Posted on: 2012-10-07
Our power system is an almost ideal voltage source. It has a fixed voltage, no matter how much current you draw. It should be impossible to lower the grid voltage by attaching too many appliances. Anyway - before you get anywhere close to that a fuse will certainly blow somewhere! The main thing electricity was used for at the time the first power grids appeared was - of course - lighting. Since a lightbulb requires a constant voltage they are pretty easy to hook up:
However, nowadays lightbulbs are being phased out. (Personally I think this is a good thing we finally get rid of those energy-consuming heating appliances which also tend to produce a little bit of light...) Compact low pressure mercury vapor lamps - also called "Energy saving lamps" or "Compact flourescent lamps" aren't really a good alternative. They contain poisonous materials, are hard to recycle, take a long time to warm up, and so on. But here comes the LED! LEDs are even more efficient - light up right away and have to potential to last tens of thousands of hours! But you can't just take a LED and hook it to the power grid. The problem is that a LED doesn't require a fixed voltage, but rather a fixed current. This concept of a current sink instead of a voltage sink can be a bit hard to grasp, especially because there are (almost) no devices generating constant current. From an AAA-battery to the high-voltage power lines powering trains - they are all based on the concept of a constant voltage.
Let's do some math! First, remember this formula: P = U x I (Watts = Volt x Amps). (Yes, it's AC voltage, but let's forget about that for now). Let's assume the lamps in the image above are 60 Watt lamps. At 230V that means they draw about 0.26A.
- So if all the lights are switched off, we have: 230V, 0A. Thats 230V x 0A = 0W being consumed. Great.
- Let's switch on one lamp. We still have 230V, that doesn't change - but now we draw 0.26A. Power draw is 230V x 0.26A = 60W .
- As soon as we switch on another lamp the current rise up to 0.52A. The voltage, guess what, stays at 230V So, 230V x 0.52A = 120W.
- Now lets assume a lamp burns out and completely short-circuity the line. That's bad. Assuming all parts of this circuit are ideal parts with absolutely no resistance, they can carry any amount of electricity. (Remember, ideal parts do not exist in real life!). That would be infinity Amps! 230V x infinity A = infinity W. Good thing that at more than typically 16A your fuse will blow before the infinite energy destroys the universe.... More likely, without a fuse, since all wires have some internal resistance, they would heat up in the wall and burn your house down.... Never try to bridge or repair a broken fuse!
But what about LEDs? In the example above you apply a fixed voltage of 230V to the the lightbulb, the current regulates itself to 0.26A. LEDs work the way round - you give them a fixed amount of current and the voltage regulates itself. A 1W-LED typically requires around 330mA - Then it will draw around 3 Volts (3V x 0.33A = 1W). It's important to know that you can't just hook a LED to a constant-voltage supply at about 3V! Tiny changes in the voltage will make the LED draw much more current and it will burn out very fast. So how do we get those exact 330mA? The answer is to use a LED-Driver. If you speak German this website might have some useful information for you! A replacement LED-Bulb contains such a driver. If you use more LED-Replacement-Bulbs you're ending up with something like this:
Each driver typically contains a rectifier, a mosfet, a transformer, some diodes and a controller-IC. Probably some more stuff... That's a lot of electronics! But what if we have constant current, not constant voltage flowing through the power lines, how would we wire things up? First of all we need to flip serial and parallel wiring. For once, the two lamps from the first picture - replaced by LEDs they need to be wired serial, not parallel. But it doesn't stop there, the whole circuit must be rearranged:
Take a good long at this circuit diagram. It all needs to be serial instead of parallel. Also the switches are reversed. A switch in the "on"-position now switched a light off. And yes, the switches actually short-circuit the LEDs! Lets do some math again. In this case we assume 330mA LEDs with a forward-voltage of 3V.
- Let's start with one LED switched on (all switches "on" but the first one). Current is 330mA, the LED draws 3V. Power draw = 0.33A x 3V = 1W
- Let's switch on another LED (turn "off" the second switch). The current stays at 330mA, but now we have two LEDs drawing 6V together. And 0.33A x 6V = 2W
- We can carry on like this, when all five LEDs are switched on we have 0.33A x 15V = 5W
- Now we switch off all LEDs (all switches "on"). Essentially we have a short-circuit. But isn't that dangerous? Not in this case. No LEDs mean 0V voltage drop. And 0.33A x 0V = 0W. No power is being consumed :)
- So it's pretty safe, right? - even a short circuit is a normal condition rather that a disaster. What could go wrong? There is a disaster case, but it's a different one: An open circuit! An ideal constant current source would try to deliver those 330mA anyway it can - by rising the voltage waay up! So, 0.33A x infinity V = infinity W! It would probably strike an arc somewhere once the voltage goes up the a few thousands volts. This arc will draw a few Kilowatts (0.33A x 10000V = 3300W) and get very hot very fast and burn your house down .... So we'd need a spark gap acting as a "reverse-fuse" which shorts out a circuit in case the voltage gets too high.
The last scenario sounds very unlikely since there are no ideal constant-current sources. The power grid comes close to being an ideal constant-voltage-source but there's no such thing for constant-current. If a constant-current supply is fed from the local 230V grid it probably won't even go up to 230V on it's output. The voltage will rise to that level and stay there. A fault in the cabling will just cause everything to go dark, there's no danger in real world scenarios and we need no spark gap:
Possible? Yes. A good idea? No! A larger constant-current grid is certainly not practical. Hooking up multiple LEDs to a central driver however may have it's advantages.
And it's always good to rethink things we now take for granted. There may be a better way of doing things - but there are a lot more much worse ways.