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Erm, right. I should probably know this, but...

J

Jamie

Jan 1, 1970
0
I apologise for the HTML, if that's not liked in this group.

Mosfets...

If is was to take say, this (http://us.st.com/stonline/books/pdf/docs/2950.pdf) mosfet, and connect like this (excuse the drawing, oh and it'll only work in Courier, or some other fixed-width font like that):

4.5V 18R
-----------------\/\/\/\------
| |
\ |Source
Switch \ ____|____
| Gate | |
------------------| Mosfet |
|_________|
|
|Drain
____|____
| |
| Load | 200mA (bunch of LEDs in parallel)
|_________|
|
|
0V |
------------------------------

Now as far as I know, say the switch is open (ie. gate is at 0V) then no current will flow between the source and the drain, and the LEDs won't be lit, right? And if the switch is closed (gate at 4.5V and higher than the source voltage) then the mosfet will be 'on' and current will flow through the mosfet and the LEDs will light up, yeah? Or not? 'Cause I soldered it all together and the LEDs just stay lit, no matter what I do with the gate voltage. It's all getting abit annoying.

Anyways, any help would be appreciated,
Jamie
 
M

Michael A. Terrell

Jan 1, 1970
0
Not only is usenet a HTML free zone, binaries are only allowed on
binaries newsgroups. The one used by the sci.electronics.* groups is
where you can post them as a
GIF, JPEG, or other common image file. Then, post a message here giving
the name of the post. If you try to post a binary file here, most
people will never see it.
 
S

Steven Bastien

Jan 1, 1970
0
Oh yes! - good catch. I didn't check the PDF. Also, (ideally) he should put
the source to ground and the load and current limit resistor on the drain
side, and then to 4.5V. This will maximize Vgs and ensure that it is
sufficiently greater than threshold voltage.

Steve B

4.5V
___________resistor_________LEDs________Drain__________Source________GND

|

|

|
 
J

Jamie

Jan 1, 1970
0
Right, hang on. Your gonna have to explain that one again. I'm confused.

Jamie
(http://us.st.com/stonline/books/pdf/docs/2950.pdf) mosfet, and connect like
this (excuse the drawing, oh and it'll only work in Courier, or some other
fixed-width font like that):current will flow between the source and the drain, and the LEDs won't be
lit, right? And if the switch is closed (gate at 4.5V and higher than the
source voltage) then the mosfet will be 'on' and current will flow through
the mosfet and the LEDs will light up, yeah? Or not? 'Cause I soldered it
all together and the LEDs just stay lit, no matter what I do with the gate
voltage. It's all getting abit annoying.
 
J

Jamie

Jan 1, 1970
0
The whole switch thing was just for simplicity, the gate is actually connected to the output from an OR gate.

Jamie
Jamie,

Use a SPDT switch so that you can ground the gate when in the off state, or use a 1K pull down resistor on the gate with your SPST switch. MosFETS have very high input impedance into the gate, and if left open as you have it, they can float in the on state (especially after they are already on). See diagrams below.

Good Luck,
Steve B


Option 1:

4.5V 18R
-----------------\/\/\/\------
| |
\ |Source
Switch \ ____|____
| | Gate | |
| ----------------| Mosfet |
| |_________|
| |
| |Drain
| ____|____
| | |
| | Load | 200mA (bunch of LEDs in parallel)
| |_________|
| |
| |
0V | |
------------------------------

Option 2:

4.5V 18R
-----------------\/\/\/\------
| |
\ |Source
Switch \ ____|____
| Gate | |
------------------| Mosfet |
| |_________|
| |
/ |Drain
\ ____|____
/ 1K | |
\ | Load | 200mA (bunch of LEDs in parallel)
| |_________|
| |
| |
0V | |
------------------------------
 
J

John Larkin

Jan 1, 1970
0
Right, hang on. Your gonna have to explain that one again. I'm confused.

Jamie

Take a look at the little schematic on the first page of the fet
datasheet. There's a parasitic substrate diode from source to drain.
Basicly, a n-fet is intended to work with the drain more positive than
the source. In your circuit, it's "upside down" so the diode is
foreward biased all the time, so you can't turn the puppy off.

So, use a p-channel fet, or, to use the same fet, rearrange things,
putting the load in the drain circuit like Steve suggests. (Nice
linear ascii schematic!)

John
 
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