Again no one said you have to switch anything high side, they said you can not switch high side with an N-channel FET and a small transistor without a bias voltage. Anyway, what followed from that was a statement that an N-channel device plus a small transistor can not be made to perform the same function on the high side as a P-channel device can. Realistically P-channel devices suck when compared against similar N-channel FETs and that is part of the reason people even bother to go about designing boost circuits to allow all N-channels devices to be used in a bridge. What was said was that given the IRL520 as an N-channel FET to compare to, a P-channel device of the same characteristic ratings would generally be applied for the purpose of switching high-side. No one but you has said power HAS to be switched high side. Zoomkat you are reading more into my reply than I put there in your attempt to show how it can all (?) be done low-side. This requirement was not included in the origional post. Pretty much a restatement of the same potentially flawed assumptions based on the assumption that the power has to be high side switched. I guess I ought to test that idea, but once again, I’ll consider any suggestion thoroughly. On the other hand, as I was writing this, it occured to me that I might not need a FET of any sort. By using the N-channel FET, I have done the opposite: The LED is on unless the switch is closed, which would mean that BOTH switches must be closed. Each whisker is hooked to a different FET, so current will flow from either one, and light an LED when either switch is pressed. Thus I thought that I could use a p-channel FET such that current only flowed when the switch was closed. What I set up was that the sensors were pulled high, and would go low when the switch is closed. Since I am more I/O line limitted than anything else, saving two lines is valuable. This process, which uses feature sizes approaching those of LSI integrated circuits gives optimum utilization of silicon, resulting in outstanding performance. For both the left and the right side, if either sensor trips, I don’t actually need to know which one it is. MOSFET - Power, N-Channel, Logic Level 50 V, 16 A, 47 m These are NChannel logic level power MOSFETs manufactured using the MegaFET process. Therefore, I don’t need four different I/O lines for these sensors, but instead only need two. Now, I have some pretty serious brainpower on this system, so I know that I can only run into something when I’m going forwards. I have four whiskers on a robot, two in front, and two in back. I’m new to electronics, so some of the jargon went over my head, but let me describe what I am doing, perhaps somebody can suggest a better solution: Is there a trick that i dont know off to drive it at logic level ? Pulling it up to source voltage seems also not ideal because for example of the IC i mention if my source voltage is 50 the voltage at the gate is 50v witch is beyond its capability of +-20v.Glad I generated some debate. If that is so why advertise PMOS's as "logic level gate" ( Digi-Key) when with common logic levels i wont be able to use it properly on high voltages(or currents). Now thats not ideal since the the highest i can run it with my 5v logic is 20v way below its rated 80v. Now at logic 5v high my mosfet will probably gget destroyed because the voltage at the gate (referenced to source) will be -45v which is waaay past the +- 20. PMOS on the other hand Works the same As NMOS but when used as a typical high side switch the Source pin is not connected to ground but to Vcc!. (Again correct me when im wrong about something) So no matter what voltage that im switching the mosfet will turn on and off with my logic singal. When both used as a switch the NMOS is pretty much staight forward, because NMOS is typically used as a low-side switch a logic-level NMOS like mention above will always turn on at logic 5v HIGH because the gate pin is referenced to the SOURCE pin which is connected directly to ground(which the Logic ground is also referenced). I have always this mindset that to drive a MOSFET you have to apply a certain voltage to the gate of the mosfet to turn it "on" But it would seem that it not entirely true, you have to apply a certain voltage into the gate with reference to the source pin not necessarily the ground pin (correct me if im wrong).įor situations stated below i will use the two ADVERTISED as LOGIC LEVEL MOSFETS the BUK9134(NMOS) and FDS8935 (PMOS)
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