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I have been using this fuse blown indicator circuit for a bit, it basically works perfectly fine with positive voltages no matter if there is or nor a load connected

enter image description here

The problem starts when I feed the circuit with negative voltages, as you can see in the picture my MOSFET never activates, It simply won't work as intended, even if I decide to turn around the LED, it uses ground as reference and it actually turns on the led but stays on permanently (meaning the mosfet doesn't open) since it's using GND as reference and providing 5v to the led and resistor in inverse polarization.

enter image description here

Already tried putting some diodes at the bottom but the MOSFET still doesn't respond. My question here is, how can I activate the MOSFET using negative voltages? Considering I will also use this circuit with -12v in the future, I'm thinking of using an inverter but I would like to hear some feedback before I do this.

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  • \$\begingroup\$ I have a hard time believing you're actually using a monstrous 100W TO-3 MOSFET to switch an indicator LED, but anyway use an N-channel MOSFET and reverse the LED. \$\endgroup\$ – Spehro Pefhany Nov 14 '14 at 1:19
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    \$\begingroup\$ hahah sorry i just picked one at random from the library \$\endgroup\$ – GoatZero Nov 14 '14 at 3:21
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This will work for negative input voltages. Take care not to exceed the maximum gate voltage of the MOSFET (add a zener diode if necessary to protect the MOSFET). Also the MOSFET must turn on sufficiently at the input voltage, so you may need to use a logic-level N-channel MOSFET for -5V in.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ By Gate protection Do you mean the zener betwen Source and Drain that usually comes in some packages? \$\endgroup\$ – GoatZero Nov 14 '14 at 4:25
  • \$\begingroup\$ Yes, zener from source to drain- the 10K resistor limits the current. \$\endgroup\$ – Spehro Pefhany Nov 14 '14 at 11:36
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Replace the PMOS with an NMOS and flip the LED around.

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  • \$\begingroup\$ tried it, didnt work \$\endgroup\$ – GoatZero Nov 14 '14 at 1:22
  • \$\begingroup\$ From looking at your schematics, it looks like you only turned the LED around and you're still using a PMOS. \$\endgroup\$ – alex.forencich Nov 14 '14 at 2:52
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Alongside the theory hopefully someone can confirm this to me

i had to sit for a while and read about how mosfets work, basically get activated by a voltage above 2.5v between Source and gate, this allows the current to flow from Source to Drain

in my circuits, using the PMOS for positive voltage and NMOS for negative voltage, i had to add zener diodes with a 5.1v value to protect the gate from any kind of transient that can kill the gate inm the NMOS the led goes fliped since im using negative voltages from the source

enter image description here

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  • \$\begingroup\$ however a little question is still in the air, A zener must be added to the gate of a MOSFET if the gate voltage comes from a supply that is above 20v. how can i be sure whats the correct zener value to pick, for example when i start using +12v and -12v power supplies my zener reverse voltage should be exactly 12v? \$\endgroup\$ – GoatZero Nov 14 '14 at 18:50
  • \$\begingroup\$ Aaaargh! Turn off the grid before screengrabbing! \$\endgroup\$ – Transistor Jan 23 '18 at 15:48
  • \$\begingroup\$ Zener is added to protect the MOSFET if gate gets too high voltage. Anything below 20V - but obviously not well below is OK. \$\endgroup\$ – V-Mark Jul 13 '18 at 7:46

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