# ROP goes to 11. Mag Tower FET mod.



## Conte (May 24, 2014)

*Let's play a game*, *it's called: Find the FET.*







Well that's looks normal. 






Hmm, what's that crap on the side ?






Some new connection, but not a FET. 






I see, a new positive connection. 






And there's something strange going on with the Negative too. 

Let's look inside.






That's looks different, well it's definitely modded. 






Wait, what am I looking at here ?






It's the FET !






I came up with a creative and clean why to pack a FET into a stock mag tower. 






The FET in question is a P-Channel IRFR 5305. 
Rated 55v 30A. Those are some some kicking numbers. 
Only problem is I can't get it to light up with less then 5v at the gate, 
so it's useless for the FM 4v Axial bulb running of 4 Nimh D cells I originally intended it for. 
It will light up but not completely. 

*It does however make this mod perfect for those inclined to build a ROP. *

You've already seen how I prepped the FET, you'll soon understand the odd resistor placement. 

Lets move on to the negative. 






That piece of scrap came out of a laptop battery pack. 






It will pass the negative control voltage to the switch. 






It's cut long enough to pass over the Positive hole and sandwich under the mag switch. 
No insulation is needed the way I modded the Positive battery terminal . . . speaking of which . . . 






You're going to want to remove it, remove the spring, and put a slight bevel on the edge. 
This will remove the bite for an easier reinstall. 
I spun it in a half in chuck and hit it with a file. 






Go ahead an pop a hole in the side of it. 






It'll make it easier to install a wire. 






When you reinstall it upside down, it makes for a way cleaner finished look then the ol' solder a wire to the end of the spring.
This is why we bevel it. It won't go in nicely this way unless you take the edge off. 






We do the usual drilling of hole to pass the new wire, but we also cut a little slit in the side of the tower base to pass this flat piece of metal. 






The source pin is going to sit on that flat. The new positive wire is connected to the other side. 
18g solid copper wire was used. Be weary, these are tricky solder joints to make. 
It's hard enough to get the tip of the iron down there to make that joint while not melting anything,
one wrong move and you'll bind the source pin to the spring cup.






Illustrated on this cut down tower, you can see how well the little FET sits and fits in the hole. 
I'm prototyping a couple other FET mods. 






Now you see how I was able to cleverly mount the resistor in such a way that it's also the control signal pickup.
The lead on the resistor going to the Gate pin will also make contact with the Mag Switch. 






And so now, we're pretty much done. As you can see, no Mag Switches were harmed in the making of this mod. 
It just slips in there like it normally does, and can slip back out for servicing. No soldering to the switch it required, just be careful to align the bottom contact as you slip it in, or it can get pushed rearward, or down into the battery contact. 
You may have to precompress to switch contact to slip it over. If done right, the negative post will span over the hole supported on either side with no risk of shorts. 






Hold on, I forgot to add the crystals. 
This mod would be pointless unless I fixed up the slug spring. 

The wire was fused in the open space at the very tip of the coil so that the spring will still evenly sit in it's respective cups. 
I filed the outside edge of the wire for a snug fit with little insertion force. 






And now, the ROP . . . she goes to 11. 






To be honest, I've never really been impressed with the 3853 bulb,






but the combination of this mod plus some kicking 26650 IMR cells really give it . . . . **One More! 

*





Too bad I don't have a 3854-H bulb on hand.

*IMPORTANT CONSIDERATION:*

*! This design uses a P-CHANNEL FET !*
*Not the common N-Channel FET that is abundant and used it pretty much every other FET mod. 

*As such, other guidelines and schematics will not apply. 

A P-channel FET switches the positive side of the circuit. 
An common N-Channel FET switches the negative side. 

A P-FET is needed for the convenience of being able to stick it under the spring cup. 
In order to execute this design with an N-channel FET you would have to reverse the polarity of the bulb contacts which is easy enough, but then you positively charge the outside of the bulb slug. 

_IF that Positively charged slug happens to touch your aluminum reflector, which is mounted in your negatively charged aluminum Mag Body, well . . . _
If you have the same kind of Fivemega reflector as I, then the slug is directly contacting the reflector via the cam system,
so that idea is out. 


Meanwhile, you could put an N-channel FET switching the negative while leaving the polarity intact, but under the same circumstances stated, contacting the reflector would bypass the switch and cause the light to come on without the use of the switch. 
This could be an option that would work for Non-Cammed systems that can be aligned to prevent the slug from contacting the reflector. However, this is a completely different design. I have a really slick prototype built on this system, but for a different bulb mount that doesn't completely address this issue yet. It pretty much depends on the reflector and head having enough anodizing intact to insulate it from the body. 

In conclusion, I leave you with a rough diagram of what going on:






*Adding a FET switch can literally result in an increase of output over a mechanical switch.


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## RichS (Jun 2, 2014)

Wow... nice mod! Thanks so much for taking the time to put up an excellent pictorial of the complete process. I'm still pretty sure this mod is beyond my skill level, but now I know who to bribe to get one of these done... :naughty:


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## Conte (Jun 3, 2014)

If it helps I figured out that you can use the common N-channel FET with the exact design, just you have to install the batteries backwards.


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## RichS (Jun 3, 2014)

Conte said:


> If it helps I figured out that you can use the common N-channel FET with the exact design, just you have to install the batteries backwards.



Good to know! If the N-channel FET was used and the batteries were inserted in the normal fashion (positive toward the head), would it cause any catestrophic failure? Or would the light just not turn on?

Also - are you running 2x26650s in a 2D host? How much brighter is it than just running on Li-ions?

Sorry for all the questions!


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## vicv (Jun 3, 2014)

26650 are li ions:twothumbs


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## Conte (Jun 3, 2014)

RichS said:


> Good to know! If the N-channel FET was used and the batteries were inserted in the normal fashion (positive toward the head), would it cause any catestrophic failure? Or would the light just not turn on?
> 
> Also - are you running 2x26650s in a 2D host? How much brighter is it than just running on Li-ions?
> 
> Sorry for all the questions!



It's hard to say. The light turns on but the switch doesn't work. It make it quick to tell that you've done it Wrong.
I believe that you would damage the FET if you continued to run it that way, but luckily with my samples it does not fail instantly and you can just fix it and carry on.

Yes, 2x 26650 in a 2D host.
They are IMR Cells, and they are definitely brighter then my 2x 18650 Config in a 2C host. Mind you I'm running older 2100 mah Sony cells. 
I would expect better performance out of 18650 IMR cells or maybe newer 3000+ Mah cells.


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## Hallis (Jun 17, 2014)

Excellent build pics and diagrams  What sort of runtime are you getting with those IMR cells? I've got several packs of ROP Low/Hi bulbs as well as quite a few loose ones that i've gotten here and there. (mostly lows from people that only used their hi bulb and donated me their low because I sometimes value runtime over output


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## Conte (Jun 18, 2014)

Thanks. I remember when posts like this were common. 

The runtime is exactly what you expect if you run the numbers. 3.5 / bulb current. 
I'd didn't time it exactly but I think I got a solid 40mins. 

I don't run a ROP hi in practise. I mostly did this to see if I could cram a FET into a mag tower and see how the ROP behaved. 

I also never had 26650 IMR's until recently. 
They really breath new breath into the incan. 

I value runtime too. IN practise I run a ROP low equiv style bulb in my lights unless they have a multi mode driver. In whch case I'm a fan of the WA1111 if I can dim it. 

That's another bulb that's really kicking with the new IMRcells.


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## Hallis (Jun 20, 2014)

Conte said:


> Thanks. I remember when posts like this were common.
> 
> The runtime is exactly what you expect if you run the numbers. 3.5 / bulb current.
> I'd didn't time it exactly but I think I got a solid 40mins.
> ...



I've got a few 1111's in my stash. May see if i can find a spare tower laying around and try a build.


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## JimmyM (Jul 16, 2014)

Nice new approach. I did something similar a few years back with a 2C Mag for an ROP-HI. I used an N-FET inside the switch tower. Worked great. The only problem was now that the switch resistance is so low. It makes all the other resistances, which were inconsequential at the time, look big. :naughty:


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## Conte (Jul 17, 2014)

I hear you. 

One you see the improvement a FET can make, you start to look at all your other lights like they are . . . incomplete.


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## cbsmith111 (Jul 30, 2014)

I am wanting to do an FET mod on my ROP, but most of the threads explaining it are now old and lack pictures if you can find them. I've stared at this enough that I think I may finally get the general concept well enough to replicate it or make my own arrangement. It looks like there is a band covered up on the resistor, would you mind telling me what type it is? What information do you use to determine the best resistor and FET values for an application like this?


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## JimmyM (Jul 30, 2014)

I just noticed in your schematic that the diagram on the right does not match the one one the left. The one on the right leaves the gate floating when off. Bad stuff. The one on the left has the gate pulled high when off, the correct condition. 

For those looking to replicate this circuit, I'd suggest using a 10k pull-up instead of 1M. It will turn off the FET faster and reduce heat during the OFF event. Depending on the gate capacitance of the FET you use, it may prevent it from getting hot when going on-off repeatedly.
It will only use 0.00072 A in addition to the bulb and 0A when it's off.

In the past, I've typically used N-FETs due to their super low Rds(on). Some lower than 1 mOhm. P-FETs typically have higher Rds(on) due simply to the fact that it's easier to move electrons than it is to move holes. It's a matter of how the P-FET & N-FET operate at a sub-atomic scale.
What P-FET are you using?

Maybe someone can check my math here, but...
I looked up a low Rds(on) P-FET rated at 30V. I found the gate charge is 120nC. At 7.2V that's about 0.000000017 Farads (17nF).
Discharging that capacitance @7.2V using a 1M Ohm resistor to 2.7V (1 time constant) will take about 0.017 Seconds. But to get to fully off (0.07V), it will take 4 time constants or about 0.68 Sec. That's not too long to be in the linear range of an FET, but it may get hot of you do it over and over.
A 10k Ohm resistor will reduce those times to 0.00017 Sec and 0.00068 Sec respectively and remove, almost entirely, the risk of getting hot.

Just my $0.02.


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## JimmyM (Jul 30, 2014)

cbsmith111 said:


> I am wanting to do an FET mod on my ROP, but most of the threads explaining it are now old and lack pictures if you can find them. I've stared at this enough that I think I may finally get the general concept well enough to replicate it or make my own arrangement. It looks like there is a band covered up on the resistor, would you mind telling me what type it is? What information do you use to determine the best resistor and FET values for an application like this?


The resistor can be as small as possible. 1/8W-1/10W resistor are common and nice and small. They carry no real power and thus can be as small as possible. The ones I use in my SMD designs are 0603s (0.060" by 0.030") devices and never even warm up. Use the smallest resistor you can find that is 10k-22k Ohms. At 7.2V it will only need to dissipate 0.005W Max.
Of course, if you have a 1/4 resistor on hand and can fit it in? Go for it.


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## Conte (Jul 31, 2014)

Oh crap, you're right. I botched up the schematic. 

OK, I've fixed it. 

I admit, I did Zero math in choosing that resistor value.
I actually did not have a 10k on hand. 
I tested a few different values from 1k up to 1m and got the same results on the bench. 
I chose 1M cause it worked and it's what I had in surplus. I'm low on the other values. 

Meanwhile, your electrical knowledge is vastly greater than mine it seems. 
The way you explained it makes sense to me, however, realistically, the difference in switching time between 10k and 1M seems to make so little difference in a design running such a low duty cycle.

I actually did a test after you posted this, and could not get the FET to heat up to any capacity that I could feel, hah. 
I guess if it was PWM it'd burn up. 

That's cool tho, I didn't know that. 
Maybe I'm fluked out with the FET I used. 
I've tested this with a couple.

The P-FET I used in the initial design is an IR FR5305.
I could not run this fet with lower than 5v. 

I since replaced it with an N-FET: AO D451. 
This one seems to work fine of lower voltages, a couple of D cells even, I wanted this tower to work with my 4v bulb off 4 NIMH D Cells. 
To run this with an N-FET I simply put the batteries in backwards.


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## JimmyM (Jul 31, 2014)

At the PWM frequency I use, it would turn on and off 4 times in the time the 1 M resistor would take to just barely start turning off. But this is a different application. If it doesn't get hot, then 1M is fine.


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## LuxLuthor (Jul 31, 2014)

This thread is what makes CPF great. Very generous sharing all the details, and same with Jimmy always being there to help out. Really great thread.


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## Conte (Aug 1, 2014)

Just like the old day, eh Lux ?

Came back to the forum after not being around for about 4 year, and was like, "What happened?".

Seems people aren't modding like they used to.


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## Conte (Aug 1, 2014)

JimmyM said:


> At the PWM frequency I use, it would turn on and off 4 times in the time the 1 M resistor would take to just barely start turning off. But this is a different application. If it doesn't get hot, then 1M is fine.



Yeah that makes sense. 

Hey, you know what just occurred to me . . . you figure the slow switch on time might as as sort of a micro speed soft start if you will ?
Slow down the start up just enough to take the initial inrush jolt off the bulb ?

This assumes that the FET ramps up as it switches on. 
Maybe this mod will add a few hours, or maybe minutes to the bulb life.


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## JimmyM (Aug 1, 2014)

You don't have a resistor inline with the switch. So the FET turns on FAST. The pullup just slows down the turn off. You shouldn't try adding a resistor though. More powerful bulbs will incinerate the FET if you slow down the turn on/off too much. AWRs old linear regulator tried to do a softstart but would burn up the FET with the bigger bulbs. Your FET switch is working just fine. Don't try making it something it isn't.


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## Conte (Aug 1, 2014)

Oh, I have no current intention of further modifying this. 

I'm more interested in playing with intelligent regulation these days, if you know what I mean, hah. 

So, what you're saying, is it works as a fast on, slow off in this setup ?
And if there was a resistor on the switch side to feed to on signal, that's how it would slow it down ?

Interesting concept. I could see how it could fry things then. 


This does however, give me another idea . . . . .


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## JimmyM (Aug 2, 2014)

Conte said:


> Oh, I have no current intention of further modifying this.
> 
> I'm more interested in playing with intelligent regulation these days, if you know what I mean, hah.


I believe I do.


Conte said:


> So, what you're saying, is it works as a fast on, slow off in this setup ?
> And if there was a resistor on the switch side to feed to on signal, that's how it would slow it down ?


Exactly. But a resistor on the switch side AND one on the high side will create a voltage divider and reduce the voltage to the gate, possibly drastically increasing the FET resistance when it should be fully ON.


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## Nano-Oil.com (Sep 12, 2014)

Bravo Conte,


What you did with the spring by shunting with what looks like braided copper wire is brilliant & is what I did to all my NiteCore D4 and I4 chargers, 
for isolation, I use Wet Noodle type silicone casing,

On those chargers the connection between Negative of battery & its solder point on PCB is actually the spring of the battery holder which just hooks on it, 


Needless to say that I challenge that as a current delivery component, 
with this approach they have basically created a very nice looking resistor, 


I have known for a long time that conventional spring do not belong into any device that draws more than 1 amp, regardless of geometry.


I find so much nonsense in the Vaping Industry even on high end mods & devices to the point that I modded & pretty much made my own spring which can take about 100 amps without ever getting warm, the only draw back is the compression set with a few millimeters of displacement but it works on all my builds.


Take care, I am glad I landed on your thread while looking for Luxluthor,


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## LuxLuthor (Sep 13, 2014)

Nano-Oil.com said:


> Bravo Conte,
> 
> 
> What you did with the spring by shunting with what looks like braided copper wire is brilliant & is what I did to all my NiteCore D4 and I4 chargers,
> ...



Great to see you too Christian! Still love your oil the best!!!


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## Nano-Oil.com (Sep 13, 2014)

LuxLuthor said:


> Great to see you too Christian! Still love your oil the best!!!


Eh ! found you !, 

You bought so much from me that please allow me send you one version FDA Approved which I developed for Musicians who became avid customers world wide even big names in French Horns, Concerto Pianists also in Europe, 

Lots to talk about, will do it in private, 
Take care and have a great Weekend,


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