MOSFET drain to source current

[mmilejski]

I have recently built a simple dummy load, the one from Dave's video.
I haven't used a logic level mosfet as Dave suggested but I used simple BUZ11. Why? In datasheet there is a graph "Output characteritics" which shows that at 4V gate to source voltage (5 volt Vcc - non rail to rail op amp clipping, what I am getting at op amp output), it should be able to pass about 2.5A. So I thought it will be ok.
The problem is that it does not do that... I am able to get 400-600mA tops, varying with Vds. Am I reading this graph correctly?

Marcin.

[JoeO]

How about posting a link to the data sheet you are using.

[Psi]

I had a quick look, and the graph does seem to show 5V GS being able to switch a few amps.

But they're probably assuming other factors are as favorable as possible to make the fet seem good.  Wouldn't surprise me if the graph was made at 100deg temp or something stupid like that.

Edit:
Also, I notice that, in the datasheet im reading, that graph was made at Vds 25V
If your circuit is 5V that may explain the smaller current. The on resistance may very well be enough for 2.5A at 25V but only 600mA at 5V ?

[alxnik]

I have recently built a simple dummy load, the one from Dave's video.
I haven't used a logic level mosfet as Dave suggested but I used simple BUZ11. Why? In datasheet there is a graph "Output characteritics" which shows that at 4V gate to source voltage (5 volt Vcc - non rail to rail op amp clipping, what I am getting at op amp output), it should be able to pass about 2.5A. So I thought it will be ok.
The problem is that it does not do that... I am able to get 400-600mA tops, varying with Vds. Am I reading this graph correctly?

Marcin.

The datasheet assumes Vds = 25V. I assume that you are not trying with 25V. Given that the threshold voltage is ~3V, I don't think you can achieve much with 4-5V Vgs

[mmilejski]

I was using Fairchild Semiconductor datasheet, the first one on google.
It has many characteristics graphs for different drain to source voltages, and I tried Vds on whole range to 30V.

Whats you current sense resistor value? That will take away some of your Vgs. Say 800mA through a 1R resistor your 4 Vgs is now 3.2Vgs.

That might and probably is the case. I did not consider voltage drop across current sense resistor (1R). I have already ordered some logic level MOSFETs so that will solve the problem.

Marcin

[alxnik]

I was using Fairchild Semiconductor datasheet, the first one on google.
It has many characteristics graphs for different drain to source voltages, and I tried Vds on whole range to 30V.

Whats you current sense resistor value? That will take away some of your Vgs. Say 800mA through a 1R resistor your 4 Vgs is now 3.2Vgs.

That might and probably is the case. I did not consider voltage drop across current sense resistor (1R). I have already ordered some logic level MOSFETs so that will solve the problem.

Marcin

Or you can increase the opamp input voltage to ~10V, generic opamps can handle it. Or lower the sense resistor. Although Dave put an 1? for easier math, 1? @ 2.5? = 6.25W

[mmilejski]

Or you can increase the opamp input voltage to ~10V, generic opamps can handle it. Or lower the sense resistor. Although Dave put an 1? for easier math, 1? @ 2.5? = 6.25W

I know, but want to use 9V battery with LM7805 as a power supply, and with 5V supply I can add microcontroller (without any additional regulators) with a rotary encoder to act as cheaper substitute for multiturn potentiometer
I also ordered 1% 1R resistors to connect 10 of them in parallel, but I don't think I'll need higher currents so I'll stick to my current 1R 5W or order precision-ish 10R (1% 0.6W) resistors to parallel them up and be able to handle up to about 2.3 amps.

[mmilejski]

Seems like you are trying to pound a square into a circle. If you are set on the battery and 5V reg then you should include RR output opamp in your order. For 2.5A through a 1R you will lose 2.5V Vgs even with RR opamp that leaves about 2.4V gate source voltage. Why not just use a wallwart to power it and then use a regulator for the micro and display?

Standard mosfets are more durable and a much wider selection is available as well as non RR opamps.

I have included rail-to-rail output op-amp in my order. Nonetheless you changed my mind about power suppply, and I probably will use standard wall wart to power the circuit. Current consumption will surely jump up when I add micro and simple 8x2 character display, and that certainly would drain battery in no time.