N-Channel Mosfet as a switch - Page 1
Forum section: electronics / beginners

author: mribble
author_rank: Regular Contributor
timestamp: July 03, 2015, 11:09:14 pm
content: I am triggering photography flashes. Today I'm using an optoisolator, but am considering changing to an n-channel mosfet. To trigger a photography flash you just need a switch. One pin from the flash provides between 3 and 12V. The other pin is common and when they are shorted the flash is triggered.I have done some testing with a BSS138 mosfet (http://www.mouser.com/ds/2/302/BSS138P-351728.pdf). It seems to work well on all the flashes I tested, but thought I'd check with you experts to see if this is a good mosfet to use or if there is a reason I shouldn't use a mosfet.As I mentioned flashes that I'm working with use 3-12V. I would connect this to the drain and connect the flash common to the ground with my micro. Then I'd use a pin on the micro (3.3V) to trigger the flash. The reason I was using an opto before is because there are flashes that run at 600V, but in reality those were destroying the optoisolator anyways and already tell people not to use those. I'm not sure how mosfets will fail, but I'm guessing it isn't as nice as an optoisolator which fails in a very safe way for my use case.The big advantage is most of the optoisolators have a rise time of around 10 us and these mosfets are much faster (this matters for certain cases).These mosfets have a drain-source breakdown of 60V. That means it is safe to use with 60V inputs, right? After that the mosfet is destroyed? Can someone tell me how a mosfet typically fails? For example where does the high voltage go. If it just creates an open circuit that's great, but if it jumps to the gate at a certain voltage that means destroying my micro which I can live with. I just want to understand how these fail with too high of drain-source voltage.Thanks in advance!

author: SeanB
author_rank: Super Contributor
timestamp: July 05, 2015, 05:23:02 am
content: Older flash units have up to 400v on the flash trigger, which will cause the MOSFET to avalanche over drain to source. Provided the current and thus power is limited to less than the package can handle it will only slowly degrade the device. The flash however will trigger whenever the unit breaks down, and over time it will eventually either fail to trigger or will go short circuit.You probably want to get a logic level device with a 400V or higher capability, the gate charge required will be higher, so you probably will want a buffer, and almost any CMOS buffer like a 4069 or a 4050 will work, using all 6 in parallel. You probably will also want a 33R resistor in series with the gate as well, and this will still switch very fast, and will work with any flash unit. Current rating of the unit is not really an issue, all devices with 400V or igher will be capable of at least 5A, and will come in either a TO220 or TO247 package, though you might be able to get some in a D pack surface mount version. Here the power dissipation is not an issue so any package will work on it's own. Flash units can have a very high peak current in the trigger circuit, as you are discharging capacitors into a transformer in most units.As many units do not always define a polarity on the flash trigger connector ( older cameras used a mechanical switch which is polarity insensitive) you also will want to have a bridge rectifier on the input to ensure it will work irrespective of polarity, using any 4 1kV rated 1A diodes like UF4007. It will survive as the pulse current is within the single non repetitive pulse rating, and the 4 take up little board space.