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I have a Arduino Pin connected to a Logic Level MosFet (IRF3708) to switch a 12V led strip which should consume 1A. The Ugs(th) voltage of the MosFet is 2V. That I don't accidentally kill one of my MosFets I put a voltage divider at the output of the arduino. It should give me a voltage of 2.04V but I just get 1.6V. The arduino pin and the MosFet gate should be high impedance and I don't understand why?

Schematic

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ There is a schematic drawing tool button on the editor toolbar. Please draw a schematic rather than describe a circuit in words. \$\endgroup\$ – Transistor Oct 13 '17 at 20:30
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    \$\begingroup\$ Maximum Gate to Source voltage is 12V. No need to put resistive divider to drive its gate with arduino. \$\endgroup\$ – Wesley Lee Oct 13 '17 at 20:34
  • \$\begingroup\$ Sorry I didn't know there was a tool in the forum. Sorry for that. \$\endgroup\$ – moessi774 Oct 13 '17 at 20:40
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    \$\begingroup\$ Did you not know 2V is the threshold for high resistance at low current? You Always need 2.5 to 3x this max Vgs(th) as a minimum! \$\endgroup\$ – Sunnyskyguy EE75 Oct 13 '17 at 21:01
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    \$\begingroup\$ perhaps a better question to ask is "Why do I keep killing my MOSFETs?" since that is the root problem. \$\endgroup\$ – Frosty Oct 13 '17 at 21:03
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The most likely cause is the Arduino pin going to 4 volts, rather than 5. Have you measured it?

With that said, you are going about your gate drive in exactly the wrong way. You should not put any sort of divider before your gate. You need to understand the FET specs much more than you do. The upper limit on Vgs(th) is not a do-not-exceed limit. Rather, it is a minimum requirement. Worse, if you look closely at the data sheet, you will see that this level is specified to produce a drain current level of something like 250 uA or 1 mA, neither of which is remotely adequate for your load. For normal loads, you should provide at least 3 times the upper Vgs(th). In this case, if your load requires relatively little current you might get away with 5 volts. Then again, you might not.

EDIT - Since you have verified that the Arduino pin goes to 4.92 volts, the most likely culprit is your meter. 1 Megohm is the standard input impedance. Consider its effect on the circuit

schematic

simulate this circuit – Schematic created using CircuitLab

which is close enough to your 1.6. And if your resistors are not 1% tolerance, such tolerances could easily explain what difference remains.

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  • \$\begingroup\$ I have read about that. But I fried some of my MosFets after driving them directly with the Arduino and thought I put a too high voltage on the gate. The load was a led stripe with ~1A @12V and I don't get how I destroyed it then. \$\endgroup\$ – moessi774 Oct 13 '17 at 20:42
  • \$\begingroup\$ And the Arduino voltage was 4.92V. \$\endgroup\$ – moessi774 Oct 13 '17 at 20:49
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    \$\begingroup\$ @moessi774 - Are you sure you've tied your +5 and +12 grounds together? \$\endgroup\$ – WhatRoughBeast Oct 13 '17 at 21:28
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    \$\begingroup\$ @moessi774 - See edit. \$\endgroup\$ – WhatRoughBeast Oct 14 '17 at 2:14
  • \$\begingroup\$ Yes I put the grounds together. But your explaination is logical. I calculated the same divider including my before used 1MOhm pull down I had on the gate and wondered why this is the same voltage which I read with the multimeter but without the pull-down resistor. Thank you very very much :) \$\endgroup\$ – moessi774 Oct 14 '17 at 5:44
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Vgs(th) is the voltage at which the MOSFET turns on, not the maximum Gate-Source voltage the part can tolerate.

Maximum Vgs is given in the "Absolute Maximum Ratings" table of the datasheet and is +/-12V so you don't need to "protect" the gate from the voltage level of your Arduino.

As to why your voltage divider doesn't work as expected: Have you verified your component values and your source voltage?

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  • \$\begingroup\$ I have read about that. But I fried some of my MosFets after driving them directly with the Arduino and thought I put a too high voltage on the gate. What could have destroyed the MosFet then? The load is a stripe of Leds which should pull ~1A @12V. \$\endgroup\$ – moessi774 Oct 13 '17 at 20:44
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    \$\begingroup\$ @moessi774 You have no idea how to select MOSFTs And the answers are duplicated on this site for Vgs, Ciss, effects on Resistance and rise time \$\endgroup\$ – Sunnyskyguy EE75 Oct 13 '17 at 21:14
  • \$\begingroup\$ I am new to MosFets I know. But I want to learn it and accept my failures when I try to. What do you mean by duplicate answers? \$\endgroup\$ – moessi774 Oct 14 '17 at 5:46

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