This is completely off topic, but I thought as you are (well, most of you) such a knowledgeable bunch of people, I thought I may ask you.
I am going to put LEDs in my radio-controlled car. I have purchased 2 white and 2 red bright LEDs (Like ones from bike lights), which are going to be front and rear lights.
I took one LED, and connected it to a 9v (rectangle) battery. It is really bright, but gets hot - very hot, quickly!
I don’t know what battery to use… If you know anything about circuits, help me please!
I think I may need something like a lantern battery (6v, maybe) or something. Can someone please tell/link to a pic, of what these are!
I don’t know too much about these, but I think there are quite a many different kind of LED’s. Those you have might be for something 4,5 V. Just a guess based on my little experiments with leds. I had four of these (bright/clear when not conected) red leds, connected to 4,5 V (3xAA) and they were really bright. They warm a little but not much. That 9V is probably too much. Where did you get those?
It doesn’t usually matter what kind of batteries they are (AA, AAA…). The voltage is the same 4,5 V (serial, not parallel). That is something to start with. It shouldn’t burn up your LED’s.
Most LEDs don’t work long at 9 volts, the ones that will have a current limiter or resistor built inside them. The ones that have a resistor in them are only good for a small range of voltages.
What you need to do is to get a resistor that is the correct value to limit the current to what the spec is for your LEDs, usually from 10 to 30 milliamps (note that you must limit it to the correct value, not that range). Or, if your LEDs are current limited to a particular voltage, use the correct voltage.
Sorry, I don’t have time right now for a more detailed explanation, I have to get ready to go work as an electronic technician.
You have 2 choices here. Either get a lower voltage battery, eg use 2 1.5V AA or AAA batteries to give 3V, which should work fine. If you want to keep the 9V battery you need to add a resistor so that some of the voltage is dropped across that rather than the LED. TO be really accurate you need to do the maximum rated current of the LED (if you got it out out of a catalouge look in that, or if you got it from a shop, ask them, they might know.) It’s generally around 25mA. You also want to know the voltage drop across the LED, which ideally you need to find out in the same way, but it’s normally around 3 or 4V (brighter(larger) LED’s generally have higher voltage) Then from my good mate ohms’ law you get R=V/I, with I being 25mA (LED current) and V being 5 or 6 (9- LED voltage) and you get either 200 or 240. Go for 240 if you’re cautious or 200 if you want brightness. Or, if you can find somewhere to buy this stuff (www.maplin.co.uk, or rswww.com, or school type places when they’re not looking are good places to start) get a few either side and a few extra LED’s and play around until it doesn’t break.
Connect the battery, the LED and the resistor in series. If you don’t have a handy soldering iron then normaill twisting the component legs together and then putting some tape around it will do the job. And if you’re feeling really technical add a switch in the same way.
Hope that helps. Apologies if I gibbered too much.
John
P.S. While we’re on the subject, does anybody have a handy design for a speed controller for a switched reluctance motor with 8A current limit controlled from a ti c6780 dsp? with software design as well? No? Oh well, it’s only 1 module.
edit: 'course, you could just wait till it melts through the floor and enjoy the nice fireworks…
I don’t know the actual voltage drop, but John says it’s about 3V-4V. And he sounds like he knows what his saying. It’s because of the LED’s own resistance.
joona’s right, the 3 - 4 volts is the voltage drop across the resistor. From your kirchoff’s voltage law (you probably don’t know it, but it ain’t complicated) total voltage in a circuit is zero, so if you have a 9V source you must have a 9V drop on the other components, so if you want 4 volts across the LED you need to choose a resistor so that you get 5 volts across that at whatever current you need. Current is equal in all components in a series circuit, so the current in the resistor should be equal to the maximum operating current of the LED (normally around 25mA) Ohm’s law is the other proper useful one, and that is; Voltage = current x resistance, which rearranges (by dividing both sides b current, just for completeness) to: resistance = voltage / current.
So if you want a voltage drop across the LED of 4 V, that means you want a voltage across the resistor of 5V, and a current through the resistor of 25mA (which is 0.025 in normal numbers) Giving: R = 5/0.025 = 200 Ohms (and the stripes on the resistor will be red, black, brown. See this site if you want to know about that).
The problem having worked all that out is that most resistors don’t come in all values, and the one’s around that value are 180, 220 or 270 ohms. I’d go for 220, with it being nicely in the middle of the 2 values I mentioned in my last post.
The other problem here is that it all depends on the values of the LED. The one’s I’ve sued are fairly typical, so they should be all right. There isn’t too much problem with slightly inprecise values.
And if all that’s too much effort then steel the heat sink of your CPU, gaffer tape it to the LED and chuck some of that paste stuff they use between the 2:)
I think that’s it. Come back next week for basic semiconductor design…
If I’m too confusing, Joona was right (sorry for repeating what you said) so read his posts.
No, I do know what I’m saying. I’m just not certain that what I’m saying is right, OK?
And yes, I’m posting at 1.08 in the morning. I was at a party, but it wasn’t too great, and my beer ran out, hence being back. That and the 9am lecture thing.
I have physics today - I love it! I asked my teacher what he thought I should do, but didnt really know.
We have been doing about resistance lately, but stuff much easier than this!!
What is easier than this. At least I think this is one of the easiest things about electricity. You just wait until you get (some day) to the AC currents. Whiii!! All those imaginary numbers are going to make you crazy at the beginning.
Getting a bit OT here, but do you mean electronic engineer, or electrician? Similar words, but quite a big difference. One means 3 to 4 years at uni, the other won’t make your head hurt so much - it’s quite difficult, but then few worthwhile things are easy (you learn’t to ride a uni, right?)
If you mean engineering, don’t worry about GCSE electronics. physics gcse and a-level covers everything you need before going to uni. As in most science-ish subjects, they tell you how things work and it all seems nice and simple, then you get a bit more advanced and they tell you they were lying.
John
(edit: Joona - 6 cans. I had to get up early the following morning, and I was going to see a band (muse-they were very good) this evening. lack of any more beer was by design.)
220 Ohm
I want to be an electrician as well… hummm… better get used to this!