I am working on a prototype project with a large electromagnetic solenoid. I want to be able to vary the strength of the solenoid via a PWM (preferred), or constant current.

The issue I’m having is that I want to turn the solenoid on and off rapidly (1-150 Hz). I am currently using a bench power supply at about 1500 W and a large solid-state relay that is triggered by a proximity switch. The bench power supply is operating the solenoid, but it’s hard on the power supply to be turned on and off rapidly. I have tried a PWM controller, but it had a soft start feature which retarded the power to the solenoid.

I need an amplifier or motor controller that can handle rapid and continuous "instant on" and "instant off". The solenoid can handle a maximum of 96 V and 500 A for short periods of time and ultimately I want to run the system from batteries.

  • 1
    \$\begingroup\$ The solenoid stores a lot of energy in the field - turning it off requires dumping it somewhere. That is going to be the main obstacle to this plan ... \$\endgroup\$
    – pjc50
    Feb 10, 2020 at 0:32
  • \$\begingroup\$ Can you reveal how fast (amperes per second) you expect the coil current to change, what's the inductance and DC resistance of the coil and how high voltage the insulation of the winding can stand? By knowing this one can find a way to recycle a part of the energy. \$\endgroup\$
    – user136077
    Feb 10, 2020 at 1:39

1 Answer 1


Conceptually, you want something like this:


simulate this circuit – Schematic created using CircuitLab

When you turn M1 on, the current rises in the solenoid, charging the magnetic field up. When you turn it off, the current decreases towards zero, flowing through the diode or upper MOSFET. If you PWM fast enough relative to the inductance, the current will be fairly steady.

There are suitable modules available for M1 (MOSFET) and D1 (Schottky diode). The diode can also be replaced by another MOSFET which may result in better efficiency. however shoot-through has to be avoided or there could be a spectacular failure.

For example, this module.

enter image description here

  • \$\begingroup\$ Thank you very much for your reply. I already have the MOSFET as well as the fly back diode. I am just looking for a PWM that would have an adjustable slew rate or soft start. I basically want to eliminate the soft start all together. I was also thinking of using an audio amplifier that runs a large subwoofer as a controller. \$\endgroup\$
    – Gevans
    Feb 11, 2020 at 3:17

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