suggestion for N-FET driven by 3V MCU

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Jepael
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Posted by Jepael: Wed. Jan 22, 2014 - 01:38 PM
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So my component-fu skills have gotten a bit outdated, and I tried to select something like 2N7002 or BSS138 to drive a bunch of relays with 3.3V MCU.

Those would work great with 5V logic levels but I am a bit afraid different manufacturers have a bit different specs and 3.3V might not be enough to turn the FETs fully on.

Any pointers for common low-side FETs that eat 3.3V logic levels and can drive 12V relays (max 100mA)?

Also I will put BAT54 schottky over the coil, it will eat the max 100mA back-emf nicely.

Too bad the 100mA includes multiple coils, so even if I put one diode per coil, one diode will catch all back-emf so the area may be large.

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Learning and Information, General Electronics
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Bingo600
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Posted by Bingo600: Wed. Jan 22, 2014 - 01:53 PM
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I'm sure i read somewhere that the BSS138 would do it with 3v3.
The 2N7002 is on the "border" ......

/Bingo

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chartman
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Posted by chartman: Wed. Jan 22, 2014 - 03:23 PM
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NDS7002A has typ Vgs of 2.1 and max 2.5V ... Id 280mA and pulsed 1500mA

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ka7ehk
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Posted by ka7ehk: Wed. Jan 22, 2014 - 05:13 PM
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BSS138 is good down to 18V, IIRC. The on-resistance may be a bit high (2-3 ohms) at such low gate voltages, but at modest power levels, it does nicely. It is readily available and relatively inexpensive.

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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meslomp
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Posted by meslomp: Thu. Jan 23, 2014 - 06:45 AM
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Jim, I assume you misplaced the comma there and it should be 1,8V instead of 18V....

NXP NX3008NBK or ...PBK for N and P type

as more and more systems go to lower supply voltages there should be a large number of fets available these days though....

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Who-me
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Posted by Who-me: Thu. Jan 23, 2014 - 07:22 AM
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Fairchild FDV3xx series, or there is also Digital / Resistor Equipped Transistors ?

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Jepael
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Posted by Jepael: Thu. Jan 23, 2014 - 12:51 PM
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Great, thanks, now I'll have something to compare with.

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ka7ehk
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Posted by ka7ehk: Thu. Jan 23, 2014 - 05:56 PM
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Yes, meant 1.8V (we use periods for decimal points on the other side of the ocean).

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Jepael
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Posted by Jepael: Thu. Jan 23, 2014 - 08:52 PM
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Which again hit me. The decimal separators.

Every frigging Excel spreadsheet from component manufacturers that are supposed to help with component design somehow (like calculating thermal design power with given power supply voltage) have no sense for regional settings of Excel/Windows in any way.

I use local regional settings where the decimal separator is a point. Some code in those Excel sheets complain that I cannot enter voltage as 3.3 because it must be entered as 3,3 but if I enter 3,3 the thing won't calculate anything as to Excel itself 3,3 is not a number but 3.3 is.

The solution is not nice, you have to change the decimal separator in spreadsheet program globally, you cannot of course select the separator just for one of the open files. This makes no sense.

Sorry for the rant, this went offtopic.

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valusoft
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Posted by valusoft: Thu. Jan 23, 2014 - 10:39 PM
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:lol: So enter whole millivolts and divide by 1000 at the end. Or should that be 1,000 :lol: :lol: Sorry.

Ross McKenzie, Melbourne Australia

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