FAQ
TL;DR: For a 12 V motor tied to ground, use a high‑side P‑MOSFET driven by a TTL‑controlled N‑MOSFET; note “relays are much better about inrush,” and 40 A at 12 V is over 1/2 HP. [Elektroda, stephen Van Buskirk, post #21668090]
Why it matters: This FAQ helps makers replace bulky relays with robust, safer high‑side MOSFET switching from TTL logic for 12 V loads.
Quick Facts
- Typical project context: 12 V loads with on/off control at 20–40 A (no PWM). [Elektroda, Nick Durkin, post #21668083]
- For a grounded load and high‑side switching, you need a P‑channel MOSFET. [Elektroda, stephen Van Buskirk, post #21668092]
- Simple gate drive: N‑MOS pulls P‑MOS gate; use ~100 kΩ pull‑up, 5.6 V zener, and ~1 kΩ resistor. [Elektroda, stephen Van Buskirk, post #21668082]
- Add flyback protection: diode across motor; for relays, a diode plus a Boucherot cell (2–5 Ω in series with 1.0 µF). [Elektroda, stephen Van Buskirk, post #21668099]
- Expect large inrush: incandescent loads can draw 10–20× rated current for ~20 ms; fuses won’t reveal true peaks. [Elektroda, stephen Van Buskirk, post #21668086]
Do I need a P‑channel MOSFET for high‑side motor switching?
Yes. With the motor tied to ground, place a P‑channel MOSFET on the positive rail for high‑side switching. An N‑MOSFET on the low side would break the ground reference, which you cannot do here. Use a suitable P‑MOSFET rated for your voltage and current. [Elektroda, stephen Van Buskirk, post #21668092]
How do I drive a high‑side P‑MOSFET using a TTL signal?
Use a small logic‑level N‑MOSFET to pull the P‑MOSFET gate low. Add a high‑value pull‑up resistor to source, a 5.6 V zener to limit Vgs, and a series resistor to shape the gate current. This lets a 5 V TTL output switch the 12 V high side reliably for on/off control. [Elektroda, stephen Van Buskirk, post #21668082]
Which small BJT works for the level‑shift/drive stage?
A general‑purpose NPN such as PN2222A or 2N3904 works for the low‑current drive stage in the shown approach. These parts are inexpensive, widely available, and easy to prototype. Keep leads short and add a base resistor from the TTL source. [Elektroda, Steve Lawson, post #21668091]
What P‑channel MOSFET part should I consider at 12 V and high current?
The IXTP140P05T is a P‑channel MOSFET option suggested for high‑current 12 V work. Check Rds(on), SOA curves, and thermal resistance against your continuous and surge currents. Budget around the referenced unit price and include a heatsink if losses dictate. [Elektroda, Steve Lawson, post #21668093]
How much heat does a 9 mΩ P‑MOSFET make at 40 A?
At 40 A, P = I²·R ≈ 14.4 W, which often exceeds passive cooling alone. Even at 10 A, dissipation is about 3.6 W, requiring a decent heatsink. Plan thermal paths and consider lowering Rds(on) or using multiple devices if space and cost allow. [Elektroda, stephen Van Buskirk, post #21668094]
Can I parallel MOSFETs to share current on the high side?
Yes, but watch switching skew and cost. P‑MOSFETs in parallel reduce effective Rds(on), yet unequal timing can cause momentary overstress. Add small gate resistors and ensure good thermal coupling. “It gets expensive fast,” so compare to an N‑MOSFET + driver solution. [Elektroda, stephen Van Buskirk, post #21668096]
Why not just keep the relay?
Relays are bulky and noisy, and coils draw power. If you use one, add a diode across the coil and consider a Boucherot cell (2–5 Ω with 1.0 µF) to tame kickback and resets. Contacts can be electrically noisy under inductive loads. [Elektroda, stephen Van Buskirk, post #21668099]
How big can inrush or stall current get, and why do fuses mislead?
Incandescent loads can pull 10–20× their steady current for ~20 ms, and motors show high stall current. Fuses react slowly and pass large instantaneous peaks, so they under‑represent worst‑case stress on semiconductors. Size silicon for peaks, not fuse labels. [Elektroda, stephen Van Buskirk, post #21668086]
Which package and Rds(on) target should I pick for 20–40 A?
TO‑220 parts are easy to prototype and mount to heatsinks. Without a heatsink, assume ~1 W dissipation. At 40 A, that implies ~0.625 mΩ—hard to meet—so plan on heatsinking or more devices. Verify Rds(on) at your gate voltage, not just at 10 V. [Elektroda, stephen Van Buskirk, post #21668090]
Can I use an N‑MOSFET on the high side without a pricey P‑MOSFET?
Yes, if you drive its gate above the supply using a charge pump or higher rail by ~5–12 V. This provides the needed Vgs to fully enhance an N‑MOSFET as a high‑side switch while keeping losses low. Gate driver ICs can also do this. [Elektroda, stephen Van Buskirk, post #21668092]
How do I switch an AC motor from a TTL output?
Use an opto‑isolated triac driver like a MOC30xx to trigger a mains triac such as a BTA12‑600. Include the recommended series resistors and observe isolation and safety clearances. This suits simple on/off control of moderate AC loads. [Elektroda, stephen Van Buskirk, post #21668082]
Are MOSFET current ratings realistic for continuous use?
Marketing currents are often pulse ratings. Use Rds(on) and Ohm’s law to estimate real dissipation, then check the SOA and thermal path. “Don’t confuse mΩ with Ω” during selection; verify in the datasheet, not only parametric tables. [Elektroda, stephen Van Buskirk, post #21668090]
What is a Boucherot cell and where should I put it?
It’s an RC snubber: a low‑value resistor in series with a capacitor across an inductive load or coil. For relay coils, use about 2–5 Ω with a 1.0 µF ceramic in parallel with a diode to suppress kickback and prevent logic resets. [Elektroda, stephen Van Buskirk, post #21668099]
How do I wire a simple high‑side P‑MOSFET switch from TTL? (3 steps)
- Connect P‑MOSFET source to +12 V and drain to the load; load’s other end to ground.
- Use an N‑MOSFET to pull the P‑MOSFET gate low; add 100 kΩ pull‑up to +12 V, 5.6 V zener gate‑to‑source, and ~1 kΩ in series.
- Drive the N‑MOSFET gate from the TTL pin; add a flyback diode across the motor. [Elektroda, stephen Van Buskirk, post #21668082]
When should I abandon MOSFETs and use a relay anyway?
If cost, thermal management, or high surge currents overwhelm your P‑MOSFET options, a relay may be simpler. Add a coil diode and optional Boucherot cell to protect your logic, and verify the relay’s contact ratings for inductive loads. [Elektroda, stephen Van Buskirk, post #21668099]
Any practical device examples I can start with?
Try IXTP140P05T for P‑channel high‑side duties at 12 V. For the small driver transistor, PN2222A or 2N3904 works. Validate with heatsinking and protection before full current testing. “Paralleling” devices can help but raises cost. [Elektroda, Steve Lawson, post #21668093]
