MOSFET drain to source current - Page 1
Forum section: electronics / beginners

author: mmilejski
author_rank: Contributor
timestamp: September 10, 2013, 09:43:02 am
content: 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.

author: JoeO
author_rank: Frequent Contributor
timestamp: September 10, 2013, 11:49:45 am
content: How about posting a link to the data sheet you are using.

author: Psi
author_rank: Super Contributor
timestamp: September 10, 2013, 11:56:38 am
content: 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 25VIf 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 ?

author: alxnik
author_rank: Regular Contributor
timestamp: September 10, 2013, 12:18:08 pm
content: Quote from: mmilejski on September 10, 2013, 09:43:02 amThe 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

author: mmilejski
author_rank: Contributor
timestamp: September 10, 2013, 12:42:38 pm
content: I was using Fairchild Semiconductor datasheet, the first one on google.It has many characteristics graphs for different drain to source voltages, andI tried Vds on whole range to 30V.Quote from: AcHmed99 on September 10, 2013, 12:00:05 pmThat 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

author: alxnik
author_rank: Regular Contributor
timestamp: September 10, 2013, 01:01:36 pm
content: Quote from: mmilejski on September 10, 2013, 12:42:38 pmOr 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

author: mmilejski
author_rank: Contributor
timestamp: September 10, 2013, 03:31:49 pm
content: Quote from: alxnik on September 10, 2013, 01:01:36 pmI 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 potentiometerI 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.

author: mmilejski
author_rank: Contributor
timestamp: September 10, 2013, 05:00:19 pm
content: Quote from: AcHmed99 on September 10, 2013, 04:26:07 pmI 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.