FAQ
TL;DR: For pulsing five electromagnets at 1–20 Hz, size for current, protect against inductive kick, and respect lethal voltages—“DC above 80 V can kill,” and “Be safe and double check always.” [Elektroda, Peter Evenhuis, post #21663907]
Why it matters: This FAQ helps hobbyists safely choose switches, voltages, and protection parts when building a tunable high‑voltage EM pulser.
Quick Facts
- Size the MOSFET’s Vdss to ≥1.5× (often 2×) your highest coil voltage to survive transients. [Elektroda, Steve Lawson, post #21663918]
- Logic‑level MOSFETs switch from 5 V gates; many others need about 10–12 V gate drive. [Elektroda, Steve Lawson, post #21663908]
- Stacking 12× 9 V batteries ≈ 108 V but only about 0.4 A—insufficient for high‑power coils. [Elektroda, Peter Evenhuis, post #21663907]
- Always add a flyback (protection) diode across the coil to clamp reverse EMF and save the switch. [Elektroda, Steve Lawson, post #21663919]
- Standard 555 timer use is typically ≤18 V; use a driver stage for higher coil voltages. [Elektroda, Aaron Hiniker, post #21663906]
What information do I need before designing a 1–20 Hz electromagnet pulser?
List each coil’s resistance, inductance, and rated current. Decide series or sequence operation. Pick supply voltage that achieves target current. Choose a switch device with sufficient Vdss and current. Plan for a flyback diode and a pulse source (e.g., 555) feeding a driver. Safety rules apply above 24 V and become lethal above 80 V. [Elektroda, Peter Evenhuis, post #21663907]
Why do experts say “it’s all about the current” for coils?
Magnet strength depends on ampere‑turns, so current through the winding dominates. Voltage is only the means to push that current through coil resistance and inductance. “If it’s inductors you are driving, then it’s all about the current.” Size your supply and switch around current first. [Elektroda, Steve Lawson, post #21663918]
How do I pick the right MOSFET voltage rating (Vdss)?
Choose a MOSFET with Vdss at least 1.5× your maximum coil voltage; 2× offers more margin. Inductive transients can exceed supply voltage during switch‑off. This headroom helps avoid avalanche and device failure. Pair it with a flyback diode to limit spikes. [Elektroda, Steve Lawson, post #21663918]
Do I need a flyback diode across the electromagnets?
Yes. Place a rectifier diode across each coil, reverse‑biased to supply. It stays off when the MOSFET is on, then conducts on turn‑off to absorb reverse EMF. Without it, voltage spikes can punch through the MOSFET. [Elektroda, Steve Lawson, post #21663919]
Can I drive a MOSFET gate directly from a 555 timer?
Yes, if you use a logic‑level MOSFET that fully turns on at 5 V. Non‑logic MOSFETs may need about 10–12 V gate drive. Keep gate leads short, and include a small series resistor if ringing appears. [Elektroda, Steve Lawson, post #21663908]
Are stacked 9 V batteries good for powerful electromagnets?
No. A dozen 9 V cells give about 108 V but only around 0.4 A. That low current starves the coils, delivering weak magnetization despite the high voltage. High‑current supplies are a better match for strong EM pulses. [Elektroda, Peter Evenhuis, post #21663907]
Is rectifying 120 V AC for this project a good idea?
Not for beginners. Mains‑derived DC is lethal and unforgiving. The forum cautions that DC above 24 V can burn and above 80 V can kill. Use isolated, current‑capable low‑voltage supplies and proper training before attempting mains work. [Elektroda, Peter Evenhuis, post #21663907]
What is a MOSFET, and why use it here?
A MOSFET is a transistor that acts as an efficient electronic switch. It handles coil current with low loss and is easy to drive from logic if it’s a logic‑level type. Choose adequate Vdss and current ratings, then add a flyback diode. [Elektroda, Steve Lawson, post #21663908]
When should I consider an IGBT instead of a MOSFET?
Use an IGBT when coil voltages approach a few hundred volts and you still need pulsed current. The thread suggests IGBTs for 300 V‑class applications, with strict safety discipline. Ensure proper isolation and protection networks. [Elektroda, Steve Lawson, post #21663908]
How do I estimate coil current from resistance?
Measure DC resistance (R). For steady current, I ≈ V/R once inductive transients settle. Ensure the supply can deliver that current continuously. Size wiring, switch, and diode for at least that current. [Elektroda, Aaron Hiniker, post #21663910]
How should I wire five electromagnets that must pulse together?
Place the coils in series if they must share the same current. Drive the series string with one MOSFET low‑side switch. Add one flyback diode across the entire string, observing polarity. Verify the MOSFET’s Vdss margin. [Elektroda, Steve Lawson, post #21663917]
What’s a logic‑level MOSFET gate requirement in plain terms?
Logic‑level MOSFETs reach low Rds(on) with a 5 V gate. Standard MOSFETs often need about 10–12 V to turn fully on. Match your driver to the device type to avoid heating and voltage drop. [Elektroda, Steve Lawson, post #21663908]
Can I limit coil current with a series resistor?
Yes, but it wastes power. The thread shows an optional series resistor to cap current when coil specs are unclear. Prefer setting current via supply voltage and proper coil design when possible. [Elektroda, Steve Lawson, post #21663921]
How do I add a safe switching stage to a 555 pulser?
Use the 555 to generate 1–20 Hz, then drive a MOSFET gate. Choose Vdss ≥1.5× supply and add a flyback diode across the coil. Keep control ground and power ground common and short. [Elektroda, Steve Lawson, post #21663918]
What’s a quick three‑step to protect my switch from coil kick?
- Wire a diode across the coil, cathode to supply, anode to the switch side.
- Size diode current to at least the coil current.
- Verify MOSFET Vdss margin and layout short leads.
[Elektroda, Steve Lawson, post #21663919]
Any final safety guidance for high voltage coil drivers?
Work one‑handed around energized circuits, verify isolation, and double‑check wiring. Respect thresholds: above 24 V causes burns; above 80 V can be fatal. “Be safe and double check always.” [Elektroda, Peter Evenhuis, post #21663907]
bases.
