N channel mosfet Question - Page 1
Forum section: electronics / beginners

author: BrokenTrace
author_rank: Newbie
timestamp: December 13, 2014, 07:02:30 pm
content: Hello!I want to make a switch to power a circuit on/off. I got the SI2302DS N channel mosfet.Datasheet:http://www.alldatasheet.com/datasheet-pdf/pdf/19134/PHILIPS/SI2302DS.htmlThis is the schematics:The question is, did I understand how to connect the N channel mosfet?R1 its a pull-up resistor, How can I calculate it? The datasheet has a lot of information, but I dont know what voltage must be on the gate to power it on, and what current will flow througt the resistor.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2014, 07:19:18 pm
content: It will probably blow the MOSFET or at least drastically shorten its life because it exceeds the maximum rated gate-source voltage of 8V.To get round this, you need a potential divider to reduce the gate voltage. If you add another resistor with the same value as R1 (call it R2), it will half the gate voltage, so give you 8V when the input voltage is 16V but you don't want to go anywhere near the maximum rated voltage, so set R1 to 100k and R2 to a slightly lower value say 82k.Is this really necessary? Can't you find a normally closed switch rated to over 600mA at 16V?

author: BrokenTrace
author_rank: Newbie
timestamp: December 13, 2014, 07:39:18 pm
content: Quote from: Hero999 on December 13, 2014, 07:19:18 pmNo, I dont find any with a small size.so you say something like:8V its the max voltage, but what could be the minimun safe value?Lowering the voltage, it lower the current flow I guess right?Appart from the voltage, how do you choose how many current will flow through the Gate? The minimun I could?Can you share a better component to do this?

author: dannyf
author_rank: Super Contributor
timestamp: December 13, 2014, 08:46:52 pm
content: QuoteYou have a circuit that will turn off the load only when the button if pushed.

author: Zero999
author_rank: Super Contributor
timestamp: December 13, 2014, 09:05:19 pm
content: Quote from: BrokenTrace on December 13, 2014, 07:39:18 pmThe minimum voltage depends on the MOSFET's threshold and the drain current. See figure 5 on the datasheet: " Output characteristics: drain current as a function of drain-source voltage" Make sure the gate voltage is high enough to allow sufficient drain current with a low drain-source voltage.Once the steady state has been reached, virtually no current flows into the gate; it looks like a capacitor.

author: BrokenTrace
author_rank: Newbie
timestamp: December 13, 2014, 10:25:57 pm
content: Quote from: dannyf on December 13, 2014, 08:46:52 pmwell its not a button itself, its a switch slider 1p2t, so it keeps on or off, i dont need to hold the button. Just to make the circuit simple.@Hero999 thanksDo you think that mosfet could handle that system? or it will get too warm?P=I^2*R(dson) = 0.6^2*0.077= 0.027W It will even get warm right?That final scheme is what you think its more convenience?

author: dannyf
author_rank: Super Contributor
timestamp: December 13, 2014, 10:37:57 pm
content: QuoteWouldn't it simplier to just put the "switch slider" in to switch on / off the circuit? What value does that mosfet add?QuoteCheck the datasheet for thermal resistance.