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Mosfet as a 12V Switch (and opto)

I've been stuck on this for awhile. I've got a MOSFET that is not behaving the way I expect.

I was hoping it would act similar to the following animation where source to drain is controlled by the gate:

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But when applying 12V between the source and the drain, my load (a light) slowly illuminates as if a capacitor is powering up. I did not expect anything to happen since nothing was applied to the gate.

Is there any way to get it to work like an on/off switch like the link above applied I can do?

Is there any way someone can draw or point me to schematics that specifically describe the proper way, maybe even with an optoisolator involved? Sorry for asking for that but I've really frustrated. I got this to work with normal transistors, but I'm hoping the FETs will perform better.

I'm using this kind of MOSFET:

formatting link

And (eventually when I get the basic part figured out) this kind of optocoupler: http://www.ortodoxism.ro/datasheets/vishay/83717.pdf

I appreciate anyone's help and patience as this is my first attempt at electronics work.

Reply to
ProgrammerChicago
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When the gate is floating it can/will charge up since there\'s
nothing to drain the charge away and, eventually, slowly turn the
device ON.


>Is there any way to get it to work like an on/off switch like the link
>above applied I can do?
Reply to
John Fields

The Gate is very high impedance, It's acting on surrounding static fields. You need to pull the gate down to the SOURCE side of the fet. A resistor to lower the input impedance.. then you'll get what you

1 meg sounds like a round number.

If you're doing a sink switch (that's one that pulls the load to common), you use a N-channel with the source on the common.. If you're trying to do a source type of switch (+ volts on) to a load. you use P-channel on the + side output with the SOURCE on the + rail. then you simply pull the Gate to common which will turn it on.. Beware of the fet that is being used. Make sure you read the spec's on what the GATE (Vgs) can handle and should have for voltage. that's about it in a nut shell I guess.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

Thanks, I'll try your suggestions and follow up with the results asap.

Reply to
ProgrammerChicago

Thanks JF, and Jamie. It works perfectly now... best of all, the FET is generating virtually no heat unlike all the regular transistors I tried which would have needed heat sinks.

Reply to
ProgrammerChicago

That's because of the turn on state resistance between the drain and source. You can saturate a bipolar transistor and get good results.

The key is saturation which simulates low resistance paths that generates the lowest heat possible for that component.

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5
Reply to
Jamie

That's good to know in case I want to make this device AC capable. For now I've set my mind on FETs. Hopefully they'll have some advantage down the road. Thanks again.

Reply to
ProgrammerChicago 

--
Do you want to know why?
Reply to
John Fields 

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But not as good as you can with a MOSFET.

Bipolars can\'t generally get to Vce(sat) below 0.3V, while MOSFETS
can get to milliohms so, local dissipation for a given current load
will generally be less for a MOSFET.
Reply to
John Fields

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