Depletion mode Mosfet

Thread Starter

spike1947

Joined Feb 4, 2016
167
Hi
Can anyone point me to a simple basic schematic drawing that contains a " n-channel mosfet depletion mode transistor " in the circuit .

I just really want to know how to turn it off , I am drawing a circuit ( not finished as yet ) for a project , but the info I have about this mosfet is that the Gate voltage has to be around 0.7v min above the source voltage to turn it off !, not sure how to get round that , that is why I am asking if anyone knows of a circuit using a depletion mode mosfet in it , then maybe I can work it out , or if they know how to turn it off in a very simple term .
.
sorry if I am not giving all the info that maybe you are wanting , but maybe can if required .

cheers

spike
 

dl324

Joined Mar 30, 2015
11,314
Vgs needs to be negative to turn off a depletion mode N channel MOSFET. If you apply 0.7V, it will operate in enhancement mode.

What is the part number?
 

dannyf

Joined Sep 13, 2015
2,197
By putting a sufficiently high drain resistance.

Most RF MOSFETs are of depletion type. They are not for switching applications so hard to imagine the need to turn them off.

They are biased with a resistor or led.
 

Marley

Joined Apr 4, 2016
356
In an N-Channel depletion mode MOSFET, the gate needs to be negative of the source to switch it off. The device will be fully on if the gate is the same voltage as the source.

A common type is the J111 series. See the data-sheet here.

You can see that the gate-source cut-off voltage is about -3.0V to -5.0V depending on type. See the graph "figure 5" in the above data-sheet.

Have a look at this typical circuit.
 

Thread Starter

spike1947

Joined Feb 4, 2016
167
I am doing a infrared alarm system , were the ir led is on all the time into the ir receiver , then the feed from the ir receiver will feed into the DN2530 to turn it off , when the beam is broken ( ie the path of the ir led is blocked ) there will be no feed to the gate of the DN2530 so then it will operate a relay ! , that is the theory I have ! .

Thanks all for all your input , i think I am getting to deep here ! .

cheers

spike
 

Thread Starter

spike1947

Joined Feb 4, 2016
167
The datasheet says you need Vgs of -1 to -3.5V to turn the MOSFET off. Below is the typical IV curve that shows you need a voltage more negative than -1.5V.
View attachment 103823

If you post a schematic, members can help you with your design.
Thanks for that info ,my schematic is a bit messy at the moment , I need to redraw some of it ,,this is done by hand on graph paper , will have to scan it to put on here .

cheers

spike
 

dl324

Joined Mar 30, 2015
11,314
Thanks for that info ,my schematic is a bit messy at the moment , I need to redraw some of it ,,this is done by hand on graph paper , will have to scan it to put on here .
If you haven't already purchased the depletion mode MOSFET, you can use a less expensive enhancement mode device.
 

crutschow

Joined Mar 14, 2008
25,269
You don't need to use a depletion-mode device which requires a reverse-polarity to turn it off (gate polarity opposite the drain polarity. Easier to use a common enhancement-mode device.
What is the signal voltage from the alarm?
 
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