Switch Ratings and Safety for Unregulated Box Mods

[WKS01]

I would like to preface this post with a disclaimer. I am not claiming flawless knowledge on the subject of switches or electricity. The information provided below is combined from multiple sources, including but not limited to experience, switch manufacturers and textbooks on the various subjects. If someone has a correction or better explanation please share that information.

With the recent influx of unregulated box mods being created, special attention is needed regarding the safety of the switch. At the time of this writing there are few (if any) momentary switches that can handle 20 or more amps using a DC voltage source that would suit most builder's needs. However, this has not stopped people from attempting to rerate switches from the manufacturer’s specifications. This document has been written to help explain the common method used and why it is incorrect and very unsafe. This may be a long read, but is extremely important if you are deciding to build an unregulated device.

Switch ratings can be quite confusing for someone without experience or knowledge of them. This information will not go into extreme detail regarding switch ratings, but it will attempt to give enough information for builders to make informed decisions.
Two types of electricity exist, alternating current (AC) and direct current (DC). In the case of box mods, DC electricity is in use. This is important to note and will come up later when discussing the build of switches. Switch ratings tell the maximum amperage and voltage that the switch is certified to handle. You can tell what voltage type the switch rating refers to by looking at the letters that follow the voltage rating. If the switch rating contains VAC it is AC voltage, if it contains VDC the switch is DC voltage. There also maybe times no lettering follows the voltage rating ( Example: 2A/48V ), this type of rating usually means the rating is for both AC and DC voltages. It is advised to always refer to the manufacturer data sheet for the switch and not simply a reseller’s description because they may not properly or accurately relay the information.
A common error has been to rerate switches for use at lower voltages using the basic power calculation, Joule’s Law. The belief is that one can simply determine the power dissipated at the switch and use that as a constant value to determine the new amperage rating. Below is an example of this equation being used to INCORRECTLY to rerate a switch.

THIS METHOD IS NOT CORRECT AND SHOULD NOT BE USED FOR DETERMINING A SWITCH RATING!!!

Switch Rating: 10A/125VAC
Power = Voltage * Current (P=V*I)
P = 125*10
P = 1250 watts (Theoretical Power based on switch rating)

1250 = 4.2*I
I = 297.6 amps (“New” current rating for the switch)​

According to Joule’s Law, when using the switch at 4.2VDC the switch can handle nearly 300A, despite only being rated for 10A originally. Without even examining the multiple errors in this method, the current rating and wattage only should be a red flag that this is not correct. If the switch dissipated 1250W that would be roughly equivalent to a standard space heater. And an amperage rating of nearly 300A is outrageous. For comparison, most modern homes receive 150-200 amp service, for the entire home!

From an circuit analysis standpoint applying Joule's Law in such a way is simply incorrect as the voltage used is not the correct value. When determining the power loss in electrical components the voltage to be used in the equation is the voltage drop across the component. In the case of a switch in an unregulated box mod this is not 4.2V. If that was true then when using the switch you would have a voltage reading of 4.2V on one pole and 0V on the other pole when the switch is in the closed position. Meaning no voltage is seen by the atomizer further down in the circuit. The power dissipated by a component is related to two factors, the current passing through the component and the internal resistance of the component itself. With those two factors and a rearrangement of Joule's Law (P=I²*R_Internal), power dissipation of a component can be found. Though for switch ratings power dissipation is not very important as if you are operating within the manufacturer's specifications you should within the heat dissipation the switch can handle.

In addition to examining the feasibility of the values of power and amperage, switch ratings are not linearly dependent on voltage. More than a basic equation goes into determining the amperage and voltage ratings of a switch. Contact geometry, contact material, distance between the contacts, switch material, load type, etc. are all parts of the equation, too much to address in a single sitting. There are ways to derate a switch, but they are in no way 100% accurate. A rough estimate according to some switch manufacturers is that a current rating of a 125VAC switch on a resistive only load would be suitable at 25-30 VDC.
Notice that the amperage rating is the same despite the voltage dropping nearly 75% of the original rating. This is due to the drastic deference between AC and DC electricity. When a switch is being opened or closed, the current does not simply stop once the contacts are apart, the contacts must reach a certain distance to stop the current flow. For AC switches, this is much easier since the current is zero 120 times a second for 60 hertz service or 100 times for 50 hertz service. This zero current allows for the arcing between contacts to be terminated much easier. With DC switches, these zero points do not occur, allowing for the arcing to continue between the contacts. If one examines multiple switches rated for AC and DC circuits, you would notice that DC switches tend to have a snappier and quicker action when being used compared with AC switches. This is purposely designed as another way to help overcome the constant current experienced in DC circuits.
There is a lot of information above and yet this just barely scratches the surface of switch ratings. Below are a few key points to take away from this information:

• DO NOT use a switch at a amperage or voltage above the ratings given in the data sheet.

• DO NOT attempt to rerate a switch using Joule’s Law (power equation), EVER!

• When possible find a switch that is rated for your application. If it will be used in a DC circuit, find a switch with a DC rating.

• If you are ever unsure about a switch’s rating, contact the manufacturer for clarification.

 

[Moxienator]

Wow. Nice info! Thank you!

sent via Droid Mini

 

[Johnmath]

So does a mosfet assist in keeping the switch from overloading?

 

[edwinoey]

Is this where the usage of mosfet is crucial? I mean im not an electrical engineer but from what i understand, mosfet is taking the amp load, making it act like a switch, so that the real switch only taking several mili amps and will not broken the switches cmiiw. Since i wanna buy a box mod, im pretty sure i need a boxmod with mosfet

 

[turbocad6]

the best way to connect a high amperage load with a remote switch is with a relay of some sort, a mosfet is a sort of electronic relay, the high amperage is switched within a sealed electronic circuit rated to handle this switching repeatedly, many thousands of times with no degradation, but for those that choose to do it the crude way of a mechanical sort, what exactly is this "danger" you speak of?

I mean what's the danger of using a switch beyond it's limitation beyond just eventually damaging the switch? I know it's theoretically possible for a switch to internally arc itself together causing a stuck on switch, but short of that which almost never happens and even if it did you'd have plenty of time to react, whats the real danger here?

 

[Zanderist]

I mean what's the danger of using a switch beyond it's limitation beyond just eventually damaging the switch? I know it's theoretically possible for a switch to internally arc itself together causing a stuck on switch, but short of that which almost never happens and even if it did you'd have plenty of time to react, whats the real danger here?

I'd imagine that the piece of metal making contact would get pretty hot rapidly to melt the insulation, it would probably become a small fire hazard.

 

[JimmyDB]

the best way to connect a high amperage load with a remote switch is with a relay of some sort, a mosfet is a sort of electronic relay, the high amperage is switched within a sealed electronic circuit rated to handle this switching repeatedly, many thousands of times with no degradation, but for those that choose to do it the crude way of a mechanical sort, what exactly is this "danger" you speak of?

I mean what's the danger of using a switch beyond it's limitation beyond just eventually damaging the switch? I know it's theoretically possible for a switch to internally arc itself together causing a stuck on switch, but short of that which almost never happens and even if it did you'd have plenty of time to react, whats the real danger here?

Switches are rated to act as switches, instead of additional loads on a system. The inherent safety of a switch, that which prevents the electricity/heat from contacting anything besides the intended surfaces, is based on/around the rating goal at design time.

One potential issue is the switch itself can become part of the load on the power source, this may not be a big deal in regards to your safety (but it could be a factor), but it may have the result of adding resistance to your load. Since the additional resistance will vary with the load... you can't even easily compensate for it... hence it effects your end result of vapor/flavor production and even your battery run time. You wouldn't knowingly touch a hot coil... and you probably don't want your switch to heat up like that either. Obviously, that example would require a metal switch in which the portion touched to actuate it can conduct heat from one of the contact points.

The switch can fail in a latched state and depending on the total load you are putting on your batteries and the rest of the build quality... what completely fails first is up in the air. The coil could catch your wick on fire [probably no big deal] before it breaks, your internal wiring could sag/melt [possibly no big deal], your batteries could fail [could be a big deal, hopefully not]. Some super smart people that think ahead, even put heat sensitive glue (hot glue) inside their mods to hold wires and such in place... becomes a very fun waste of time to clean out and if used to prevent various parts from coming in contact... well, it could fail to do that once it liquefies because the wire it's holding down is joule heating under constant current. Did I mention it smells great when it starts burning, and would probably taste great if you happened to be vape some of that smoke

The switch can fail in a latched open state... leaving the person to resort to their backup device until they can repair it.

All in all, I would expect the person who is going to risk their switch to apply that same reasoning to the rest of their build... undersized wiring, pushing their batteries to the 3 second load limit, etc. But hey, people who want to build such a thing and then hold it near their face while using it... or put it in their pocket... that's up to them, but I hope they don't forget the liability assumed by them if they let anyone else unknowingly operate such a device, or should anyone get hurt at any time due to it. A good buddy may never take their friend to court, but someones health insurance company sure will. Not to mention how bad we would feel if our poor choices lead to getting a friend hurt, even if it wasn't a serious injury... we would probably look like idiots.

As far as how often switches fail in a closed state, that is directly related to how often the switches are pushed and to what extent, but most importantly... most devices are built to handle stresses far above their stated rating and when it comes to [temporary] switches, the norm is for them to be designed in such a way as to prevent closed state latching as much as possible... and that tends to lend itself to them becoming a load on the system versus just failing closed (because they are built to exceed their rating safely).

As far as electrocution... to be very realistic, we are dealing with low voltage systems [most of the time] here... and they are systems where you couldn't even get a 'dangerous' amount of current to pass through a person if you held the wires in your hands. This is one of the reasons we can handle batteries without the worry that we will short the leads in our hand and die from it (besides not having a route to vital organs if only one hand is involved). Grab your DMM and check the resistance across hand, on your tongue or even between 1mm on your thumb... did you get something higher than 100k ohms? Let's do the math for 100k Ohms... ( (4.2^2)/100000 ) / 4.2 -> .000042 Amps at 4.2V. (I took the long way)

So... should we only use components within their ratings, of course we should! The greatest danger comes from all the other mistakes that would be common in a design that allows for out-of-spec use... such as a lack of concern for general safety or quality and failsafes.


As far as the original post... I'm not even sure what type of person would think that a switch is for dissipating a load or that watts come in to play in the manner described?! Switches aren't rated to conduct and prevent the conduction of a set amount of power as such... the volts and the amps are distinct ratings. Given the voltage being followed, it can also handle the amps rating... and given the amps rating is followed, it can handle the voltage listed... and even then temporary switches will often also have a duty-cycle listed... and that's an important factor too. Exceeding the voltage is what in reality often leads to arcing and exceeding current ratings is what often leads to mechanical failure of the housing (due to joule heating). Exceeding both the voltage and amperage ratings often leads to me laughing out loud or getting very upset depending on the outcome and who is involved

We are all responsible for our own actions... mistakes can happen, but negligence isn't just an accident.

Whatever we all choose to do... I just hope the outcome is something we can laugh about later.

 

[WKS01]

As far as the original post... I'm not even sure what type of person would think that a switch is for dissipating a load or that watts come in to play in the manner described?!

Knowing just enough to get yourself in trouble may be the issue. Perhaps since many vapers know Joule's law to determine the power at the coil, they also attempt to apply it with switches. I see many new "companies" pop up each week attempting to make unregulated mods and make ridiculous claims about their switches. Most I have talked with attempt to use the incorrect logic above to determine how much current a switch can handle. It seems to be a lack of knowledge on the subject or they are receiving incorrect information from other people. The more frustrating part is some of these "companies" do not even listen to why they cannot apply Joule's law and the irrelevance of wattage . They continue to sell an unsafe design and make false design claims selling to people who do not know any better, I have seen a few claim they handle down to 0.02 ohms. That is putting roughly 180A - 210A through a switch rated for 3A, that is simply an accident waiting to happen.