voltage dropping HELP!

[docnjoj]

Awright all you electrical folks. I need to drop my 5 volts from my d/c converter to 3 volts for my tail light. I get 2 ohmsR@ about 5 watts by my insipid calculations! What is the REAL answer? Or should I just leave the 5volts on and see how long the leds last? Thanks in advance.
otherDoc

 

[Jeremy Harris]

Do you have any idea how much current the tail light draws?

If it's a standard bike one that runs on a couple of AA cells, then it probably only uses a few mA. Your best bet would be to just get three diodes, something like 1N4001, and put them in series with the supply. This will drop your 5V down to about 2.9V to 3.2V which should be good enough. These diodes are good for 1 amp, which should be more than enough. Wire them up in series, all facing the same way. The diode at the end of the string with the band marked on one end goes to the light positive connection, the unmarked end of the diode on the other end of the string goes to the DC-DC converter positive output. The light negative connection goes directly to the DC-DC converter negative output.

If you have some other light, like straight LEDs with no form of current limiting, then the answer will be a bit different!

Jeremy

 

[John in CR]

I just use the LED wizards. Be careful of overvolting your expensive LEDs as it will lead to their premature death.
http://led.linear1.org/led.wiz

John

 

[Roy]

You could live your traction batteries alone. Just find a place to strapon a couple of D-cells and wire the 3volts to your light. That will last a very long time and no silly resister burning up batteries. I have used this approach with 3 D-cells where I pull off both 3 and 4.5 volts running 3 different lights.

 

[rkosiorek]

if you must use the output from your 5V dc-dc converter maybe you should consider a 3 terminal LDO regulator. typical parts would be LM7803, BA03CC0T, MCP1825S, MCP1826S, MCP1827S.

available from Digikey for a buck or two each.

since you are only dropping from 5V down to 3V efficiency would not be a big issue.

rick

 

[docnjoj]

Wow! Thanks folks. Those parts (diodes and LM7803) are actually availible at my local RadioShack. Thanks to all! I am using the DC converter simply because it is already set up for my 5 volt LED headlights. My "expensive" tail light only cost 6 bucks!
otherDoc

 

[John in CR]

Wow! Thanks folks. Those parts (diodes and LM7803) are actually availible at my local RadioShack. Thanks to all! I am using the DC converter simply because it is already set up for my 5 volt LED headlights. My "expensive" tail light only cost 6 bucks!
otherDoc

Why the diodes and LM7803, when all you need is the correct resistor? It's my understanding that regular LEDs need a resistor anyway for long life of the extra bright normal LEDs, even if you supply the right voltage. I did a series parallel combo that avoided needing a resistor for some in my office that shine directly down on my keyboard, and their brightness dimmed greatly after only a month. They need a resistor to prevent over-current from what I understand.

John

 

[Jeremy Harris]

Why the diodes and LM7803, when all you need is the correct resistor? It's my understanding that regular LEDs need a resistor anyway for long life of the extra bright normal LEDs, even if you supply the right voltage. I did a series parallel combo that avoided needing a resistor for some in my office that shine directly down on my keyboard, and their brightness dimmed greatly after only a month. They need a resistor to prevent over-current from what I understand.
John

Depends on what the light is, John. IF this rear light is just an LED, then yes, all that's needed is a different current limiting resistor. BUT if this rear light is an ordinary 3V bike one that normally runs on two AA (or two AAA) batteries, then it will need a constant voltage supply (bike LED rear lights usually have a built in pulse drive circuit to get good battery life).

Without more information we can't be sure that a different value current limiting resistor will work (it probably won't work for a pulse-drive unit without a smoothing capacitor), but we can be sure that a 3V supply will work.

BTW, I measured the current on my small battery-powered rear light. It was just 35mA average, but had spikes that were around 300mA or more. It obviously runs the LEDs with high current pulses to get high brightness, and relies on persistence of vision to appear like a steady light.

Jeremy

 

[docnjoj]

Double thanks, Jeremy. Your explanations are crystal clear, and that really helps the "electronically somewhat challanged" folks like myself! RS only had diodes and the LM 7805, so I went with the diodes!
otherDoc

 

[rkosiorek]

sometimes simple is good. but when you get a general question without any specifics like current, or specific wiring information you can't just give a simple answer.

possibly he could get away with a resistor, but how could anyone calculate a value with the information given?

i have tried several cheap LED taillights. if they are the simple kind that just turn on and off a resistor works fine. if they are the type that flash or blink in verious sequences than aresistor will not work well. if the resistor is large enough to limit the current when it is operating steady state, it is too large for the bright flashes.

rick

 

[docnjoj]

Thanks Rick! I kinda figured that the resistor would do someting like that, so I went with diodes. Seems to work fine with 3 in series. Will try it with 2 next, to see if the light gets brighter
otherDoc

 

[John in CR]

Diodes in series is exactly what I used and my LED's dimmed 80-90% very prematurely. Other LEDs were the diodes I used. I understand a resistor not working if there are some electronics in front of the LEDs for blinking or whatever, but for a steady on LED, I've been told that they need a resistor. It's a case where brighter isn't always better.

John

 

[Jeremy Harris]

John,

The reason your LEDs dimmed prematurely is to do with the way you chose to power them. LEDs on their own (not ones built in to an assembled lamp that is designed to run on batteries) are current driven devices. The supply voltage is unimportant (as long as it's above the LED forward voltage drop) as it's the current that needs to be regulated.

Most small LEDs need around 20mA to 30mA to run, any more and they will tend to prematurely fail. It doesn't matter if the supply voltage is 5V or 100V, as long as the LED current is limited to the right value.

One way of limiting current cheaply is to use a series resistor. This works OK for low voltage drops, but is problematic at high voltages because of the power loss (the resistor wastes a lot of power and gets hot).

Most ready-made, battery powered, bike LED lights use an array of LEDs with internal circuitry to allow the lamp to run on just one or two cells. This circuit provides a regulated current to the LEDs and allows the lamp to run consistently right up until the batteries run down. A resistor wouldn't work well for a lamp like this, as red LEDs need a minimum of around 2.2V or so to turn on. Here's an example that shows why these bike lamps use a proper current regulator:

- If the two cells in the lamp are new, they will give a voltage of about 3.2V.
- For 30mA through the LEDs, the dropper resistor would need to be (3.2 - 2.2)/0.03 = 33 ohms (for new batteries).
- Two end of life cells will be down to about 2.8V, so this 33 ohm resistor would now reduce the LED current to (2.8-2.2)/33 = 0.018, or 18mA. The result is a more than 50% drop in LED current that may reduce the brightness by an unacceptable amount.

In our case, most people will want to run their lights from their main battery pack. There are two ways to do this:

- use a ready-made bike lamp, intended to run on batteries, and then fit a voltage regulator to reduce the bike battery voltage to the bike lamp voltage (typically 3V), or;
- use ordinary LEDs and drive them with a current limited supply, either a dropper resistor or better still a decent current regulator.

For those who want to build their own lights from discrete LEDs, then a neat way around the current/voltage drop problem is to wire all the individual LEDs in the light in series. If you have 20 LEDs, with a forward voltage rating of 2.2V (check this for the particular LEDs you buy, the forward voltage varies quite a lot with LED type and colour) then if you wire them all in series you need at least 2.2 x 20 = 44V to start to turn them on. If you have a 60V battery, then you can get away with using a resistor to restrict the current through the chain of LEDs. If you wanted 30mA through the LEDs, then the resistor you'd need for this example would be (60 - 44)/0.03 = 533 ohms. The nearest preferred value is 560 ohms, which will work fine in this case. The resistor wastes a bit of power, so needs to be suitably rated. In this case the resistor power loss can be worked out by knowing the voltage across the resistor (60 - 44 = 16V) and the current flowing through it (30mA = 0.03A). Power loss is 16 x 0.03 = 0.48W, so a 1/2 watt resistor is just about OK. In practice, I'd go for a 1 watt rated one to give a bit of a margin.

Similarly, the power input to the LED array can be worked out, from the LED forward voltage drop and the current flowing through the array. For our 20 LED series array, this is 44V x 0.03A = 1.32W. This shows the relative power losses of using a resistor well, the total power used is 1.32 + 0.48 = 1.8W, and we are wasting 26% of this in the resistor.

There are fancier ways of regulating LED current, like the very efficient pulse drive circuits that many ready-made bike lamps use, but hopefully the above should make some of this LED stuff a bit clearer.

We still don't know what sort of LED light the OP is using, so we are still guessing as to whether or not the voltage regulation or current regulation approach is the right one in this particular case.

Jeremy

 

[rkosiorek]

Doc,

what a wonderful answer. i wish that the person posing the question in the first place put 1% of that effort into framing the question properly. I guess they assume that someone here is psychic or something. i have been accused of being psycho on occasion but never psychic. either that or they are looking for an answer that is half-assed.

my other pet peeve is - how often do you see the same question over and over again. every few weeks there is the round of questions, how high a voltage can i use on my(fill in blank yourself) or how big a wire gauge is used, or how do i figure out which wire is which on the bargain barrel throttle i bought. of course it never occurs to anyone to use the search engine first and find out if anyone else asked it before. that would make too much sense.

rant over, it's friday, having a bad day, just got a registered letter from Revnue Canada. note that this is an explanation - no appology for the comment.

rick

Hat's off to you doc. you have more patience than I.

 

[docnjoj]

Sorry Rick! I believe I was the OP. The light works fine with 3 diodes in series! If it burns out or goes dim, I have 4 more! Sorry about the revenueeeeers too! In this country in the South they used to shoot them! Have a better day!
otherDoc