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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: Diode question
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Author Topic: Diode question  (Read 14820 times)

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Offline MidnighToker

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Diode question
« on: January 09, 2012, 02:20:33 PM »
Last time I messed with diodes was in HS electronics class, so I'm in the dark on this one....I think it's stupidly simple, but want to run it past some others that know WTF they are doing. LOL

I came across a 4-way switch @ allelectronics that I thought would be great to use in a voltage display on a VV mod (using Murata circuit) to switch between battery voltage, vaping voltage and display off (with an extra position in there).  Didn't think it completely through (or pay as much attention to the description as I should have  :wallbash:) and wound up with a switch that could fry an atty in a heartbeat...

The switch only has three wires: White, Red, Black.   The positions go: Off - R/W - R/W/B - R/B.   Now when I first thought of using this, in my head it was a 4-wire setup.  Then when I got them and looked at it I figured red to pin 2 of the display, white to battery +, and black to VVo+, no problem...until I realized that position 2 connects all 3 wires which would feed 8.4v straight to the atty bypassing the fire switch and VV circuit completely.  POOF

So my question is this:
What can I put inline to prevent the "backfeed" from the 4-way?  Since this is going to the voltage display, even a small Schottky diode will affect the readout and I think going with anything more complex (i.e. current limiting) will just increase the variance.

(Link to switch http://www.allelectronics.com/make-a-store/item/PB-168/4-POSITION-PUSHBUTTON-SWITCH/1.html)

Offline CraigHB

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Re: Diode question
« Reply #1 on: January 09, 2012, 03:36:15 PM »
The only way to rectify a current without the drop of a diode is with a MOSFET.  But then you have to provide control for the MOSFET.  Might be able to do it simply with one of the switch outputs.  Not thinking too clear right now, tired, didn't sleep well.

Offline MidnighToker

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Re: Diode question
« Reply #2 on: January 09, 2012, 03:55:04 PM »
The only way to rectify a current without the drop of a diode is with a MOSFET.  But then you have to provide control for the MOSFET.  Might be able to do it simply with one of the switch outputs.  Not thinking too clear right now, tired, didn't sleep well.
Actually, that's a thought.  The display for the "Vaping voltage" would be triggered by the fire switch which I could also use for the MOSFET control.  Just wondering if this would effect the voltage drop in any way.

Ack!!!  I gotta get this thing on a breadboard and start poking... (and then try and figure out how the hell I am going to cram all of this into the space that I want to. :wallbash:)

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Re: Diode question
« Reply #3 on: January 09, 2012, 04:19:14 PM »
I found a Diode with only a 65mv forward voltage drop. Unfortunately it's $14.30.
Wouldn't it be easier to just use a 3 position slide switch?

Offline Todd

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Re: Diode question
« Reply #4 on: January 09, 2012, 08:55:02 PM »
Wouldn't it be easier to just use a 3 position slide switch?

That is what I would use. A simple on/off/on switch.

Offline MidnighToker

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Re: Diode question
« Reply #5 on: January 09, 2012, 09:35:53 PM »
I found a Diode with only a 65mv forward voltage drop. Unfortunately it's $14.30.
Wouldn't it be easier to just use a 3 position slide switch?
Yeah, MUCH easier. (and yeah, I stumbled on a few diodes, current limiting and such, going up to $130!!) But this is kind of a pet project as a gift and want to keep it as sleek as possible...all flush/near-flush buttons.
(this is why I was asking you about the digital pots the other day)

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Re: Diode question
« Reply #6 on: January 09, 2012, 10:53:44 PM »
I am assuming this is the switch you are referring to.

http://www.allelectronics.com/make-a-store/item/PB-168/4-POSITION-PUSHBUTTON-SWITCH/1.html

I actually drew it out and tried 6 wiring configurations to see if I could figure something out for you.  Mo matter how you wire it, you will always need to break the circuit somewhere with another switch.  Too bad there is not 4 wires, 1 for off, 1 for just the red, 1 for the red and white and one for red and black; then it would work.  Having all three positions turn on at 1 time causes a problem for what you want to do.

Offline MidnighToker

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Re: Diode question
« Reply #7 on: January 10, 2012, 12:12:20 AM »
Too bad there is not 4 wires, 1 for off, 1 for just the red, 1 for the red and white and one for red and black; then it would work.
That is exactly what was in my head when I grabbed them.  I dunno WTF you would use this thing for...bridging power sources?

I'm gonna keep scribbling with the MOSFET idea (just what I need more crap in a box that already can't fit all the crap that I'm stuffing in there LOL) as that seems to be the only viable solution if I don't wan't to "settle" with a V drop from the diode.

Offline Todd

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Re: Diode question
« Reply #8 on: January 10, 2012, 12:52:33 AM »
I dunno WTF you would use this thing for...bridging power sources?

Here's an idea.  I have seen a few mods that have dual atomizers.  You could run your VOUT to the red wire, one positive atty pin to the black wire and one positive atty pin to the white wire.  Then in position 1 no atty would fire, in position 2 the atty with the white wire would fire, in position 3 both the attys would fire and in position 4 the atty with the black wire would fire.

Why one would want two attys on one mod I do not know unless ofcourse you are using two cartos or atomizers with different flavors and want a quick change without changing over or I guess for double the vape of the same flavor.

You know this is something I would not do, but like I said I have seen mods with dual attys, LOL. :laughing2:

Offline MidnighToker

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Re: Diode question
« Reply #9 on: January 10, 2012, 12:56:01 AM »
Here's an idea.  I have seen a few mods that have dual atomizers.  You could run your VOUT to the red wire, one positive atty pin to the black wire and one positive atty pin to the white wire.  Then in position 1 no atty would fire, in position 2 the atty with the white wire would fire, in position 3 both the attys would fire and in position 4 the atty with the black wire would fire.

Why one would want two attys on one mod I do not know unless ofcourse you are using two cartos or atomizers with different flavors and want a quick change without changing over or I guess for double the vape of the same flavor.

You know this is something I would not do, but like I said I have seen mods with dual attys, LOL. :laughing2:
You might be on to something there...keeping this one stashed for some future deviations LMAO

Offline MidnighToker

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Re: Diode question
« Reply #10 on: January 10, 2012, 10:16:52 AM »
OK, if I go the MOSFET route (looking at this P-Channel right now http://www.vishay.com/docs/71076/71076.pdf) and I put the Black wire from the switch as the Drain on the FET (VV board would be Source, and fire button would be the Gate) I think I would wind up with the first 2 clicks showing battery voltage, then the third click only showing the VV value when the fire button is pressed.

I think I have this right in my head, but would appreciate some input.  I know that there would be a resistance loss from the FET, but it is minimal (.28 ohm @ 4.5v) and I doubt if it will effect the display enough a make a difference.

Offline CraigHB

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Re: Diode question
« Reply #11 on: January 10, 2012, 03:27:46 PM »
Need a wiring diagram.

To use a MOSFET as a rectifier, you have to reverse the drain and source.  Think of it in terms of the body diode that's sometimes shown in the schematic symbol.

Offline MidnighToker

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Re: Diode question
« Reply #12 on: January 10, 2012, 05:21:28 PM »
OK, I know that this is Butt ugly, but the wife is ragging about running to the store and wanted to get this up.  I think I drew it out how I have it in my head....



Offline CraigHB

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Re: Diode question
« Reply #13 on: January 10, 2012, 06:34:52 PM »
You need a pull-up resistor between the gate and source.  You never want to let the gate on a FET float.  With the very high impedance a gate pin normally has, it can actually pick up enough stray voltage just from inductance in the wiring to flicker on and off partially. 

Also, like I said before, to use a MOSFET as a rectifier, you need to swap the drain and source.  Otherwise, the body diode will conduct in in the direction you want to block.

I'm not positive on the wiring for the switch since it has a wierd pole configuration.  You've studied that a lot more than I would.

Offline MidnighToker

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Re: Diode question
« Reply #14 on: January 10, 2012, 07:53:01 PM »
You've studied that a lot more than I would.
LOL.  Yeah, I can kinda get obsessive when I get something in my head.  When I found the switch that light in my head went on and then when I realized that it wasn't exactly what I expected it pissed me off enough that i was determined to find a way to make it work.

You need a pull-up resistor between the gate and source.  You never want to let the gate on a FET float.  With the very high impedance a gate pin normally has, it can actually pick up enough stray voltage just from inductance in the wiring to flicker on and off partially. 

Also, like I said before, to use a MOSFET as a rectifier, you need to swap the drain and source.  Otherwise, the body diode will conduct in in the direction you want to block.

I'm not positive on the wiring for the switch since it has a wierd pole configuration. 
Thanks...almost forgot about the resistor.  It's been a while since I worked with MOSFETs, but it pretty much just has to be any resistor to prevent it from floating.  (i.e. like a 10K would be fine?  Or will a 220 work?....that seems low to be too close to a dead short to me)

Not quite sure what you're saying about switching the drain and source though.  In a FET, doesn't the current flow from source to drain?  (Feel free to call me an ignorant tard at any point....FETs have always tripped me up for some reason.  Back building FistPacks I was in IRC with Nuck asking him the same questions over and over about them. LOL)

As far as the pole config goes (that is still pissing me off...), I essentially just want to block the white wire from making contact with the black on that annoying 2nd pole where all 3 wires are in contact.  I am not concerned with it basically repeating the 1st pole connecting red and white, just so long as the black wire does not connect with the white...not what I WANT, but I can deal with it.

Offline CraigHB

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Re: Diode question
« Reply #15 on: January 11, 2012, 12:54:29 AM »
10k would be fine.  There's a pretty wide range you can use.  You can actually go as high as 1Meg without issue.  You don't want to go too low or you're wasting power for no reason.

I understand what you're saying about the drain current on a MOSFET.  There was a time (not all that long ago) I would have asked the same thing.  I found out at some point that drain current can go either way and it makes no difference.  It's only the body diode you have to consider.  In fact, a MOSFET with the extra substrate tied to the source (all 3 pin MOSFETS are like this) is pretty much just a diode until you start applying gate-source voltage.  Practically, the only difference between a P and N channel MOSFET is an N-channel requires a positive VGS to turn on and a P-channel requires a negative VGS to turn on.  Also, the direction of the body diode is opposite in terms of drain and source.

Offline MidnighToker

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Re: Diode question
« Reply #16 on: January 11, 2012, 06:56:52 AM »
10k would be fine.  There's a pretty wide range you can use.  You can actually go as high as 1Meg without issue.  You don't want to go too low or you're wasting power for no reason.

I understand what you're saying about the drain current on a MOSFET.  There was a time (not all that long ago) I would have asked the same thing.  I found out at some point that drain current can go either way and it makes no difference.  It's only the body diode you have to consider.  In fact, a MOSFET with the extra substrate tied to the source (all 3 pin MOSFETS are like this) is pretty much just a diode until you start applying gate-source voltage.  Practically, the only difference between a P and N channel MOSFET is an N-channel requires a positive VGS to turn on and a P-channel requires a negative VGS to turn on.  Also, the direction of the body diode is opposite in terms of drain and source.
Thank you!  That is the first time on any of the forums that anyone ever explained to me what a MOSFET actually was/did.

So it is basically a diode that is sitting backwards between the gate and source (so the current won't flow) that the gate over-rides (in VERY simple terms).  That would also explain why I always had so many problems when I kept trying to use a N-channel on the positive leg....I was pretty much just putting a diode inline so it blocked nothing.

Offline CraigHB

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Re: Diode question
« Reply #17 on: January 11, 2012, 08:45:58 AM »
Yep, you can think of a MOSFET as a diode with a valve on it (the gate).  However, this only applies to 3 pin MOSFETS.  The ones that have 4 pins allow you to use the extra substrate as required to eliminate the body diode, but they're rarely used for DC applications.  That extra substrate is what forms the body diode and the MOSFET can latch up under heavy load without it connected to the source.

Offline jester

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Re: Diode question
« Reply #18 on: January 18, 2012, 08:17:25 AM »
hi i am hoping that some one can help me out here as i am only just starting out making mods. Well here goes i am making a 5 volt mod box with a MC78T05C Fairchild 5 volt regulator could anyone please tell me what capacitors or resistors that i need to use please thank you jester and ann marie  8)

Offline CraigHB

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Re: Diode question
« Reply #19 on: January 18, 2012, 02:10:25 PM »
According to the data sheet, no external components are required.  You don't need resistors since voltage is internally set to 5V.  Resistors are used only for regulators with adjustable output.

Offline jester

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Re: Diode question
« Reply #20 on: January 18, 2012, 02:18:14 PM »
 :thankyou: HI THANK YOU FOR REPLYING I LOOKED AT THE DATA SHEETS BUT I FOUND THEM CONFUSING, NICE ONE FOR SIMLIFYING IT JESTER AND ANN M  :thumbsup:

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