[SOLVED] Can I light a single LED via battery taken from a case fan ?


Jan 31, 2003
I want to install a single LED light in an N scale building. It will be battery driven. I have extra case fans and want to go a budget route. Any thoughts on this?
Huh, never thought of it that way, always assumed the voltage was a necessary constant.
Elementary dear Watson, elementary physics that is. Voltage drops with resistance, every conductor (including resistors) has some resistance or we wouldn't have problems with long distance power transmission. where voltage has to be boosted to 100 000V+ just to allow for losses.


What is constant (but see below) is Resistance. That is a property of the material the item is made of. So when you apply an electrical Voltage across the item, that causes some current to flow through it. The quantity of current flow is determined by the material's resistance. That's Ohm's Law: V = IR, relating Voltage, Current, and Resistance.

If you have an item as load connected to a source of Voltage, there at two ways you can manipulate the resulting current flow. You can insert an added resistance in the circuit in series, so that the total resistance of the circuit is increased, causing current flow to be reduced. Instead, if you add a second resistor in PARALLEL to the first load, that DEcreases the resistance of the total load and the current flow INcreases, although this does NOT change the voltage applied to the first load, and hence that current does not change. BUT in real life no Voltage source can provide infinite current. Any source acts as if it has its own resistance inside it, so the lower the resistance of the external circuit is (and hence the higher the current allowed to flow), the more the output Voltage of the source device is REDUCED by the Voltage drop due to the source's INTERNAL resistance. So this DOES actually reduce the current through the external load because it reduces the real Voltage applied to that load. That reduction of Voltage (and hence current) at the load depends on the nature and magnitude of the effective internal resistance of the Voltage source device. For example, if your Voltage source is a small 9V battery, the impact of a second resistor in PARALLEL with the first load will be much greater than if the Voltage source were a huge 12 V car battery. Or, in many practical electronic devices, the Voltage source really is one designed to control to a tight range the VOLTAGE it puts out, so it ensures that its effective internal resistance is altered automatically as the resistance of the load changes, preventing any change in the source's output Voltage. But there is a limit to how much current the power supply module can deliver, so as the total load resistance decreases due to the added low-value resistor in parallel with the primary load, eventually the source output voltage falls despite its attempts to maintain. Thus with a very low resistance in parallel with the primary load, you still can reduce the voltage to the total load and impact its performance.

As always, there are exceptions to the rules about a fixed resistance value as a property of the material. We're aware that the actual resistance of even a plain piece of wire will change slightly when its temperature is changed. And we've all heard of special cases in which a metal brought to really low temperatures suddenly becomes a Superconductor with zero resistance. That is due to a substantial change it the arrangement of atoms and electrons in the crystalline structure of the metal. Perhaps fewer of us are aware that any real wire (including any resistor) is not purely a resistor, but actually has a very small inductive property. This becomes a small factor when the current flowing through is not DC, but an alternating current that induces in any inductor a current trying to flow in the opposite direction, effectively increasing the resistance to current flow in the main direction.

It happens that years ago I did research in this area applying high-frequency AC - from audio frequencies up to above microwaves into the Far-Infrared region - to materials to study the movement of the molecules in the samples. The samples can be viewed as the dielectric insulating material in a capacitor measuring cell. The dielectric material can be modelled as an ideal capacitor in parallel with an ideal resistor. In those high-frequency regions, the frequency of the applied electrical field can match the natural frequency of rotational movement of many molecules, or parts of them. When the frequencies match approximately, the molecules absorb energy from the applied field to increase their energy, and this amounts to acting like a resistor absorbing energy and converting it to heat. But that only happens when the frequencies are close together, so in effect the apparent resistance of the material changes according to the frequency of the applied electromagnetic signal. We see an application of this this every day in our kitchens. A microwave oven uses a strong field at about 2 to 5 GHz to excite certain parts of organic molecules in food and water to higher rotational energy states, which we observe in macro scale as heated food.
Last edited:
Reactions: CountMike