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enter image description hereI am currently trying to rework a circuit I have to make it more efficient/ correct. Here is the objective, I have a sensor that is ground activated in an internal computer circuit. It is open util it is grounded, at which point the circuit is active. Well, I have 24-28v signal that I need to activate this circuit. What I've done in the past, is use a relay. Powering the coil with the 24-28v signal to switch the circuit to ground. This works ok, but it is large and far more than what is needed. I am trying to convert this to a Mosfet or transistor of some type. But I am unsure what would connect to what, and if it would work. My current idea was to connect the 24v signal to the gate of an NMOSFET, Drain connected to the computer and source connected to ground. But, since there's no real load and I'm not sure which way the current flows in the circuit, I'm unsure what to do. I would experiment, but I don't want to destroy the computer. Thanks

EDIT: Voltage is DC and the ground is shared.

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  • \$\begingroup\$ Unclear what you are asking. A diagram of the circuit you're trying to rework would help, along with a description of its quantitative requirements. \$\endgroup\$ Commented Mar 16 at 21:17
  • \$\begingroup\$ I've attached what the old circuit was. Hopefully that clarifies it a bit. I would like to get rid of the mechanical relay, mainly for space and reliability since it is frequently switched. Essentially, I need to ground a pin on the computer using a 24v signal. Thank you for your help. \$\endgroup\$ Commented Mar 16 at 21:27
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    \$\begingroup\$ 28V has the potential to damage a mosfet gate if connected directly \$\endgroup\$ Commented Mar 16 at 21:41
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    \$\begingroup\$ Two critical questions. 1 - is the 24 V source AC or DC? 2 - Do the 24 V source and the downstream computer share a common ground? If yes, then this can be done with 1 transistor. If no, then you still need some form of isolation. This can be a relay or a opto-coupler. Please update your question with this information, and I'll convert this comment to an answer with a schematic. \$\endgroup\$ Commented Mar 16 at 21:42
  • \$\begingroup\$ What supply voltage is the computer running? Knowing that will help you select the VCEmax (for BJT) or VDSmax (for MOSFET). \$\endgroup\$ Commented Mar 17 at 3:23

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Sssumin that that signal to be grounded is a positive DC signal then something like this. exactly which transistor or mosfet will depend on how much power you need to switch.

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

If it's a negative signal or an AC signal then something else will be needed.

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  • \$\begingroup\$ Thank you Jasen, this helps clarify a lot. \$\endgroup\$ Commented Mar 16 at 21:54
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You can use a N-MOSFET with source connected tor ground, and with a sufficient current and voltage rating for whatever signal it needs to ground (which you have not determined). You could experimentally determine that with a multimeter to measure the open-circuit voltage, and the current to ground when configured as an ammeter with one meter probe to ground.

But 24V is to high for most MOSFET gate-source max voltage ratings, so you can use a resistive divider at the gate input to reduce it to about 10V.

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  • \$\begingroup\$ This is great information. Thank you. Would a NPN transistor be better or any different in this case? \$\endgroup\$ Commented Mar 16 at 21:53
  • \$\begingroup\$ You can use either an NPN or an N-MOSFET. The BJT has a small saturation voltage of perhaps a 100mV from collector to emitter when on, whereas the MOSFET has a strictly ohmic voltage drop equal to its on-resistance times the source-drain current. \$\endgroup\$ Commented Mar 17 at 5:09

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