Is this switch safe? Dual 26650

[Levitys]

Hey Guys!

New Member here. I'm looking for some help.
I am a electrical retard.
Anyways, I am trying to build a mechanical box mod ecig.
Basically, it's two batteries a 510 connector and a switch.
Studying up on how to build this mod some people say the switch is safe while others say the switch is not safe.
I was wondering if you guys could take a look and let me know if this switch would be safe or if I am going to blow my hand off.

Thanks!

 

[Gryd3]

Take a look at two numbers.
Look at what the batteries can put out Amperage wise, and look at what the switch handles.

To do the math for you, that coil will allow anywhere from 2A to 20A through the circuit. You tell me if you think it's safe and we can go from there.

 

[Levitys]

Hey Gryd3,

Thanks for the quick response.
The Battery is rated for a continuos 20a ora pulse up to 60 amps but this is only at 4.2 volts max.
The switch is rated at 5amps but at 250 volts.
This is where I get unsure... because of the volts.
I'm not sure how they play in.

Thank you!
I really appreciate your help!

 

[Bluejets]

Unless I am sadly mistaken and my eyes are playing tricks on me, that switch looks like it is a dead short across the battery.

 

[Gryd3]

Unless I am sadly mistaken and my eyes are playing tricks on me, that switch looks like it is a dead short across the battery.

It most definitely is

Hey Gryd3,

Thanks for the quick response.
The Battery is rated for a continuos 20a ora pulse up to 60 amps but this is only at 4.2 volts max.
The switch is rated at 5amps but at 250 volts.
This is where I get unsure... because of the volts.
I'm not sure how they play in.

Thank you!
I really appreciate your help!

The volts and amps that it is rated for it it's highest rating.
So you could use 2V on it just fine if it's rated for up to 250V.
Same with current, you can use 1A on it fine because it is rated for 5A...
The part that does not work well... is because of the circuit there, if built properly, it could draw up to 20A which is 4 times higher than what your switch can handle.
and building the circuit like in the picture will short out the batteries... which could be up to 60A ...

 

[KrisBlueNZ]

Yes there's a mistake in your circuit. The pushbutton is connected straight across the batteries. Also the "coil" at the top is permanently connected across the batteries. You need to connect them all in series. But...

Your battery voltage is 7.6V nominal (two 3.8V cells in series). If your coil is at the low end of the possible range of resistances, 0.2Ω, then it will draw 38 amps! You can calculate this using Ohm's Law which says
I = V / R
where
I is current, in amps;
V is voltage, in volts;
R is resistance, in ohms (Ω).

Edit: WRONG! The cells are in parallel, not in series!

In this case, V = 7.6 and R = 0.2 so I (current) comes out at 38 amps. That's a LOT of current, and the pushbutton won't last long.

If the coil resistance is 2Ω, though, the current will be only 3.8A.

The obvious solution would be a high-current relay, but relays with contact ratings of 40A and higher aren't available with coils that will operate at 7.8V.

Another solution would be one or more MOSFETs, driven by the pushbutton. These can switch very high currents, and they're pretty cheap, but they need a driver circuit to switch them very quickly and cleanly. A suitable MOSFET is at http://www.digikey.com/product-detail/en/PSMN1R0-30YLC,115/568-6721-1-ND/2674288. You would use two of these in parallel.

If you're interested in pursuing this, let me know.

 

[hevans1944]

And what is "a mechanical box mod ecig" pray tell?

 

[Levitys]

First off I really just want to say thank you.
Although most of this is going over my head...
I really appreciate you guys taking the time.
So, Thank You.

To answer Hevans question....
An Ecig "Mod" hales back to the days when no one was making ecigs so you had to make them yourself.
They were called "mods" because you had to mod them out of other things... such as flash lights, copper tubing, etc.
A "Mechanical Mod" is an Ecig mod that has no electronic components.
Basically just the battery and a switch. Most of the time no wires.
(Battery in copper tube with 510 at the top and a mechanical switch at the bottom)

If you're interested in learning more about ecigs and vaping here are some pretty good ecig tutorials

Back to my schooling lol

Ok I believe I have addressed the issues from before with adding this MOSFET.
Hopefully I have taken care of the shorting issues as well?

I do have a couple questions/concerns though...
1.) Do I need to add a heatsink on top of the mosfet?
2.) What is the lowest coil (resistance) that could be built safely with this schematic? (pulses of 1-5 seconds)–"A Hit"
3.) Does the buttons light require additional wiring somehow?

I look forward to reading your responses!

Thanks!

 

[KrisBlueNZ]

It doesn't seem like this is going over your head! You have the circuit design basically right.

You have the cells connected in parallel. I didn't notice this before. That means the supply voltage is 3.6V. It's good for the coil, which obviously may need a LOT of current, but 3.6V gate-source voltage is not guaranteed to be enough to make the MOSFET saturate very well. Here's the graph from the data sheet. This graph shows TYPICAL characteristics, not guaranteed performance.


The trace with the red arrow relates to a V_GS voltage of 3.5V. At 40A the trace is off the graph, V_DS less than 0.1V, but these are just TYPICAL characteristics. R_DS(on) is only guaranteed at V_GS voltages of 10V and 4.5V, not 3.6V. So using 3.6V to drive the gate is risky. You could use a 9V PP3-type battery for the gate supply.

I recommend using a lower gate-to-source resistor, to improve the turn-off time of the MOSFET and reduce the time spent in the linear region (which causes a lot of power dissipation). Assuming you use a 9V battery for your gate supply, a gate-source resistor of 220Ω rated for 1W would dissipate about 0.4 watts, so it would get warm with continuous use but would be fine for a 1.5 second hit. It would draw about 40 mA from the 9V battery which is significant but no problem if it's only being used intermittently.

It's not ideal driving the gate directly from the pushbutton because of contact bounce (Google it or look on Wikipedia). It's quite possible that, on some random day, the pushbutton will produce an undefined level for long enough that the MOSFET will remain in its linear region, dissipating as much as 70 watts, for so long that it internally overheats and the magic smoke leaks out. So if I was designing a product, I would use a driving circuit. But you might get away with it.

Also strictly speaking, the heating coil is an inductive load and should be snubbed because of the possibility of back EMF (again, Google or Wikipedia) that could generate a high-voltage pulse that would exceed the MOSFET's 40V gate-source voltage rating and again blow it up. But the inductance is likely to be pretty low, and there will be other sources of damping in the circuit, and you would need a huge diode to clamp the back EMF, so it's probably best not to bother.

In the worst case, a 0.2Ω heater coil, and a perfect battery (no voltage droop under heavy load), the current would be 38A, which would cause the MOSFET's 0.0017Ω ON-resistance (worst case R_DS(on) with 10V gate-source voltage) to drop about 0.07V (from Ohm's Law). Power dissipation would be 0.07V × 38A = 2.7 watts, but only for 1.5 seconds at a time. I don't think you would need a heatsink.

MOSFETs are sensitive to electrostatic discharge and are easily damged. When you assemble the circuit, keep the MOSFET in its protective tube or foam, then clip a paper clip around the leads to short them together, then solder the resistor between gate and source before you remove the clip. With 220Ω between gate and source, the MOSFET will be pretty robust.

I don't know what that pushbutton needs to keep it illuminated. It probably has two extra terminals for the internal LED. It may or may not need an external resistor to limit the current to the LED. It may or may not be able to light brightly from a 3.8V supply voltage. I guess you wouldn't want to power it from the 9V battery because it would shorten its lifetime noticeably. Post the manufacturer and part number and I'll have a look.

 

[Bluejets]

Switch detail ....

 

[Levitys]

Hey Kris,

I just wanted to say thank you! You rock man!
I am still trying to understand everything so that I can ask the right questions.
I will get back to you soon.

Thanks man!

 

[KrisBlueNZ]

Bluejets, are you sure that's the right switch? I don't see a part number mentioned anywhere in this thread.

Levitys: No problem If Bluejets has the right switch in post #10, it's designed to have a 12V DC supply for the illumination. You might be able to make it illuminate properly at a lower voltage - either by changing the series resistor, if it has an LED in it, or by using a different lamp, if it uses a lamp. But it's going to drain one of your batteries if you want it illuminated all the time.

You MIGHT be able to open up the switch and replace the illuminating component with a high-efficiency red or green LED - these light quite brightly at only 1~2 mA and you could power it from the 3.6V battery (with a series resistor - LEDs need series resistors) without affecting the lifetime noticeably.

 

[hevans1944]

My wife and I are supposed to quit smoking (again) this weekend. I considered trying e-cigs, but those just facilitate a nicotine addiction without ashes. I too have "an addictive personality" and cigarettes are the hardest addiction to break. I quit drinking in 1989 after twenty-something years, but that was only after two half-hearted, employer-motivated, prior attempts. Fear of death helped finally, as did a 12-step program and professional counseling. None of these motivate me yet to quit smoking, despite two heart attacks since 2000 and numerous stents. Bupropion {Wellbutrin) did get me tobacco-free for about a year the first time I quit smoking, after the first heart attack. Chantex (varenicline) did the same thing the time after that. How stupid of me to light up again a year or so later.

My dad quit smoking at age 40 and then survived two open-heart by-pass operations before finally succumbing to metastasized lung cancer at age 84. My two years younger brother still smokes and is fighting cancer, the latest on a lung plus a lesion on his chest wall. The prognosis is not good for either me or my brother. Genetics plays a part in who gets cancer I believe, but I appear to be tipping the odds toward an early death from heart failure, or perhaps prostate or lung cancer, with my nicotine addiction.

I would still like to help @Levitys with his "mod" though! What is that "coil" made from? It is hard to believe it takes so much power to vaporize a liquid already trapped in an absorbent polymer. Perhaps you could consider making the "coil" using a thin nichrome wire with higher resistance?

 

[Levitys]

Hey Guys! Sorry it took so long to reply.
You’re basically taking me to school here.

I’ve really tried to wrap my head around all this but man… You guys are like super-geniuses lol

Reading through everything multiple times here is what Im thinking…

The switch has no part number or any usable specs I can find anywhere so I’m thinking
I will use a similar switch without the led thereby saving battery life and eliminating a hassle.
It was for kicks anyways not really functional



This is the switch Im thinking now:

3A - 250v Momentary Switch

Any Objections?


Since we are on the topic of switches…

I came across this switch here:
60A 12V Momentary Switch

Wouldn’t this switch eliminate the need for the mosfet?
I would rather go this way... it’s obviously much easier, but it seems kind of fishy to me.
I can’t find any other switch rated like this anywhere. It’s from china and again… no part number.

What do you guys think?

Kris:
I believe the 9v battery option is a great idea but everything has to be built inside a hammond 1590B Box.
With the 2 batteries, switch, mosfet and 510 connector I honestly don’t believe there will be enough room for a 9v battery.
Is there a simple way to supply the necessary voltage to the gate without an additional battery? Or is there a smaller 9v option? (like cmos small?)

Phew! lol



Hevans:
I’m so sorry to hear about your brother.
I recently lost someone to cancer almost a year ago now.
She was the best person I have ever known and to say I miss her tremendously would be a gross understatement.
I hope for the best for you and your family. I will pray he has a speedy recovery.

Like you smoking has been one of my biggest addictions.
I started very young and… well… you know the rest of the story.

In addition to being a smoker I was born with asthma so as you can imagine I didn’t breath all that great.
Terrible asthma attacks EVERY morning. Hacking up a lung! I didn’t care though I loved my smokes.
I had tried to quit many times to no avail until I started vaping.

I know there is a lot of misinformation going on from the cigarette companies and government right now about the safety of ecigs
but let me tell you from first hand experience. It is 1000% times healthier. Don’t let anyone tell you different.
My asthma is now pretty much non-existent. I feel 100x better and I don’t stink anymore.
Not to mention I was able to quit smoking in 1 day and It only costs me about $10 a week to use my ecig unlike $14 a day.


I know it’s still an addiction and ya it would be better to just stop but if you have ANY chance of starting back up again I urge you to start vaping instead.
It’ll change your life. It has mine

Back to business

The coils are made from many things depending on what people use (Ive seen guitar wire used lol) but most people (myself included) use Kanthal wire. You are right about the power it takes to vaporize. It doesn’t take anywhere near the specs Im posting here. The reason I am using the huge range of resistance is because this mod will be used with RDA’s. An RDA is short for Rebuildable Driping Atomizer. People who use these atomizers like to push the limits with what is called “Cloud Chasing”. I just want to make sure that this mod stays relatively safe if someone were to try to build super low ohm coils on it.

I look forward to reading your responses!

Thank you again guys!
You have no idea how much it is appreciated!

 

[KrisBlueNZ]

This is the switch Im thinking now:
3A - 250v Momentary Switch
Any Objections?

No objections. It looks nice!

I came across this switch here:
60A 12V Momentary Switch
Wouldn’t this switch eliminate the need for the mosfet?
I would rather go this way... it’s obviously much easier, but it seems kind of fishy to me.
I can’t find any other switch rated like this anywhere. It’s from china and again… no part number.

If the specs are real, yes you could avoid the MOSFET and the 9V battery for the gate supply voltage. But it looks just like the 3A one apart from the colour of the plastic, and as you say, with no manufacturers' reference, it could very well be bogus. Digi-Key have a huge range of products, and their highest rated pushbutton is 20A. It's a heavy duty US-made unit and they sell it for USD 34. So I'm suspicious of a 60A claimed rating for USD 13. You could ask the seller to email you a copy of the manufacturer's data sheet...

I believe the 9v battery option is a great idea but everything has to be built inside a hammond 1590B Box. With the 2 batteries, switch, mosfet and 510 connector I honestly don’t believe there will be enough room for a 9v battery.
Is there a simple way to supply the necessary voltage to the gate without an additional battery? Or is there a smaller 9v option? (like cmos small?)

You could build a voltage multiplier circuit but it wouldn't be much smaller and would be a lot of trouble.

You could use three or four CR2032 3V lithium coin cells in series, with a higher value gate-source resistor. How does that sound?

 

[Levitys]

If the specs are real, yes you could avoid the MOSFET and the 9V battery for the gate supply voltage. But it looks just like the 3A one apart from the colour of the plastic, and as you say, with no manufacturers' reference, it could very well be bogus. Digi-Key have a huge range of products, and their highest rated pushbutton is 20A. It's a heavy duty US-made unit and they sell it for USD 34. So I'm suspicious of a 60A claimed rating for USD 13. You could ask the seller to email you a copy of the manufacturer's data sheet...

I agree. It just doesn't seem right. It's odd they don't make switches rated for this type of thing though. You'd think there would be more options available in this day and age.

You could use three or four CR2032 3V lithium coin cells in series, with a higher value gate-source resistor. How does that sound?

This sounds Great!

What do you think the battery life would be like on the gate supply?
How would you go about wiring that up?
Would you stick with 220Ω resister?

Thanks!

 

[Bluejets]

http://www.ebay.com.au/itm/12V-Blac...=AU_Car_Parts_Accessories&hash=item417458fd93

Looked like this one to me.Then again,,,maybe not

 

[KrisBlueNZ]

What do you think the battery life would be like on the gate supply?

Current would be (3.6 + 3.0 + 3.0 + 3.0) / 1000 = 12.6 mA.
CR2032 capacity around 150 mAh; total run time 150 / 12.6 = 12 hours
Hit duration 2 seconds; number of hits would be (12 × 60 × 60) / 2
= 21,600 hits.

That's just for the CR2032 cells. Obviously you'll need to recharge the main cells a lot more often than that!

 

[hevans1944]

Since we are on the topic of switches…

I came across this switch here:
60A 12V Momentary Switch

Wouldn’t this switch eliminate the need for the mosfet?
I would rather go this way... it’s obviously much easier, but it seems kind of fishy to me.
I can’t find any other switch rated like this anywhere. It’s from china and again… no part number.

What do you guys think?

The seller on e-bay mistakenly listed the switch. It is NOT capable of 60 A. It may not even be capable of 6 A. The seller on e-bay directs the buyer to www.apem.com and suggests downloading a PDF for AV Series switches from that web site. I did this, and tried to upload the suggested PDF file as an attachment to this message, but it is too large for this forum. You may visit this page and download it from there. This series of switches from this manufacturer has current ratings in the range of 2 A to 4 A to 8 A, depending on voltage switched and contact construction (solid silver or silver-plated copper/brass). Think about the physics: how large a contact and connecting wire is required to reliably switch a 60 A load with minimal losses in the switch?

So, it looks like you are back to using a power MOSFET, but I doubt you will get anywhere near the battery surge amps through it. Nice thing about it though, you can mount the MOSFET directly to the inside of the die-cast aluminum case, which makes a fair heat sink.

 

[KrisBlueNZ]

The seller on e-bay mistakenly listed the switch.

Mistakenly? You're being very generous there Hop. The 60A claim, in conjunction with the high price (many times higher than the same switch from different suppliers) and the absence of any mention of the current rating in the body of the ad, suggests to me that the seller is being deliberately deceptive. And on eBay too! I AM surprised :-/