I’ve mounted a fan in my car that draws 8 amps and the temperature switch that activates it can maximum handle 8 amps which is ideal. My question is if I want to add another fan for extra coolant that draws another 8 amps and make it turn on by the same switch, will 8+8=16 amps or will it only draw the 8 amps still for both fans so it will not exceed the 8 amps?

I’m a little confused about to measure amps in this case…

will 8+8=16 amps

Yes.

If the two fans are in parallel, each will have the full 12V across them.

If they are in series then they will only draw 8 Amps but there will only be half the voltage across them so maybe not enough to make them spin.

And having an 8 A fan that is controlled by an 8 A switch is asking for trouble. Motors tend to draw extra current when they are starting up or stalled.

Have you actually measured the current drawn by the fan? It may be rated at 8 A but in use draw less. The current drawn will depend on the load which will depend on the environment in which it is mounted.

Russell.

You don't really want to control the fan directly from the temperature switch. You should use the temperature switch to trigger a relay. It's the relay that then switches power to the fan.

Most generic automotive relays are rated at 30A, and one of these should be fine with a pair of 8A fans. That said, the fans will likely pull more than 8A each when they initially start. You could always use one relay for each fan, and trigger them both from the same temperature switch.

Whatever you do, don't expect the temperature switch to last very long if it's directly powering the fan(s).

Ian.

Are you familiar with the water pressure analogy? It's surprisingly effective for understanding volts and amps in an intuitive sense.

Imagine the wires are pipes full of water. The pressure of the water is the voltage, and the flow rate is current. This is a good conceptual model - it's not perfect, but it's good for understanding how voltage and current relate to eachother.

Imagine the wires are pipes full of water. The pressure of the water is the voltage, and the flow rate is current. This is a good conceptual model - it's not perfect, but it's good for understanding how voltage and current relate to each other.

And a skinny pipe (or a valve that's part-way on) has high resistance so less water flows. A fat pipe has less resistance so water flows freely.

...But, if you cut a pipe water there is zero resistance and water flows freely, whereas if you cut a wire you get infinite resistance and electrical current stops flowing.

The 1st thing you learn when you take an electronics class is **[u]Ohms Law[/u]**, which describes **the relationship between voltage, resistance, and current.**

We don't usually know the resistance of a fan (and we usually don't care), but we can calculate it from the voltage & current specs.

Simplified, Ohm's Law says:

**More Voltage = More Current**

**More Resistance = Less Current**

**Also simplified -** When things are operating normally, **the voltage is usually constant** (or nearly constant). For example, there is 120V at your wall outlet (or 220V depending on where you live) and that voltage is there all the time. Current only flows when you plug something in and turn it on. Similarly, your car battery is (approximately/nominally) 12V and that doesn't change (or doesn't change much) when you connect one or two fans.

If you plug too many things into the wall outlet too much current flows, a circuit breaker blows, and the voltage goes to zero.

Or, if the resistance connected to an Arduino output pin is too low, you'll get too much current, the voltage will drop and you might fry the Arduino.

The voltage MUST drop if the circuit can't supply the current because Ohm's Law is a law of nature and it's ALWAYS true.

Ohm's law is NOT always true.

Its certainly not true for active electronic devices. Its not true for insulators. It doesn't

apply to vacuum devices or discharge tubes.

It does apply to conductors and other conducting materials and even to semiconductors

if of uniform composition (not diodes/transistors/ICs) provided they don't self-heat much.

(In other words its not true for a filament bulb - the current is a non-linear function of

voltage due to the change in resistance with filament temperature)

If Ohm's law were always true electronics would be incredibly boring!

MarkT:

Ohm's law is NOT always true.In other words its not true for a filament bulb - the current is a non-linear function of

voltage due to the change in resistance with filament temperature

When I was first taught Ohm's Law it started "Provided the temperature of a given conductor remains constant . . .etc." so it is true for a filament bulb although difficult to verify.

If Ohm's law were always true electronics would be incredibly boring!

Agreed.

Russell

russellz:

When I was first taught Ohm's Law it started "Provided the temperature of a given conductor remains constant . . .etc." so it is true for a filament bulb although difficult to verify.Agreed.Russell

Ohm law is always true. What is not true is that the resistance is constant, for resistors it is constant but for other devices you may change voltage and the resistance changes, but if you measure R,

V = R * I is always true, or he generic expresion for impedance, V = Z * I

mart256:

V = R * I is always true

No it isn't. What is true is that dV/dI = r (incremental resistance) but that is beyond anything that Ohm covered in his papers which only referred to conductors, not semiconductor, thermionic devices, or any of the multitude of electronic devices we use today.

Russell.

russellz:

No it isn't. What is true is that dV/dI = r (incremental resistance) but that is beyond anything that Ohm covered in his papers which only referred to conductors, not semiconductor, thermionic devices, or any of the multitude of electronic devices we use today.Russell.

Now I remember semiconductors and stuff dont follow ohm rule, you are right.

russellz:

When I was first taught Ohm's Law it started "Provided the temperature of a given conductor remains constant . . .etc." so it is true for a filament bulb although difficult to verify.Agreed.Russell

The whole point is a filament bulb temperature doesn't stay constant, it varies from ambient to 2500K or so!

It's true enough.

The point is that with semiconductors, resistance is simply not a **constant**, so it is not **meaningful** to discuss whether **in this context**, Ohm's Law is "true" or not.

The problem is not the Law, but the discussion.

A slab of uniform semiconductor with ohmic contacts behaves according to Ohm's law pretty

well - Hall sensors are an example of such. The temperature dependence of resistivity of intrinsic

semiconductor is very large, but for doped slabs it is small and stable, and uniformly doped

slabs are what is needed for a Hall effect sensor.

I think we are all saying the same thing in different ways:

Georg Ohm stated that the current through a conductor is proportional to the voltage across it. He didn't define the constant of proportionality (conductance).

The modern teaching of "Ohm's Law" is that V = I x R which is true for materials where the resistance is independent of the voltage or current. That is (more or less) true for most bulk materials but not for semiconductor junctions or thermionic devices. However Georg Ohm knew nothing of those.

So Ohms Law as stated by Georg Ohm is always true as it was only intended to apply to conductors in the normal sense of the time.

Ohms Law as taught at a basic level today is not always true as some materials and devices do not exhibit constant resistance.

An extension of today's "Ohm's Law" involving calculus and incremental changes is always true.

I think that covers it.

Russell.

MarkT:

Ohm’s law is NOT always true.

I can’t believe I just heard someone say that … dV / dI is nothing short of ohms law in relation to time. Yes the variables change but I have yet to see anyone make any case against ohms law short of mis-using it.

As to the original question; 8Amps + 8Amp = 16Amps required to operate both fans at full speed (wired in parallel). As someone else mentioned if you go from + to - to + to - (Series) both fans will work at half speed (in theory) and only use 8Amps together and be 1/2 as strong. IE. Amps is the energy the drives the motor. 0-Amps = 0-rotation and amps cannot exist without voltage pushing the amps through 1/2 Voltage = 1/2 Amps.

tgit23:

I can't believe I just heard someone say that

Believe what you like but engineering is about fact not beliefs.

.. dV / dI is nothing short of ohms law in relation to time.

That has nothing to do with time. It is the rate of change of voltage with current or the slope of the V-I curve.

Yes the variables change but I have yet to see anyone make any case against ohms law short of mis-using it.

Then read the last few posts.

As to the original question; 8Amps + 8Amp = 16Amps required to operate both fans at full speed (wired in parallel). As someone else mentioned if you go from + to - to + to - (Series) both fans will work at half speed (in theory) and only use 8Amps together and be 1/2 as strong.

Interesting. Now you're saying that by halving the voltage on each motor the current through each remains the same?

IE. Amps is the energy the drives the motor.

Current is measured in Amps. Energy is measured in Joules. They are not the same thing.

Russell.

There seem to be two camps here,

- Ohms law is always true.
- Ohms law is not always true.

In fact the actual truth is:-

**Ohms law is NEVER true**

First off think what Ohm was trying to do. He was wanting some way of characterising the voltage / current relationship in a circuit. In other words for a given circuit how many amps per volt characterised the circuit. He did this by saying that "voltage is proportional to current" and anything that is proportional can be made into a equivalence by using a constant of proportionality. Hence

E = kI

Where E is the electro motive force measured in volts, and I is the current measured in Amps. The constant of proportionality k he gave to a constant which was called resistance, but it is just a constant of proportionality.

Where this is fundamentally wrong for ALL materials is that k is not a constant, meaning that resistance is NEVER a constant.

Sure for some materials it is close to a constant but it never is a constant. The truth is that what we call resistance is a function of many things, these things include, but are not exclusively limited to, temperature, voltage, current, frequency, atomic structure, and time.

Let's look at a case where most people think ohms law works. Take a lump of carbon, at low voltages and currents it is constant enough but it has a temperature coefficient, as does most materials. So it is only a constant at a fixed temperature. As you increase the current through it, it heats up and so the resistance changes. Therefore ohms law is not obeyed because the temperature change introduces a deviation from what would have been predicted at a lower current. Sure it is pretty dam good and well good enough for working with electronics, but it is not a fundamental law of physics and it does not hold.

The problem is that all real materials do not have a linear relationship between voltage and current. Take a gas for example, that has a very high resistance for small voltages. As the voltage increases the resistance stays quite constant until it reaches a point where the voltage is sufficient to start to remove electrons from the outer orbits of the gas molecules. This doesn't happen at a single voltage but is spread over a very small range. What happens is the normal thermal energy in the gas is added to the pull by the electric field it is in caused by the voltage. Sometimes this thermal force is in the opposite direction of the electric field and sometimes in the same direction. When it is in the same direction it combines with the electric field to detach an electron. When enough of these events happen electrons can pick up enough energy from the electric field to be involved in collisions with other molecules and help to dislodge them. All the time the resistance of the gas is dropping. A point will be reached when the gas breaks down and these collisions form a continuous discharge and the quantity we call resistance has dropped to a very low point. Clearly the voltage / current relationship is not constant and is very non linear.

So in conclusion for any situation you can think about resistance is never constant so ohms law is never true.

But for something that is never true it is very useful because it's deviation from true is so small it is not important, especially if it is applied correctly. That is for materials that exhibit a near constant resistance OR over a small enough section of the restive function that equates to a straight line.

Ya'll have fun rambling on how "Ohms law is wrong", how "rate" doesn't have anything to do with time and how series motors have different current flow through them. Good luck on your endeavors.

Very good attempts at spinning everything on its head A+ :). I'm sorry if you find "ohms law" offensive. lol..