Mosfet cir.

[conntaxman]

Im trying to use a IRF510 mosfet as a switch and cant figure out the wiring to make it switch on and off. I look at a few cir. using them and tried ,but didn't work. Isn't it like a Relay? isn't the Gate like the coil on a relay?and the source like the Ground.It looks like they use it to Switch the Ground in a cir. I put 3.3vdc to the G and S to ground and a led with a 270ohm from D to 12vdc and nothing happened.
ha ha Next step is a Hammer,
just cant figure out how they work.
tks
John
I want to use it in place of a DPDT relay in a PIR cir. to switch on some leds,3amp worth x 12vdc

 

[BobK]

The gate of the MOSFET requires as much as 4V to turn on even a little bit (200uA) and 10V to fully turn on. Look for a logic-level MOSFET if you want to turn it on from 3.3V or use a level translator to switch 12V to the gate.

Bob

 

[conntaxman]

Bob so what ill do is use a LM317 off of the 12vdc supply and feed the gate with 10vdc.But thanks for telling me that it takes that much voltage to open. But are these kind of like a relay with a coil? Like the gate is the coil? might as well ask.
This is for a simple pir cir, that is on the net.I bought the sensor pir from ebay.
here is the project.
http://www.electroschematics.com/6907/security-light-switch-with-pir-sensor-update/
tks
John

 

[conntaxman]

Hi Bobk.. i just look at the cir. Couldn't i just put a lm317 in place of the led .

 

[BobK]

You could use the 12V without regulating it down. The trick is to switch the 12V circuit with the 3.3V signal. An LM317 does not help with that. This is simply done with an NPN transistor and 2 resistors, but then the signal is inverted, so you would need two of these stages.

A better idea is to find a MOSFET that will switch at 3.3V. These are called logic-level MOSFETs. Do a search on the forum, there have been a couple of threads where people discussed them.

Oh, and no, a MOSFET is not like a relay, though it can sometimes perform the same function. A MOSFET is voltage driven, whereas a relay is current driven.

Bob