I did I kill my mosfet ? - Page 1
Forum section: electronics / beginners

author: safarir
author_rank: Regular Contributor
timestamp: September 26, 2010, 08:14:33 pm
content: Hi,I from Quebec, Canada soo my english is very not that good.I was working on pwm on a fpga development board when suddenly, I was unable to stop my circuit. After verification, I discover that my mosfet now have a 1.3ohm resistance when the gate is at 0v.The circuit was the following:12v-FAN-Mosfet-Groundwith a 1n914 freewheel diode across the fan and a 470uF cap connect between the FAN-Mosfet and the Ground.My oscilloscope was connect at the Fan-Mosfet connection and I never detect a peak more that 13v. The max amp I see is 1amp when the fan was stuck.The mosfet datasheet:http://www.aosmd.com/pdfs/datasheet/AO3400.pdfI am just trying to figure what I did wrong since I don't want to kill the others mosfet...

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2010, 09:04:49 pm
content: Yes, it sounds like the MOSFET is dead, the gate should not be connected to the source.Just one question: did you take the MOSFET out of the circuit or at least disconnect the gate before testing it? If not, it's possible that the gate output was connected to either rail and the meter was just measuring the resistance of the gate output.

author: safarir
author_rank: Regular Contributor
timestamp: September 26, 2010, 09:31:11 pm
content: Hi Hero999,No, I didn't remove the mosfet from the board. I only connect the gate to the ground and mesure the resistance on the Drain-Source. The orders mosfet gave me 1.5mohm and this one gave me 1.6 ohm...I just did some manual test to see what happend at the drain (replaced by a switch) when I open the circuit. I have a love of difficulty to explain what I see, I have join the image in attachement.

author: safarir
author_rank: Regular Contributor
timestamp: September 26, 2010, 09:36:42 pm
content: There is a order image, I forget the diode in the last one...The 2 image are without the capacitor, I think I probably remove it when the diode died

author: safarir
author_rank: Regular Contributor
timestamp: September 26, 2010, 09:43:08 pm
content: I add a other 1n914 to simulate the body diode and I got something more similar to what I sawThe second Image is the same thing but with the 470uF capacitor

author: Waifian
author_rank: Regular Contributor
timestamp: September 26, 2010, 10:51:55 pm
content: Schematic?

author: safarir
author_rank: Regular Contributor
timestamp: September 27, 2010, 01:33:02 am
content: There is my ghetto style shematic

author: safarir
author_rank: Regular Contributor
timestamp: September 27, 2010, 01:35:02 am
content: I just look at my shematic and I was wondering if is was possible that the current that come from the dischar from the capacitor was too big when the mosfet begin to conduct.

author: KTP
author_rank: Frequent Contributor
timestamp: September 27, 2010, 02:57:48 am
content: Hmm, you have a mosfet that can handle peak currents of 30A for very short times, but if the esr of your 470uF cap is quite small there could be 80 to 100 amps being discharged from the cap through the mosfet when it first conducts.Edit: Here is a mosfet with similar characteristics (I think) discharging a 470uF cap.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2010, 07:33:54 am
content: The capacitor in parallel witht he MOSFET is a really bad idea, get rid of it. All it does it charges when the MOSFET is off and rapidly discharges when it's turned on which has a good chance of killing it.Pleasedo not use BMP formatfor posting images, it takes up too much space and takes too long to download, use GIF or PNG which generally uses an eight of the space.

author: RayJones
author_rank: Frequent Contributor
timestamp: September 27, 2010, 08:23:29 am
content: Yes the cap is in a real bad spot, shouldn't it really be across the supply?Also I would not be using a 1N914 as a back EMF clamping device, you'd be much better off with a 1N40xx style *power* diode.

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2010, 08:29:15 am
content: Quote from: RayJones on September 27, 2010, 08:23:29 amIt depends on the current: the 1N914 has a speed advantage over the 1N40xx so in low current applications it's not so clear cut, especially given that it's the diode's surge current which needs to be considered not the continuous current.

author: Unknown
author_rank: Unknown
timestamp: September 27, 2010, 08:37:58 am
content: I would prefer an 1N914 for low(ish) current applications, too (most 12V fans I know are <1A, if not <<1A). Duty cycle is pretty low, so it should be fine as long as peak current spec is not exceeded. I consider the 1N40xx way too slow for clamping, your MOSFET could be long dead before the 1N40xx has recovered. Use some fast higher-current diodes (eg. UF4004) if the 1N914/4148 is not enough.

author: sonicj
author_rank: Frequent Contributor
timestamp: September 27, 2010, 03:41:43 pm
content: Quote from: Hero999 on September 27, 2010, 07:33:54 ami made a little mac app for this... it converts the 320px x 240px Bitmap to a 640px x 480px PNG and places the old bmp in the trash. its attached if anyone cares to try it. just drag the file(s) onto the app icon. cheers!-sj

author: tecman
author_rank: Frequent Contributor
timestamp: September 27, 2010, 10:09:43 pm
content: 1N914 is a bit wimpy for the likely current. Put a 1N40XX in.I also agree whole heatedly, dump the cap.paul

author: Zero999
author_rank: Super Contributor
timestamp: September 28, 2010, 01:33:13 pm
content: Quote from: sonicj on September 27, 2010, 03:41:43 pmWhat I'd like is a PNG optimiser.Too many picture editing programs inefficiently compress PNGs or always select 24-bit or 32-bit modes when it's just a line drawing.The program should count the number of colours used and if needs be reduce the bits/pixel and save with the appropriate palette, if the alpha channel isn't being used remove it, if it's just greyscale save in greyscale mode and if it's a black and white line drawing save in monochrome mode (1-bit per pixel.

author: sonicj
author_rank: Frequent Contributor
timestamp: September 29, 2010, 02:05:36 pm
content: Quote from: Hero999 on September 28, 2010, 01:33:13 pmthats like 8 bajillion times more complicated than my app! lol!the thing i "wrote" is just a 3 step automator workflow in disguise. you might be able to make a workflow script that handles most, if not all of the variables you listed... i really don't know though as that was the first and only time i've messed with it.-sj

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2010, 06:22:34 pm
content: I suspect a Gimp script is probably the best route to go down, rather than trying to write such a program from scratch.

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 30, 2010, 03:38:41 am
content: I believe ImageMagick will do exactly what you want. And it is designed to be used in shell scripts.