C on these datasheets (1.43 for the AO3400A and 1.8 for the 2N7000, from their tables). The calculator raises it with the junction temperature and repeats until the two agree. Example: an AO3400A driven by an ESP32 switching a 2 A LED strip at 12 V. At 3.3 V the guaranteed figure is the 2.5 V one, 48 mΩ: 2² × 0.048 = 0.19 W, about 0.21 W once warm. At 125 °C/W the junction reaches about 52 °C.

Why the IRF520 module fails at 3.3 V

The common red IRF520 module connects the signal pin almost straight to the gate of an IRF520. Its threshold can be as high as 4 V and its RDS(on) is guaranteed only at 10 V. From a 3.3 V ESP32 it may not turn on at all; from a 5 V Arduino it conducts only partly and heats up with a few amps. From 5 V you can replace it with a logic-level MOSFET with the same pinout, such as the IRLZ44N or the IRLB8721. From 3.3 V use a MOSFET guaranteed at 2.5 V or a gate driver.

Motors, relays and solenoids: the flyback diode

A coil resists changes in current. When the transistor turns off, the coil keeps the current flowing and its voltage shoots up until something breaks down, usually the transistor. A diode across the load, cathode to the positive supply, gives that current a path. For relays switched on and off a 1N4148 (small coils) or a 1N4007 is enough; with PWM on a motor use a Schottky diode such as the 1N5819 (1 A) or the SS34 (3 A), rated for the load current.

High side: P-channel MOSFET and level shifter

Sometimes the load must keep its ground connection and the switch goes between the supply and the load. With a P-channel MOSFET the source goes to the positive supply and a resistor pulls the gate up to the same voltage (off); a small NPN driven by the pin pulls the gate down (on). The gate then sees the whole supply voltage: above 80 % of the MOSFET’s gate limit the calculator adds a divider. A pin can’t drive the P-MOSFET directly when the supply is higher than the pin voltage: the MOSFET would never turn off.

PWM and switching losses

At every edge the MOSFET spends a moment half on, with both voltage and current. A microcontroller pin moves the gate charge with a few tens of milliamps, so an IRLZ44N (48 nC) needs a few microseconds per edge: harmless at 1 kHz, a few watts at 20 kHz with 5 A. The calculator estimates it as ½ × V × I × (rise + fall time) × frequency, with the whole gate charge, which errs on the safe side. For fast PWM use a small MOSFET with a low gate charge or a gate driver.

The transistors in the list

Guaranteed maximum figures at 25 °C from the makers’ datasheets (onsemi, CJ, Alpha & Omega, Vishay, Infineon). At 3.3 V and at 5 V: the RDS(on) guaranteed with that gate voltage. θJA was measured on each maker’s test board, about 6.5 cm² of copper for the SOT-23 parts: with less copper they run hotter.