Mosfet question - Page 1
Forum section: electronics / projects

author: neo
author_rank: Super Contributor
timestamp: September 26, 2016, 01:17:35 am
content: How can i use a mosfet to switch positive voltage, such as to say when its base gets 5 volts its a direct short across it allowing 20v through, my problem is that this isn't an led its a vfd tube and all it needs is voltage on its pinsPost note, this is in no way related to multiplexing post, and no a driver chip isnt an option this is for a cmos clock that im making and want to avoid micro-controllers on it

author: moffy
author_rank: Super Contributor
timestamp: September 26, 2016, 01:51:12 am
content: As long as the Source is connected to ground and the 5v Gate signal is referenced to ground a logic level N channel MOSFET should do the job. Just choose the one to suit your voltage and current.

author: neo
author_rank: Super Contributor
timestamp: September 26, 2016, 01:58:57 am
content: but how does that switch on the voltage? i dont need to connect the vfd pin to ground i just need to get it 20v

author: Skashkash
author_rank: Regular Contributor
timestamp: September 26, 2016, 03:07:58 am
content: You might possibly simplify your circuit by running the vfd cathodes at a negative voltage.If you are designing the HV circuit, it's often just as easy to make it a negative voltage.Then your anode switches just need to turn on to +5 (or whatever) to light the anode.If your circuit is already designed and you have a positive HV, then nevermind. Use p-ch fets as high side switches as has already been suggested

author: neo
author_rank: Super Contributor
timestamp: September 26, 2016, 03:09:54 am
content: Quote from: Skashkash on September 26, 2016, 03:07:58 amyou my friend are a genius, so they will run just fine if its -20v? so how would i do that ground the positive side to the metal box? perhaps if you dont mind give me an example circuit as to how id wire it or a link to one

author: Skashkash
author_rank: Regular Contributor
timestamp: September 26, 2016, 03:24:15 am
content: It's a pretty standard technique. I'm not sure what you mean by grounding the metal box.Might be some mis communication here.The filament on a vfd is the cathode. That's your most negative point relative to the display. It does get a little more complicated if the ac drive for your filament is not isolated.Some notes are here, but sorry I don't have a schematic to share.https://www.noritake-elec.com/display/vfd_operation.htmlIf I can find a good link to a complete design I'll post it.

author: neo
author_rank: Super Contributor
timestamp: September 26, 2016, 03:35:21 am
content: what i meant is how do i ground it , i cant ground a negative supply to one of the regular ground points as i understand it

author: neo
author_rank: Super Contributor
timestamp: September 26, 2016, 07:47:07 am
content: let me put it this way i know where to put positive voltage but i dont know how to ground it, the best idea i can come up with would be to use the filament ground (the filament on these runs best on dc as ive found so far) Based off of google grounding it to the filament seems convention and lt spice tells me it will work so unless someone has something to add or knows better than me i think this conversation is done