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In the attached circuit with a 5V relay and a 30V load across it, I've noticed the FQP30N06L MOSFET triggers with even electrostatic energy (touching the gate). The difference between the circuit under test and this one is rather than a 10K pulldown I'm using a 2K pulldown. Is there a way to prevent electrostatic energy from triggering the MOSFET? Is it just a matter of the pulldown resistor value? The voltage coming into the gate is 3.3V -- the coil voltage is 5V. Any help would be greatly appreciated. GND and GNDA are attached to the 3.3 V circuit and the 5V circuit, respectively.

enter image description here

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  • \$\begingroup\$ If the GNDs are different (GND and GNDA) it is not gonna work properly (hence touching it turns it on). For this to work you need both GNDs to be tied together (not sure how thats gonna affect the rest of your design. If its isolated you can use an optocoupler to keep isolation. \$\endgroup\$ – Wesley Lee Oct 8 '19 at 12:52
  • \$\begingroup\$ I'm sorry if this is a dumb question but: why do the grounds need to be tied together? What difference does that make exactly? As an additional detail -- the two grounds in question come into being because the supply starts at 5V and then I divide that down to 3.3V. The 3.3V goes into a development board I am using that has a pinout to ground. So, in a sense the two grounds are really from the same supply, just at different ends. \$\endgroup\$ – jsinglet Oct 8 '19 at 13:43
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    \$\begingroup\$ Shunt Gate to Source with a cap. <= 0.1uF \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 8 '19 at 13:53
  • \$\begingroup\$ ... just don't touch the gate. After all, why would you want to and risk damaging it? \$\endgroup\$ – Andy aka Oct 8 '19 at 14:24
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    \$\begingroup\$ @SamGibson -- yes, you nailed this. I was troubleshooting this and hooskworks pointed this out and suggested an LDO. Thanks for pointing it out. \$\endgroup\$ – jsinglet Oct 8 '19 at 23:27
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I would recommend connecting R2 on the other side of R3 closer to the gate of the MOSFET. Your schematic also suggests that the pull-down resistor uses a different ground to the source of the MOSFET (GND and GNDA respectively); i would connecting the pull-down resistor as close a possible to the MOSFET between the gate and source terminals.

If you still have problems then, since you're driving something slow like a mechanical relay, you can add some capacitance between the gate and the source of the MOSFET so that more charge is required to raise the gate-source voltage to the threshold level. An alternative to this is to add a Schottky diode in parallel with R3 to give a low impedance pull-down path when the MOSFET is blocking. Vgs(th) for the MOSFET is quoted at 1 V so you don't need to use a Schottky diode but it'll give the solutiuon the best chance of working for you.

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  • \$\begingroup\$ Another posted suggested the problem is that the grounds are different -- I have the 5V GND separated from the 3.3V GND. Is your suggestion to tie the grounds together and move the resistor up? \$\endgroup\$ – jsinglet Oct 8 '19 at 13:13
  • \$\begingroup\$ @jsinglet not quite, they should be the same ground too unless you have a good reason for separating them. The ground attached to the source of the MOSFET should be the same ground as is attached to R2 (the pull-down resistor). Additionally R2 should be physically close to the MOSFET and connected with a minimum trace length to the MOSFET gate and source terminals. \$\endgroup\$ – hooskworks Oct 8 '19 at 13:18
  • \$\begingroup\$ As an additional detail -- the two grounds in question come into being because the supply starts at 5V and then I divide that down to 3.3V. The 3.3V goes into a development board I am using that has a pinout to ground. So, in a sense the two grounds are really from the same supply, just at different ends. \$\endgroup\$ – jsinglet Oct 8 '19 at 13:19
  • \$\begingroup\$ Thanks @hooskworks -- that's very helpful. In general when would I want to keep the grounds separated? Is there ever a good reason? For example, I think it makes sense that I wouldn't want to connect to the ground of a high voltage load I am switching, but other than that, are there guidelines? \$\endgroup\$ – jsinglet Oct 8 '19 at 13:22
  • \$\begingroup\$ Also, in terms of the resistor value -- does 2k seem sufficient or should I really be thinking about 10k? \$\endgroup\$ – jsinglet Oct 8 '19 at 13:23

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