Electrical HELP!!!

[DrinkMore]

Got a problem that I can't wrap my brain around.

Problem - I have a Christmas tree that has 10 lights (battery powered). Regular clear lights. Not leds or anything. I converted it to USB powered. I plugged it in and all worked well.. for a while anyways.

The lights were unusually WHITER and BRIGHTER. I knew then that they were getting TOO MUCH power. So, one by one they blew out. Now it's dead.

The issue is - I don't know what resistor I need to prevent that from happening. Here is what I know thus far..

USB = 5v

Battery Powered Lights (10 lights) = 3v / 4watts

I need to go to rat shack tomorrow and get a resistor but I have no clue which one to get. Most all are in ohms.

Please help! Thanks!

[theknub]

and ohm is equal to the voltage divided by the ampere

power (in watts) can be defined as voltage squared divided by resistance

using those two formulas, you should be able to arrive at the resistance you need

[DrinkMore]

Originally Posted by theknub

and ohm is equal to the voltage divided by the ampere

power (in watts) can be defined as voltage squared divided by resistance

using those two formulas, you should be able to arrive at the resistance you need

Actually no. It's all gibberish to me. I read about these formulas but they again, are greek. No clue.

[DrinkMore]

Anyone else have any ideas?

[MrBogey]

variables-
watts:3-4 per bulb
volts:5
resistance=x

watts=volts^2/ohms

results in:

ohms=volts^2/watts

results in:

x=5^2/(30 to 40)

results in:

x=5/8 or 0.625 ohms or
x=5/6 or 0.83 ohms

edit: nevermind what I wrote past this...made a mistake.

Here...this should help-
http://showcase.netins.net/web/ducks...s/res-code.gif

[DrinkMore]

I picked up a pack of resistors. Has a bunch in them. I guess I will just try them until I find one.

[DrinkMore]

Thanks. I tested each one and found one that looks like it works. It's brown, black, black, gold.

When used, the lights look "normal" and not like they are cranked up and going to burst!

We will see how it goes.

[crackinhedz]

Voltage divided by current equals ohm (resistance)

[DrinkMore]

How do I find out what current these things draw?

The resistor I used, started to get super hot. Hot to the point it burned me. Something is wrong with that.

The bulbs say they are 2.5v 170mA. I have 10 of these. If I run full blown USB current, the bulbs are over driven and burn out in a few minutes. If I use a resistor, the thing gets super hot.

What am I missing?

[MrBogey]

Are you putting the resistor in parallel or in series?

[DrinkMore]

Originally Posted by MrBogey

Are you putting the resistor in parallel or in series?

No clue.

I have a black wire and red wire. Red is power and I am putting the resistor there so it's full power --> resistor --> reduced power --> power to lights.

Is that wrong?

[Mr. Hanky]

You are correct in noting that you need a resistor with an appropriate current rating (essentially, you are looking for a resistor with a suitable minimum or greater power dissipation), as well as a particular resistance value.

Those little ant-sized resistors you got may be the correct ohm value, but they are WAY over their head to handle the current that would flow through them to power a 4W device at 5 volts (close to 1 amp, but not quite). I think you should be able to look up those color codings on some chart to determine maximum allowable current ratings. It probably will turn out to be in the low milliamp range. If you are deadbent on using those little resistors, you would need to construct a network of them connected in parallel to build up the current capacity, but then you also need to use higher ohm values for each element to maintain an effective resistance value for the circuit. This is probably more complication than you are looking for, but it is indeed, possible.

What you need are larger "power" resistors, intended for high-power (relatively) applications. The size gives them higher current ratings (higher power dissipation capacity) for a given resistance value. They are usually recognizable as a finger-sized ceramic block construction. Perhaps, you can find a single piece power resistor at RS that has the correct ohm value and a suitable current/power rating (at least 1+ amps, I would guess). If not, then you may have to combine 2 or more elements in parallel to get the correct ohm value. Either way, it will be much safer than using those little resistors you have now.

[DrinkMore]

Damn. Ok. I am not hell bent on using these guys. I just figured that's what I needed.

I saw some ceramic/block ones. I guess I am having trouble understanding what I need. I picked up some colored battery lights, same thing but these use 2 C batteries. I plugged them in without any resistors and they seem just fine. Not over bright or anything.

Very weird. I am just really confused as to how and what I need for this to work out.

[Mr. Hanky]

The battery-powered lights should be a safer option for you to play around with, but it may still be a bit uncomfortable running at 5 v (2 C batteries in series would indicate a nominal 3 v operation).

With all due respect, you really need to become accustomed to doing the math involved, in order to play around with the electrical stuff you are playing with. It can be a rewarding and intriguing project, but also a very dangerous one (potential fire hazards, damaged equipment, personal injury), if you are not careful and cognizant of the circuits you are creating. You can ask for quick answers online, but you are still on the line to evaluate if advice you "got off the internet" will be safe and practical for your uses. Hence, that is why I suggest you learn more about the math before hooking stuff up.

The formulas are as follows:

V=I * R
P=I * V

V is voltage
I is current
R is resistance
P is power

That is the minimum you will need to grasp before you should experiment with simple electronics. Beyond that, it is also highly recommended to do some reading up to gain a sense of what sort of values are "high", what is "safe", and building things with a safety margin. This will give you the grasp over involved mechanics of the project so that you can identify dangerous scenarios, impractical approaches, and build things that are going to stay alive.

Short of all that (no pun intended), you are literally "playing with fire". So BE CAREFUL with what you are doing, and never leave this stuff unattended, if you haven't evaluated all of the math and safety concerns of the design.

EDIT: ...and don't go nuts hooking up more lights to a USB socket. Even a powered USB socket has a current limit, after which it will shut down or burn-out if you exceed the limit. If you are hooking up devices intended for 3V to a 5V power supply, that is already creating a situation where this can happen (the 3 V device can potentially create excessive current flow because its internal resistance was not designed to be fed at 5 V, hence, more current will flow than was intended). 1+ amps of current on a USB line can be pretty "iffy".

EDIT2: Maybe the obvious safe approach is to simply buy 2 sets of these lights and connect them IN SERIES to the USB. That will double the resistance w/o having to mess with any power resistors to pad the circuit. The lights may be a bit more dim, but they will stay alive, and the USB power supply will surely be operating in a safe domain. Just be sure to connect them IN SERIES, not in parallel (parallel will create situation even more dangerous than what you have now).

[DrinkMore]

Thanks for the info. I have found a set of lights that runs perfectly fine on a straight shot from the USB connection. No issues, no burn outs and the lights are perfectly lit.

I am all about learning what I need to know. I am also aware of the safety issues that I could be creating. That's why I am getting some opinions and info.

I have found a solution that works for me and I think I will stick with it.