I have made this circuit: Schemantic

Ignore "high voltage circuit" is not used.

It's a prototype built in a breadboard with jumpers, and i have a problem with the mosfet "IRL540N", is not passing the optimal current and spark gap must be very close with almost no gap. When using power directly without a mosfet spark is very powerfull (see "SPARK-SW"), noisy and big gap, but the mosfet cut all the potential. I'm using a full charged lithium battery at 4.0V on MCU side, also i tried with 5V power supply and i get the same result (very weak spark). Transformer pull 3A max of current when connected properly. I also tried with other TTL mosfet (same result).

Looking at datasheet the IRL540N should work good even with 2.5V for that current (3A), but in fact is not.

Additionaly i have used a 16A relay to troubleshoot the circuit, i activate the relay with the 7.4V power supply and the mosfet and it works well, pulling 2A (maybe because of weak jumper cables). Still i don't want to use the relay, it consumes 30mA and as the voltage drop on batteries relay may not work good.

More images: https://dl.dropboxusercontent.com/u/18602178/IMG_20160612_224111.jpg https://dl.dropboxusercontent.com/u/18602178/IMG_20160612_224120.jpg https://dl.dropboxusercontent.com/u/18602178/IMG_20160612_230414.jpg

What can cause that problem?

  • \$\begingroup\$ One thing that you might want to be aware of is that assuming the tesla transformer is similar to a normal transformer, you are trying to stop the current very quickly, and your load is inductive. When you turn off the mosfet, the voltage at its drain will rise very quickly, very fast. This voltage may rise above what the mosfet can handle, causing issues (although I would assume that this would just break the mosfet, rather than make it not work that well). \$\endgroup\$
    – BeB00
    Commented Jun 16, 2016 at 1:16

2 Answers 2


You may not be driving the mosfet into full saturation. I see the controller runs from 3.7 volts (4.2V hot off charge). A little low for proper gate drive voltage.

Try using a gate driver to drive the gate of the mosfet. I found one on digikey or mouser that not only drives the gate, but does level conversion as well. You could drive it's input with 3.7 Volts and use the 7.4 Volts to drive the gate. Check out the UCC37322

  • \$\begingroup\$ From looking at the datasheet, it seems like this mosfet is ok at around that voltage. At 4V, it gives a max Rds of 0.063Ohm, which is not bad. \$\endgroup\$
    – BeB00
    Commented Jun 16, 2016 at 1:18
  • \$\begingroup\$ Yes, but i tried with 5V too. this mosfet is TTL level and work with low voltages (2.5v), i can't read the datasheet proper but how many voltage do i need for have 3A current in mosfet? I will check that drivers, still can i use 2n2222 or pnp transistor as i got some here? \$\endgroup\$ Commented Jun 16, 2016 at 1:20
  • \$\begingroup\$ I have adquired this mosfets from eBay, can they be fake and not acting like a real IRL540N? Because datasheet gives very current for a 5V \$\endgroup\$ Commented Jun 16, 2016 at 1:54
  • \$\begingroup\$ I purchased 7805 regulators from china. Originally I designed a circuit using USA made parts. While the China 7805 did regulate it ran much hotter and in some cases was 0.5V lower than 5 volts. \$\endgroup\$
    – user66377
    Commented Jun 16, 2016 at 2:16

By experience you need to provide at least 5Vgs to mosfet for fully saturation, you can use a power transistor like D44H11, TIP120 or similar with proper base current Ib and do not depend of voltage supply.

  • \$\begingroup\$ Have you read the IRL540N datasheet? its a TTL mosfet, can pass lot of current with low voltage on gate. Also i said i used 5V from power supply to test. \$\endgroup\$ Commented Jun 18, 2016 at 15:35
  • \$\begingroup\$ yep, have you considered inrush current of tranformer? I think even at 5V vgs there is not enough voltage to drive the tranformer inrush or the VDS increases as see Fig1 measure VGS of the mosfet; infineon.com/dgdl/… \$\endgroup\$
    – ise5755
    Commented Jun 19, 2016 at 4:34
  • \$\begingroup\$ Ok i will drive mosfet with 12V directly on gate to test. \$\endgroup\$ Commented Jun 19, 2016 at 15:18

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