# How does a electrical device draw its rated current?

I am learning electronics and this question has been bugging me quite a bit. If I have a device, let us say rated at 6 volts and 350 mA, this means the device will work when there is a potential difference of 6 volts across the lamp and it has 350 mA flowing through it right?

In household circuits, the power outlets are usually at 220V and the current it will draw will depend on the resistor of our device no? How will our lamp with the above rating work in this case?

• A lamp will present a resistance (or impedance if AC) to the power lines such that the correct amount of current flows as per ohm's law. Commented Oct 29, 2022 at 18:02
• "6 Volts and 350 mA, this means the device will work when there is a potential difference of 6 volts across the lamp and it has 350 mA flowing through it right?" Note quite. Since most things are powered from (constant) voltage sources, not (constant) current sources, it is usually meant to be interpreted in the following context: Being rated at 6V and 350mA means that it requires 6V to run and if you apply 6V to it, 350mA will flow through it in response. Commented Oct 29, 2022 at 18:07
• If it were intended to be powered by a current source, things would be reversed: It requires 350mA flowing through it to function and to force that amount of current requires 6V be applied. It might seem like a subtle difference for a lamp which is constant power but not all loads are (like a computer or a motor). So for things expected to be powered by a constant load that 6V must remain the same, but the current could change in operation. Again, reversed for things expected to be powered by a current source. Commented Oct 29, 2022 at 18:08
• @user1850479, but wouldn't the lamp have constant resistance/impedance? I don't think it can change that to ensure the right current flows though it Commented Oct 29, 2022 at 18:30
• it may help if you think about it this way ... the 6 V is able to push 350 mA through the load ... it also means that a higher voltage will be able to push more current through the same load Commented Oct 29, 2022 at 18:56

First, ratings can be weird. Some devices (like consumer-grade light bulbs and vacuum tubes) have the nominal ratings given. Some devices (like semiconductors) have the maximum ratings given. Really well-specified devices will have both nominal and maximum ratings, and for things like light bulbs, may even come with charts giving you a performance vs. voltage and a lifetime vs. voltage, so you can make your own decisions.

If you buy a light bulb that's advertised as 6V, 350mA, then it should have around* 6V applied, and it will draw around 350mA. Apply more voltage and the current will probably go up, apply less and it'll probably go down. If that bulb is being lit up by mains power, then either there's a box in the middle that drops the voltage, or there's a bunch of them in series (i.e., 36 or 37 6V bulbs, in series, should run off of 220V as long as they all warm up about the same speed).

Ratings aside, in general any two-terminal load will pull a certain amount of current given a certain amount of voltage. This is just basic physics -- put a voltage on something, and it determines the amount of current it'll consume. A corollary to this is that if you force a certain amount of current through a two-terminal device, it'll determine how much voltage it generates to oppose the current. With only access to the two terminals you cannot arbitrarily choose both the voltage and current that'll flow.

Usually a simple device like a light bulb will pull more current with increasing voltage. However there's other things -- like anything with a switching regulator in it -- that may have a section of its operating curve where increasing voltage will make it pull less current, or the current will be flat for a while.

Getting back to your light bulb: if you plug a 6V plain old incandescent bulb directly into a 220V source, then it'll die in milliseconds, possibly violently enough to damage things around it. Ditto almost any 6V LED bulb -- although typically LED bulbs have driver circuits, and one could, conceivably**, design a driver circuit to work from 6V to 220V.

* "Around" in this case meaning $$\\pm\$$20%, but it's a squishy number -- you can get a dim glow from a 12V bulb run on 6V, and a 3V bulb on 6V will be really bright, possibly for hours or days before it burns out.

** Either as a joke or because there were really stupid specifications to be met.

• Thank you to everyone who answered this. It helped me a lot. I was under the impression that we make randomly rated devices in terms of voltage and current because there are so many different voltages that the outlet could be at (and the device on the inside did something to drop the volts). But I guess for an entire countrty, that outlet voltage is the same, so all devices will be rated at 220V Commented Oct 29, 2022 at 19:39
• More and more devices are being designed to work from 100V to 240V, so they can be plugged in anywhere. But yes -- because voltages tend to be the same country-wide or region-wide, the market for things like light bulbs or juice mixers can be big. Commented Oct 29, 2022 at 19:45

There are 4 things to consider for a lamp. The rated voltage, the rated current, the resistance of the lamp and the power produced by the lamp.

In simple terms what is fixed is the resistance of the lamp. As you increase the Voltage across that resistance the current increases. These values are related mathematically by Ohms law. This states that V=IR. Voltage is current times resistances this can be rearranged to give you the current for a resistance at a particular voltage as I=V/R.

Your 6V lamp rated at 350ma therefore has a resistance of R=V/I of 6/0.35 or about 17 ohms. It will produce an output power of Volts x Amps = 2.1W

If you connect this resistance to 240V you get 240/17A = 14A. This produces power of about 3388W which explodes the bulb.

A 240V 100W bulb is made with a resistance of about 780 ohms giving you a current of about 300ma.

By making bulbs with different resistances you can make them suitable for different voltages.

A 6 V lamp plugged into a 220 V socket will draw roughly 37 times its rated current, or about 12.8 A. However, it will do so for a very short period of time before the filament is vaporized and the lamp is destroyed.

• Possibly destroyed explosively. Learnt this plugging my lego light brick into the mains at a very young age. Don't try this at home.
– RoyC
Commented Oct 29, 2022 at 18:09
• But we still do it right? Why is it that lamps don't melt when drawing power from the main outlet? Commented Oct 29, 2022 at 18:31
• The actual amount of current it'll draw depends on the device -- although at 220V vs. 6V, "lots and lots more" will be accurate. For instance, if it's an LED with a simple bridge, then the current will (briefly) rise exponentially with voltage. If it's an incandescent light, the resistance will go up with temperature, so the current will (briefly) rise as something like $\sqrt[n]{V}$, where $n$ is something like 3 or 4 (I can't remember). The key word in all of this, however, is "briefly", perhaps accompanied by "ball of plasma". Commented Oct 29, 2022 at 18:33
• "But we still do it right?" No, if we want a lamp to run off 230V we use one designed to do that, not one designed to run off 6V. Commented Oct 29, 2022 at 18:43