Question about wiring in leds on my first mosfet box mod.

[nwooley83]

First is the start of the Outer Casing.

Second is the start of my wiring diagram.  The leds are outside of the circuit since I have not figured out exactly where to wire them into the circuit so that they can get the full power needed without suffering a load loss when firing an atty. 

I do not think that the box is solid copper since it is not particularly soft to bend or work, but to get solder to stick to the inside I have to flux and torch it to damn near glowing to get it to stick.  I can build basic plastic boxes with no problems but this one is giving me some trouble.  On my plastic mod I wire my switch into the negative polarity side of the circuit which makes wiring the Leds in rather simple since I can connect the anode to the positive of the battery and the cathode to the atty side of the switch. 

The part I am confused about is wiring the Led into the circuit with the mosfet.  It has been over 12 years since my schooling in electronics and I have not used it much since then except for general repair.

My other problem is rebuilding this circuit into this box without using the box as the common ground source as I would if I wasn't using the mosfet.

I am also going to wire a cutoff switch in on the right hand side of the fire switch if that is the right place for it.

Any and all help would be appreciated.  I have run the search option on the site and can not find the answer.  I may be missing it but I have read several hundred pages of posts and topics in the last 3 days on this and several other sites. This is what my eyes feel like 

[wa9w00d]

Hey nwooley,

What are the part numbers of D1 and D2?  And what do you want each to indicate (firing, powered up, etc)?

Wayne

[nwooley83]

NTE30039 is what I am looking at using.  I have my 47 ohm resistors for them.  I thought I needed 68 but the guys at the supply house figured it a different way than i did.

VF - Typ. 2.25V  /  Max 2.6V
VR - 5V
 IF - 30mA

Data Sheet http://www.nteinc.com/specs/30000to30099/pdf/nte30037_43_45.pdf

they will cause the rod between the two brass pieces to light up when the atty is fired(at least when the fire switch is pressed). I may wire in a second switch to light them up continous.

[wa9w00d]

I would think that 47 ohms is about right for the current limit resistors. And I would put each (series diode and limit resistor) in parallel with the atomizer, observing polarity, of course.

Wayne

[nwooley83]

Roughly like this?

[Breaktru]

Why can't you use a single resistor for a number of LEDs in parallel instead of one each?

[wa9w00d]

Yeah, that's the idea. As Breaktru points out, you don't have the limit resistors drawn in there.  You can put them on either side of each diode, doesn't matter, as long as each diode has it's own.

Your led's will light the brightest when firing without an atomizer load, and dim according to how much battery sag occurs when loaded.

Wayne

[nwooley83]

Yea I didn't draw them in but they are there... the software I used for that was a trial on my phone that didn't let me save so I will rework the schematic when I get a chance on my computer and put it in the post so if anyone else is looking it will be there.

Of course it makes feel a little foolish that I did not pay attention that I was wiring them in parallel in the other box and that I should do the same thing here....

BTW I absolutely love this site.  I have found and learned so much.  It has given me some great ideas for builds.  My wife is going to hate me lol.

[CraigHB]

It's possible to wire LEDs in parallel with a single bias resistor, but the problem you run into is that not all LEDs have the same voltage drop.  Even two identical parts will vary a little bit.  What happens is the diode with the lowest voltage drop will conduct the most current and may be noticeably brighter.  For a green and a red LED in parallel, you would get a big difference in brightness between them.  The various colors have the most difference in voltage drop, red and green being the largest.

In any case, if you really must use a single bias resistor for a set of parallel LEDs, you can try it and if they are all the same brightness, then no problem.  Obviously the value of the resistor will be lower since the bias current is larger.

[nwooley83]

Yea I have a resistor on each diode in the circuit I will have the updated circuit uploaded in a little while.

[Meanyman]

Ok, this is the third thread I've read through and each has been more than helpful. Thanks all

[CraigHB]

On the topic of parallel LEDs, there's an interesting thing about parallel diodes that applies not only to LEDs, but all of the other common diode types.  It's tricky to run any diode in parallel because unlike other solid state devices, voltage drop goes down as diodes heat up. 

So with two identical diodes in parallel one will initially carry more current as it will have a slightly lower voltage drop.  As it heats, the drop further decreases and the diode conducts more current.  For high current applications, heating can result in one of the diodes conducting the vast majority of current.  For an LED it may or may not have an impact, haven't tested it.  LEDs don't usually get that hot so there shouldn't be a big effect.

MOSFETS don't do this.  They self correct as voltage drop increases when they heat up.  So if one gets hotter it will carry less current making two MOSFETs in parallel self-equalizing.  As BJTs (bipolar junction transistors) are basically two diodes back to back, they also suffer from the cascading current effect.  IGBTs are commonly used for high current switching applications and there are issues in running those in parallel as they are a variant of the BJT.

[nwooley83]

Finally got an updated schem drawn up.

The reason for the GND - VIN boxes is that this is setup for pcb layout and those are the items for the pads, but you can just substitute a switch and the battery in the appropriate place.

[nwooley83]

Wanted to check with you guys and see if this PCB layout would work with this wiring schematic?

I am definitely going to use different parts for the actual production to make this smaller but that is the generic parts that were available on the online builder.