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So i'm creating an electromagnet using copper wire that will be powered by a 9v source. My basys3 board will output 3.3 volts which will switch on my mosfet which will then enable the external 9v power source provide current to my electromagnet. I'm using a Irl540 mosfet and here is my schematic with a 9 to 3.3 regulator(L78L33 part) to simulate the process. Right now the DC power supply in the lab im using is capping out with around 3.1 amps and 3.0v. Is there anything I can do to reach the 9v on my DC power supply that will be best for me other than strengthening my electromagnet with more copper coils?

enter image description here

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    \$\begingroup\$ How big is this magnet? 3 amps at 3 volts means 9 amps at 9 volts. If you get to 9 amps at 9 volts you will be creating 81 watts of heat. This will probably melt your electromagnet or make it catch fire, if it's a small one. \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Mar 2, 2018 at 2:39
  • \$\begingroup\$ fresh 9V battery will have this limit. ( 6V drop /3A= 3 ohms) 3 LiPo cells would be better with < 50 mohm per cell \$\endgroup\$
    – D.A.S.
    Commented Mar 2, 2018 at 2:40
  • \$\begingroup\$ @jonk, actually tho OP did say so ... my schematic with a 9 to 3.3 regulator(L78L33 part) to simulate the process \$\endgroup\$
    – jsotola
    Commented Mar 2, 2018 at 4:13
  • \$\begingroup\$ @jsotola Thanks. I must have been distracted and not engaged. I've removed the comment. Thanks. \$\endgroup\$
    – jonk
    Commented Mar 2, 2018 at 5:54

1 Answer 1

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There are two obvious problems.

1) Unless you are using a logic-level FET, 3.3 volts is not enough to reliably turn on the FET.

2) At the same time, opening the switch will not reliably let the FET turn off. The gate is floating (being neither pulled up or down), so the state of the FET is not defined.

Both of these issues can be addressed with a small change:

schematic

simulate this circuit – Schematic created using CircuitLab

When the switch is closed, the gate is pulled up to 9 volts, which should provide adequate gate drive. When the switch is open, R1 will pull the gate to ground.

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  • \$\begingroup\$ Even a 'logic-level FET' may struggle to turn on fully with 3.3V. The FET should have Rdson specified at 3.3V or lower (eg. 2.5V). \$\endgroup\$
    – Bruce Abbott
    Commented Mar 2, 2018 at 6:25
  • \$\begingroup\$ @BruceAbbott - Which is why I suggested using 9 volts in my schematic. \$\endgroup\$
    – WhatRoughBeast
    Commented Jun 23, 2019 at 21:37

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