list In: Technique person Posted By: Renzo Cattaneo

Ohm's law: they're 3!

So many times in the world of vaping we have talked about Ohm's law, in general we think of the first, sometimes we talk about the second, but rarely has the THIRD been mentioned.

So let us look at the development of Ohm's third law, which is of enormous importance for knowing the safety of non-circuit boxes and mechanical tubes in particular.

So many times in the world of vaping we have talked about Ohm's law, in general we think of the first, sometimes we talk about the second, but rarely has the THIRD been mentioned (I apologise to those who know electrical engineering well, but this small article is intended for those who are approaching vaping and have perhaps never explored these concepts).

So let us look at the development of Ohm's law in what is very often referred to as Ohm's third law, which is of enormous importance for knowing the safety of non-circuit boxes and mechanical tubes in particular.

Let us begin with a little discussion of the first two in order to be more complete:


With this law, we understand the link between voltage (V) and current (I) as a function of the user (the coil), or resistance (R) enunciated by this law. In addition to inferring the link between the physical quantities, we come to understand that the current required (battery discharge current measured in amperes) by the system is a function of the resistance, and that for safety in the use of PVs we must not exceed the nominal discharge current of the battery. a practical example:

20A battery 0.3 ohm coil, battery voltage 4 V: writing the formula as I=V/R 13.33 A available from the battery as a minimum are required. In this case, IF the battery is actually able to deliver the Amps indicated in the specification, we are in the range of use. 20A battery, 0.1 ohm coil, 4 V battery voltage: if we write the formula as I=V/R and write it as 40.0 A, it is clear that we are well outside the range of use of the battery, putting us at risk.

I would say that at this point it is mandatory to make sure that the batteries you buy are of a quality that allows us to consider the actual specification data and, above all, use them within their delivery limits. Beware of batteries that indicate, through unscrupulous marketing, inrush currents that last only a few fractions of a second instead of the operating currents, i.e. those they are capable of delivering in the operation required for vaping..


In this case, let us consider the general characteristics of the coil. This law explains that the resistance we are going to build depends on the physical characteristics of the material used. The resistivity ρ is in fact typical of solids and in conducting metals represents the ease with which the electric current flows through them. Resistance is obviously proportional to the length of the conductor and is the inverse of the cross-sectional area. Banally, for the same conductor used, the longer the conductor, the greater the resistance, the larger the diameter and inversely the lower the resistance.

Just a small hint: so-called temperature-controlled systems do not measure the temperature of the coil, but its resistivity, since resistivity is linearly and directly related to temperature.

Temperature-controlled (TC) systems therefore have a small programme which, depending on the type of metal selected, understands what the coil temperature is as the resistivity changes. This is the reason why the metal used in the circuit must be set absolutely in order for it to work properly. I would add that this is also the reason why it is complex to make these systems work in dual coil, in fact it is very difficult to make resistances that are perfectly identical and such as not to fool the circuit which would read the resulting value despite having coils with different resistance values. This would in fact make the whole thing operate inconsistently with the programmed calibration curve.

There are a number of applications available on the web which allow us, by entering a few pieces of data, to design the resistance we want and basically understand whether it will be suitable for the system we have at our disposal.

..... but let's get to the point....

OHM'S THIRD LAW: P = V I or replacing V with Ohm's first law P = R I2

In this equation P is the power and we see that it is the product of voltage and current, let's take an intuitive example to better understand. Let us imagine that we are talking about the power of falling water: of a waterfall for example. The power of the falling water is directly proportional to the height from which it falls (this is called potential) and to the amount of water falling. So a 'low' waterfall with lots of water can have the same power as a 'high' waterfall with little water. The principle is the same for electrical power: a higher voltage with a lower current has the same power as a system that linearly inverts the values of voltage and current. That is to say, with a little mathematical forcing, that by swapping the order of the factors, the product does not change :-) . This operation of changing the voltage is done automatically by devices equipped with a circuit; in these, the constancy of power is adjusted as the available performance of the battery is reduced and a small survoltor handles the reduced current availability by automatically increasing the voltage. In the case of 'mechanics', these variations will result, as the battery discharges, in the perception of different aromatic tones of the liquids used. In fact, due to different heating effects, the aromas release all the flavourings they contain differently. This is why many people like mechanics, claiming that they perceive a wider aromatic range, which in any case happens using resistors of different values... in reality it would be enough to adjust the power of the box over time .. :-)

The passage of current generates heating of the conductor, this is called the Joule effect, this physical phenomenon is precisely the development of heat that occurs when electric current flows through a conductor. The intensity of the effect is a function of the thermal capacity and resistance of the conductor itself, i.e. its physical characteristics. What does this mean for a vaping system? Simply that the materials and cross-sections used to pass the current (and not just the coil) must be adequate to ensure that the system does not overheat due to the Joule effect. This is because electrical resistance can be seen as the ability of a conductor to transform the electrical energy flowing through it into heat. In the case of vaping, we want all the heat to develop in the resistor and not in the atomiser and its physical connections to the battery.

Let's think about why in household power distribution we use large sockets for users such as ovens or washing machines and small ones for less 'demanding' uses. In this case, conductors and sockets are designed and constructed to allow currents of up to 16 or 10 amps respectively. As a rule, the 'wires' are 2.5 mm2 for 16 A and 1,5 mm2 for 10 A precisely to prevent them from heating up due to the Joule effect, making it possible to transfer the power intended for the specific consumers even without a voltage drop. Let us also consider why a part of the protection in flat cabinets is called magneto-thermal and is activated precisely due to the heating effect. However, let us remain in the field of direct current so as not to complicate matters.

The heat is therefore directly related to the intensity of the current, not even linearly, but in quadratic form and as a function of time; in fact, the relationship Q = R I2 Δt applies, where Q is precisely the heat and Δthe time interval. Therefore, the conductor, or rather the entire system, must be able to avoid getting too hot during use due to the Joule effect, which would compromise its integrity.

In practice, what do we need to check when using a mechanical tube or a non-circuit box (I am not going to go into the long-standing questions about their safe use, just saying that I prefer to use circuits) ? Simply that the structure of the system allows the passage of current in an adequate manner. Let us therefore avoid, for example, point contacts with rectilinear foils resting crosswise on cylindrical elements, or fire buttons of inadequate power (very often I have seen those approved for lift pushbuttons used) which are generally certified up to 5A. Do you remember that in the examples given earlier, however, due to Ohm's first law, the currents we use are much higher going sub-Ohm? Under these conditions, therefore, we are not using safe systems, but systems which have a critical point precisely in the button or its contact and which should therefore be coupled with electronic regulation and control devices in order to function safely.

Another issue is that of so-called hybrid contacts, in which the positive pin of the atomiser rests directly on the positive pin of the battery. In this case it is imperative to check the perfect flatness of the positive pin of the battery as well as the distance between the contact of the atomiser pin itself and the remaining part of the casing (negative). If they are too close together, in the event of deformation of the positive battery pin, this would easily lead to a short circuit and not simply to the passage of current through an inadequate section.

The last element I would like to say a few words about is short-circuiting. Technically, short-circuiting occurs when the resistance between the positive and negative poles is zero, in which case the current has no limits (it tends to infinity) and the battery delivers power in an uncontrolled manner and obviously outside the limits of use. Extreme sub-ohm resistances tend to this condition and it must be considered that in these cases even an imperfection in the assembly or in a contact leads to a decrease in resistance and therefore potentially to the more dangerous condition of a short circuit.

Safety: this is a subject that has been discussed a lot and with completely misaligned opinions, some stating that to use certain systems you have to know what you are doing and that they are not for everyone. Personally, I prefer to talk first about the suitability of systems with the obvious fact that the human component is fundamental, but I think it is clear that if the human factor leads by definition to error, the system must be intrinsically safe and that the limits of use must be indicated in all devices. Enjoy.... :-)


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