# MOSFET question?

Discussion in 'General Electronics Discussion' started by 24Volts, Mar 10, 2013.

1. ### 24Volts

164
0
Mar 21, 2010
Hello,

In attachment below, the amp meter reads 4.78ma.

I always figured that when R2 would drop to 20 ohms, the current would increase. But it simply stays the same!!!

Why??

Wouldn't lowering the resistance allow more current through the circuit??

In other words, the current is always 4.78ma weather 51 or 20 ohms is connected! So I wouldn't even know why I would need any R2 resistor there..... except for the odd situation where the mosfet would allow its channel to fully open hence R2 would be there for security purpose....!!!!

So my question is, why does the IR2 always stay the same no matter what resistor we use?

24v

File size:
24.3 KB
Views:
92
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
Review the graphs of Id vs Vgs for this device.

edit: perhaps not...

Last edited: Mar 10, 2013
3. ### 24Volts

164
0
Mar 21, 2010
I don't understand what you want to get at..... I don't see any such graphs

Is it fig5?

My point is, if G stays at 3.9Vdc, should IR1 change depending the resistance we put as R1?

Last edited: Mar 10, 2013
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
For a given Vgs, Id has a maximum value. The Id vs Vgs graph indicates this for a variety of temperatures. The Id vs Vds graph indicates it for a variety of Vgs values.

Figure 1 and/or figure 5 give this information http://www.fairchildsemi.com/ds/2N/2N7000.pdf

However it appears that the limiting value of Id should be higher than 4.78mA -- but the Vds, Id, and Vgs (the latter to a smaller extent) are in areas of the graph that are difficult to read.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
My point is, you need to understand the device. Just telling you the answer isn't going to help you.

Understand that in certain regions of operation, a mosfet is a voltage controlled current source. (And there you are -- I gave you the answer)

But I suggest you prove that this is the case by comparing your results with what is shown in the graphs in the datasheet.

The main problem is that for the values you have, the graphs may not be easy to read.

6. ### 24Volts

164
0
Mar 21, 2010
thanks Steve!

24v

Last edited: Mar 11, 2013