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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Tact switch and mosfet transistor questions
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Online Breaktru

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Re: Tact switch and mosfet transistor questions
« Reply #50 on: April 21, 2014, 04:37:57 PM »
Charlie,
You can use a STD30PF03L. It comes in a DPAK or IPAK footprint. Your choice of physical size.
See the P-Ch diagram in Reply #13 in this thread.

Here are P-Ch Mosfets with lower internal resistance: FDD3706/FDU3706 (lowest resistance) and FDS6576

Offline CharlieC

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Re: Tact switch and mosfet transistor questions
« Reply #51 on: April 22, 2014, 12:29:27 AM »
Thanks Bteaktru

I am excited to make something with this

Offline rdwilliamson

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Re: Tact switch and mosfet transistor questions
« Reply #52 on: April 28, 2014, 06:10:02 PM »
Where would someone put a zener to cut the votlage of when it reached 3. volts or so?

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #53 on: April 29, 2014, 02:03:30 PM »
You can use a STD30PF03L. It comes in a DPAK or IPAK footprint. Your choice of physical size.
See the P-Ch diagram in Reply #13 in this thread.

Here are P-Ch Mosfets with lower internal resistance: FDD3706/FDU3706 (lowest resistance) and FDS6576

Unfortunately the ones hot off the press only come in the newer surface mount packages.  You need to mount those on a host board.  Not a big deal to make a host board and mount the part, but not as simple as soldering wires to pins.

The latest models with the lowest on resistance are not being made available in the the leaded packages.  You can still find parts in leaded packages that work well enough, but if you want the best performance possible, you have to go with one of the newer packages.  They all go by different names from different makers, for example Vishay calls them PowerPAK SO-8, Fairchild calls them PQFN 5x6 8L, and TI calls them Q5 or Q5A. 

Here's an example; http://www.vishay.com/docs/62860/si7157dp.pdf

You can also find some nice power FETs in a 3x3mm DFN type package, but those are a bit hard to work with since they're pretty small.  Though they're good if you want to absolutely minimize the footprint.

Here's an example; http://www.vishay.com/docs/62909/si7655adn.pdf

Offline mrbump82

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Re: Tact switch and mosfet transistor questions
« Reply #54 on: May 19, 2014, 10:38:44 AM »
Ok I got a few mosfets mc7805ct will this work to wire in my tact switch I can't figure out if it's a p or n channel

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Re: Tact switch and mosfet transistor questions
« Reply #55 on: May 19, 2014, 11:16:34 AM »
The MC7805CT is a Voltage Regulator w/ a 5V output. Not a Mosfet
http://www.lmgtfy.com/?q=MC7805CT

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #56 on: May 19, 2014, 03:04:35 PM »
Pretty much all electronics vendors provide links to data sheets with their parts listings.  When you have questions about a part, just post a link to the data sheet and someone here can answer them for you.  Don't really know how you got from a MOSFET to a regulator, you probably got the part number wrong.

Offline Madyicstik

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Re: Tact switch and mosfet transistor questions
« Reply #57 on: August 01, 2014, 10:37:47 AM »
Apologies for reviving an old thread. Is there someone who can help with the above post? Im also i terested on  doing one of this mosfet / tact switches that can go sub ohm on a  3.7v/4.2v input. Can anybody help?

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Re: Tact switch and mosfet transistor questions
« Reply #58 on: August 01, 2014, 10:49:31 AM »
No such thing as an old thread/topic if it is relevant.

See Reply #13

Offline Madyicstik

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Re: Tact switch and mosfet transistor questions
« Reply #59 on: August 01, 2014, 12:32:47 PM »
Break as always you and the vets are an awesome wealth of knowledge. Appreciate it. I just bought a who,e bunch of the irl 1303 it should be here on monday!!

Offline BOFH

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Re: Tact switch and mosfet transistor questions
« Reply #60 on: August 04, 2014, 08:09:05 AM »
Hi

Is it possible to use a Zenner Diode with the N CH Mosfett to provide a low voltage cut off?  As im planning on using 2 Lipo packs in parallel probably go with something like this http://uk.mouser.com/ProductDetail/NXP-Semiconductors/NZX3V3C133/?qs=sGAEpiMZZMtQ8nqTKtFS%2fISyHb0gtGyWnMdca58vn6A%3d .

Placing the Diode in the circuit as shown on the attachment? I assume this would prevent the on signal to the mosfet?

Thanks
« Last Edit: August 04, 2014, 08:12:20 AM by BOFH »

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #61 on: August 04, 2014, 04:03:51 PM »
I sometimes get asked technical questions in PM where I feel my answers could be a general benefit to others.  Don't really understand why people feel they need to contact me in PM for technical info, kind of the point of the forum, but that's what they do sometimes.

In any case, I got a series of questions about using a MOSFET and I thought I would post my answers in the event they can be generally helpful.  This was the newest applicable thread so I'm posting in this one.  Here's the exchange;

Quote
I got into modding and been working on a dual 26650 mech box mod I finally learned enough from reading about all the things you've done to put a n channel mosfet in my mod but the voltage drop seems a little much I'm using the irlu3114 with a 10k resistor the vgs is 1v to 2.5v is it possible the gates not opening all the way or can I run the mosfets in parallel.

In general, you need to ensure the voltage between the gate pin and source is equal to or greater than a value specified in the part's data sheet;

http://www.mouser.com/ds/2/200/international%20rectifier_irlr3114zpbf-342321.pdf

If you look on page 2 of that data sheet, you'll see the RDS(on) value specified at 4.5V.  So to get a resistance of 5.2 mOhms, you need at least 4.5V between the gate and source pin.  If you go lower than that, "on state" transistor resistance goes up quickly.

When using a single cell (or parallel cells), you need a transistor that is specified at 2.5V.  The IRLR3114 is specified at 4.5V.  Here's an example;

http://www.fairchildsemi.com/ds/FD/FDD3706.pdf

Also, you need to ensure you're using the correct polarity.  If you want to switch the high side (positive), you need a P-channel MOSFET.  If you want to switch the low side (negative), you need an N-channel MOSFET.

An N-channel requires a pull-down resistor between gate and source.  Pull-down simply means the resistor is connected to battery negative along with the transistors's source pin.  The other side of the resistor is connected to the switch and the transistors's gate pin.  The opposite side of the switch is connected to battery positive.

A P-channel requires a pull-up resistor between gate and source.  Pull-up simply means the resistor is connected to battery positive along with the transistor's source pin.  The other side of the resistor is connected to the switch and the transistors's gate pin.  The opposite side of the switch is connected to battery negative.

Quote
would you know of a n channel mosfet that can handle around 100a or at least 75a that would be good.

You need to go to a leadless package if you want a small MOSFET able to handle high currents while also driven by a single cell or a set of parallel cells.  They don't have leads so you have to make a little host board for one.  Here's an example;

http://www.vishay.com/docs/64815/sir404dp.pdf

100A is a lofty requirement.  You won't find a 2.5V specified MOSFET with a current rating that high.  The one above would be about the best you can do in terms of a 2.5V specified part.  You can find ones in a small leadless package 100A and over, but they're 4.5V or 10V specified.  Though I don't really understand the need for such a high current rating.  A 60A MOSFET should be more than enough for anything you could do with an e-cig.

Quote
and where can I learn about host boards

A host board is nothing special.  It's just a small PCB that allows you to tie wires to a component that does not have leads.  You can simply get some FR4 copper clad off eBay and cut out a small piece with a metal saw or Dremel tool.  Then you can probably just cut the pads for the MOSFET with the Dremel tool as well.  It's only 3 pads.  Then you can reflow solder the part to the board.

If you want to etch your host boards, there are a ton of articles on the net about making your own PCBs.  Do a search on "PCB toner transfer method".  Also search on "toaster oven reflow soldering".  There's tons of articles on that as well.

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Re: Tact switch and mosfet transistor questions
« Reply #62 on: August 04, 2014, 05:08:34 PM »
Very good Craig. That should help a lot of folks.

If I said it once, I said it 100 times to members who PM me on questions as well. Please post it so others can benefit as well.

Offline david4500

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Re: Tact switch and mosfet transistor questions
« Reply #63 on: August 05, 2014, 02:08:40 AM »
Hi

Is it possible to use a Zenner Diode with the N CH Mosfett to provide a low voltage cut off?  As im planning on using 2 Lipo packs in parallel probably go with something like this http://uk.mouser.com/ProductDetail/NXP-Semiconductors/NZX3V3C133/?qs=sGAEpiMZZMtQ8nqTKtFS%2fISyHb0gtGyWnMdca58vn6A%3d .

Placing the Diode in the circuit as shown on the attachment? I assume this would prevent the on signal to the mosfet?

Thanks

I am wondering this as well. I have ordered some 3.0v zener diodes.

Offline Madyicstik

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Re: Tact switch and mosfet transistor questions
« Reply #64 on: August 06, 2014, 01:16:43 AM »
I have a couple of irlb3034 from previous project (airsoft mosfet) will i be able to use those and wire them same as irl 3103? Like the diagram on reply #13?

Offline banshee

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Re: Tact switch and mosfet transistor questions
« Reply #65 on: August 06, 2014, 08:02:20 AM »
The 3034 should work fine as it is also an N-ch mosfet and with lower resistance than the 3103

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Re: Tact switch and mosfet transistor questions
« Reply #66 on: August 06, 2014, 08:50:56 AM »
thanks banshee, how much of an amp draw does the "mosfet switch" would allow? im planning to use it in an un regulated box, that will mostlikely do sub ohm.

Offline banshee

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Re: Tact switch and mosfet transistor questions
« Reply #67 on: August 06, 2014, 11:27:25 AM »
Here is the datasheet for it: irlb3034

Offline dc99

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Re: Tact switch and mosfet transistor questions
« Reply #68 on: August 10, 2014, 05:34:54 PM »
This is how it should be wired. Note the part numbers
I have a couple of question about the master switch and the resistor. Im doing this on a duel 18650 with an std30pf03l mosfet. Is the 15k the same on a duel battery and how much current does the master on/off need to carry?

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Re: Tact switch and mosfet transistor questions
« Reply #69 on: August 10, 2014, 06:23:20 PM »
The 15K should work fine with 2 series or parallel 18650s if using it with a tact switch. If using it as a touch, then 100meg maybe in order.

You won't be switching the on/off under load which can cause arcing and be detrimental to the switch contacts.
A 3A @ 125V on/off which equates to 375W. If using 8.4v with 2 series batts the switch should be able to handle 44.64A at 8.4v

See THIS for switch amps/volts.

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #70 on: August 10, 2014, 06:24:14 PM »
For parallel cells, there's no difference, you can think of them as one bigger cell.  For series cells, voltage is doubled so current for a given resistance is also doubled.  There may be considerations there, but the 15k resistor will work fine with either battery configuration.

In reviewing this thread to answer that, I noticed an unanswered question;

You can not simply use a Zener diode on the gate pin of a MOSFET to provide a low voltage cut-out.  This is actually the third time I've seen this idea, don't understand why people keep coming up with it.

You have to provide a digital voltage level to the MOSFET gate, either on or off.  A Zener diode is an analog part that is not able to generate a digital voltage level.  It ramps current up and down with voltage.  Using one in a divider on the MOSFET gate will put the transistor in its active region. 

The active region of a transistor is where it behaves more like a variable resistor than a switch.  Remember the primary function of a transistor is to provide a variable current source.  Though MOSFETs are designed to be used as a switch, they still have this active region where they behave like a traditional transistor.

It's possible to use a Zener with an array or transistors to generate a digital signal.  You can use a comparator chip to do that more simply.  The most simple way is with a voltage detector which is a chip designed exactly for that purpose.

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Re: Tact switch and mosfet transistor questions
« Reply #71 on: August 10, 2014, 06:44:20 PM »
Thanks. Its just a simple parallel box. Just wanted to make sure. I actually have a bunch of 20 and 30amp round switches from the car stereo linear actuator days. Im pretty sure they would handle almost anything I would throw at it but the mosfet sounds a lot safer than just hoping the switch doesn't stick

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Re: Tact switch and mosfet transistor questions
« Reply #72 on: August 13, 2014, 08:15:26 PM »
After reading for the last few days, Im now hooked. Would you tell me if all this works together for a duel 18650 box.
Already ordered a crap load of tact switches
low voltage cut-NCP300HSN30T1G
mosfet-STD30PF03LT4
fuse- assume 2 for each batt-30R300UPR or 16R300GU

Offline T8ternutz

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Re: Tact switch and mosfet transistor questions
« Reply #73 on: August 15, 2014, 02:43:34 PM »
This is how it should be wired. Note the part numbers

http://breaktru.com/smf/index.php?action=dlattach;topic=616.0;attach=1366;image

Would this be wired the same way if replacing the tact with a standard N/O Switch?

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Re: Tact switch and mosfet transistor questions
« Reply #74 on: August 15, 2014, 03:04:56 PM »
The switch used in the diagram is already an NO (normally open) momentary switch.  A momentary NO switch operates the same electrically whether that's a tactile or a high current switch.

You can use any type of switch you like there.  However, the primary advantage of using a transistor is it allows a small current to control a big current.  With a transistor you can use a small low current switch.  With the inclusion of the transistor it still consumes much less space than a switch able to handle high currents directly.

Also, a transistor is more reliable than a switch and with careful selection can waste less power.  Transistors capable of supporting as much as 50A current are still relatively small.  There's the wear on a tactile that has to be considered, but generally tactiles have much higher cycle life than heavy switches.

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Re: Tact switch and mosfet transistor questions
« Reply #75 on: August 15, 2014, 06:47:52 PM »
The drawing was great and all but it didn't have fuses or the low voltage cut. (are the ones I listed ok and which do you like better)  I was making sure I didn't order a bunch of stuff that didn't work together.

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Re: Tact switch and mosfet transistor questions
« Reply #76 on: August 16, 2014, 04:30:34 PM »
I assume you mean two parallel cells when you say "dual 18650 box".  The parts you listed will work for a single cell or dual parallel cells.  Though they will work, you can make a better selection on the fuses and MOSFET.

You can select a MOSFET in a similar package with a lower gate-source threshold.  That will provide lower "on" resistance.  It wastes less power and reduces voltage loss in the transistor. 

You do have to be picky with MOSFETs.  They can vary quite a bit in terms of "on" resistance and threshold voltage.  There's actually good reasons why that's the case, has to do with speed and voltage tolerance.  Speed matters for certain applications, but not for a user on/off switch.  Voltage is low with a single cell or parallel cells so a higher gate-source or drain-source voltage tolerance is not required.

There's a few ways to spot those low threshold voltage MOSFETs.  First, they often have an RDS(on) specification for 2.5V at the top of the data sheet.  Another way to spot them is they'll have a maximal gate-source tolerance of 12V or less stated in the absolute maximums.  Finally, you can look at the gate-source threshold listed in the data sheet, it will have a minumum less than than .8V and a maximum not greater than 1.5V.

You can use two 3A hold fuses on each or cell or one 6A hold fuse on each cell.  You'll get lower resistance with four 3A fuses in total, but that may or may not be better in terms of the fit inside the enclosure and of course there's more parts.  Your call on that. 

You can find fuses with lower resistance than the ones you listed.  You want to look for the lowest resistance available with the required hold/trip currents. 

There's a also a matter of the fuse's physical size.  The higher the voltage and current tolerance, the bigger they are.  You can reduce size by looking at ones with a lower voltage tolerance.  The lowest they go is typically 6V and a Li-Ion cell is only 4.2V so just look for the smallest ones with the lowest resistance.


« Last Edit: August 16, 2014, 05:01:41 PM by CraigHB »

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Re: Tact switch and mosfet transistor questions
« Reply #77 on: August 16, 2014, 07:08:32 PM »
Thank you. Well back to the drawing board, back to mouser anyway

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« Last Edit: August 26, 2014, 12:17:20 PM by david4500 »

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Re: Tact switch and mosfet transistor questions
« Reply #79 on: August 17, 2014, 07:42:09 PM »
Nice job david  :rockin smiley:
Very neat work. Looks very professional  :beer-toast:

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Re: Tact switch and mosfet transistor questions
« Reply #81 on: August 17, 2014, 09:39:38 PM »
Lower resistance, less voltage drop
« Last Edit: August 18, 2014, 06:37:26 AM by david4500 »

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Re: Tact switch and mosfet transistor questions
« Reply #82 on: August 18, 2014, 03:50:52 PM »
It's not a problem to parallel MOSFETs.  There are some derating issues that have to be considered in some cases, but nothing significant for a simple power switch.  The derating issues only become significant in rectification applications where you rely on the transistor's body diode.

That's a 4.5V specified MOSFET so it's going to have higher resistance at the voltages you see with a single cell or parallel cells.  "On" resistance is about five times greater at 3V gate-source voltage than it is at 4.5V gate-source voltage.  If you look at figure 3 in the part's data sheet you can see that.

You could actually do better with a single 2.5V specified MOSFET than two 4.5V specified MOSFETs in parallel.  However, it's hard to find the 2.5V specified ones in a leaded package.  They're almost strictly found in a leadless package.  That being the case, using two of those in parallel may be the best you can do in a leaded package.  Of course for series cells it's not an issue since gate-source voltages are double.

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« Last Edit: August 18, 2014, 09:17:01 PM by david4500 »

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Re: Tact switch and mosfet transistor questions
« Reply #84 on: August 19, 2014, 03:54:10 AM »
No, that one is not a 2.5V specified part.  Even though it says logic level, that term can be interpreted loosely.  For modern electronics, logic level means a 3.3V supply which calls for a 2.5V specified MOSFET.  Older electronics used a 5V supply so a 4.5V specified MOSFET could be called logic level back then.

If you look at figure 7, you'll see that the "on" resistance curve doesn't flatten out until about 4V which is typical for a 4.5V specified MOSFET.  You can see on page 5 that the gate source threshold is 1.3 to 2.2V which is also typical for a 4.5V specified MOSFET.  2.5V specified parts have a threshold that ranges typically from .6 to 1.5V.

The best graph to look at for behavior with respect to gate-source voltage is the one shown in figure 7.  Most of the newer parts have that chart, but for some reason the older data sheets don't always show that.  The Infineon part mentioned before does not have that chart in its data sheet.  In that case, the chart shown in figure 6 can give you an idea, but it's not going to define "on" resistance specifically.   Figure 6 shows the transfer characteristic which is how the part reacts to a VGS pulse with no resistance in series with the drain and source.

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« Last Edit: September 01, 2014, 05:04:39 PM by david4500 »

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Re: Tact switch and mosfet transistor questions
« Reply #86 on: August 25, 2014, 05:14:37 PM »
That's a good one, you can see the curve is pretty much in the flats at 3.0V which would be a typical minimum for a single cell.  Though it's a 4.5V specified part, it has a curve that gets in just under the wire for a single cell.  Typically, the 4.5V specified parts don't get into the flat part until about 4V, but that one has a curve more to the left than most for some reason.  That chart is the one that matters so whatever that one says is what goes.

Offline david4500

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Re: Tact switch and mosfet transistor questions
« Reply #87 on: August 25, 2014, 07:42:03 PM »
Incredibly helpful. Thank you!

Offline dc99

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Re: Tact switch and mosfet transistor questions
« Reply #88 on: September 01, 2014, 04:55:53 PM »
Ok guys, I know this has been went through but cut me some slack. Will this be ok or do you have some better suggestions for a duel parallel 18650 box.
mosfet: STD30PF03LT4
Fuse:16R600GU  (2)
LVC: NCP300HSN30T1G
Res:MF1/4 CCT52R1502F
and FR4 board
I have 125w 3v on/off and tact switches

Offline david4500

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« Last Edit: September 01, 2014, 07:30:42 PM by david4500 »

Offline dc99

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Re: Tact switch and mosfet transistor questions
« Reply #90 on: September 01, 2014, 07:42:05 PM »
I actually looked at that mosfet but to be honest Im not sure I even know how to mount it, let alone wire it.  Im not new to electronics but I am new to this. My background was in car audio. Its way more forgiving. I am dam... and determined to learn everything I can. I do thank all of you for being patient

Offline david4500

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Re: Tact switch and mosfet transistor questions
« Reply #91 on: September 01, 2014, 07:59:09 PM »
Check out this n-channel mosfet and diagram instead:

http://www.mouser.com/Search/ProductDetail.aspx?qs=5XOdhvmYM0MZtKYUGpYp1Q%3d%3d



Instead of the 2x15 amp fuses, you can use 2x6 amp. The device will be limited to 12 amps when using two batteries in parallel, 6 with a single battery.

You can use 18 gauge wire instead of 14 from the fuses to 510 & source pin to batt negative.

Also 20 gauge instead of 18 from each battery to fuse,
« Last Edit: September 01, 2014, 08:15:07 PM by david4500 »

Offline CraigHB

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Re: Tact switch and mosfet transistor questions
« Reply #92 on: September 01, 2014, 08:06:32 PM »
Those SMD parts can be a bit intimidating I'll admit, but once you've actually done one, you'll be surprised that you were concerned about working with them.  The steps required are not difficult, just more involved than soldering a wire to a lead and heat-shrinking it. 

Just two steps really, make a small PCB with 3 pads and reflow solder the part in a frying pan or toaster oven.  You need a few tools and supplies, but nothing major, some solder paste, some copper clad, and you need some number drills to make mount holes for your wires.  You can cut your pads in the copper clad with a Dremel which you probably have already if you're making your own enclosures.

It that's all too much, there's a couple leaded TO-220 parts that have been mentioned on the forum that have pretty good specs for a single or parallel cells.  However, I have yet to see a leaded P-channel part with specs as good as the SMD P-channel parts.  If you go with one of those few leaded N-channel parts, you should be able to do just about as well as you can with an SMD part.

ex. http://www.mouser.com/Search/ProductDetail.aspx?qs=5XOdhvmYM0MZtKYUGpYp1Q%3d%3d

Offline dc99

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Re: Tact switch and mosfet transistor questions
« Reply #93 on: September 01, 2014, 08:36:22 PM »
Well, if Im going to learn it then I might as well learn it right. Hopefully there are a few videos on the subject. The good part is that Im not in any hurry to do this. Guess I might better order a few extra parts.

Offline T8ternutz

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Re: Tact switch and mosfet transistor questions
« Reply #94 on: September 02, 2014, 11:32:37 AM »
when running mosfets parallel would that require 2  15K resistors or just the one mounted to the top mosfet?

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Re: Tact switch and mosfet transistor questions
« Reply #95 on: September 02, 2014, 01:49:42 PM »
Just need one pull-up (PMOS) or pull-down (NMOS) resistor for the both of them.  You can use the same value as you would for a single MOSFET.

Offline longwhiteclouds

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Re: Tact switch and mosfet transistor questions
« Reply #96 on: September 02, 2014, 06:55:53 PM »
Ordered some psmn1r9-40pl. Going to make one based on the circuit david linked and one driven with a LTC1157. See what works best.

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Re: Tact switch and mosfet transistor questions
« Reply #97 on: September 02, 2014, 08:20:18 PM »
Craig, do you have a recommendation for an smd p-chan?
I have mf1/4dct52r1502f resistors
ncp300hsnt1g low voltage cutoff
rgef600k-ap fuses

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Re: Tact switch and mosfet transistor questions
« Reply #98 on: September 02, 2014, 08:41:32 PM »
If you're going to use a high side NMOS driver, you can use any MOSFET really.  The driver will provide a 5.4V gate-source voltage so you don't need a low threshold voltage.  Go for the lowest on resistance.  This one is better;  http://www.nxp.com/documents/data_sheet/PSMN1R1-30PL.pdf 

The best P-channel I've come across for use as a user power switch is this one; http://www.vishay.com/docs/62860/si7157dp.pdf

Offline david4500

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Re: Tact switch and mosfet transistor questions
« Reply #99 on: September 02, 2014, 09:49:10 PM »
when running mosfets parallel would that require 2  15K resistors or just the one mounted to the top mosfet?


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