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Simple Electronic Millamp question

  1. Sep 27, 2009 #1
    This is my first post and I am really excited to find a forum that is full of people who know WAY more than I do about electricity!

    I am wiring up a lighting rig that will mount on my fire helmet. I am part of a great fire department and we are unhappy with what is available to provide us with light while we work. I have taken it upon myself as the station officer to figure out something that will work better for our crews.

    My question is simple... I think.... If I have an LED that the white paper says is rated at a peak milliamp of 20ma. and I hook it up to a 700ma battery will it overdrive the LED (burn it up) or will the led only draw 20ma? The voltage matches what the LED wants and what I am providing it. I just can't figure out if I have to add resistors to drop the millamps into the 20ma range or not.

    thank you for your help!
    Lt. Pat Merlino
    SJI #3 Fire Department
  2. jcsd
  3. Sep 27, 2009 #2


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    Hi Pat, and welcome.

    You would have to know the voltage of your battery to work this out.

    Suppose it is 12 volts and assume the LED is a white one.

    The voltage across the LED will be about 3.5 volts.
    See this article:
    So, if you put a resistor in series with the LED to limit the current, it will have 8.5 volts across it. (12 volts - 3.5 volts = 8.5 volts).
    With 20 mA flowing in the resistor as well as the LED, the resistor needs to be about 425 ohms (8.5 volts / 0.02 Amps = 425 ohms). You can buy 430 ohm resistors.

    The 700 mAH rating on the battery gives you an idea of how long the battery will last before it goes flat.
    It means that
    (number of mA ) * (time in hours before the battery goes flat) =700 mAH

    so....... (time in hours before the battery goes flat) = 700 mAH / 20 mA = 35 hours.

    However, a 20 mA white LED is probably not going to be much use in a burning building.
    Last edited: Sep 28, 2009
  4. Sep 28, 2009 #3
    my question was more of a boiled down simple math question. I am still trying to wrap my head around all the electrical concepts

    I was looking at the CREE series of star w/ optics led for more light but they are too big for our application. The luxeon rebel SMD (3v forward voltage 350ma rated http://www.philipslumileds.com/pdfs/DS56.pdf") looks quite nice but I am not sure if I need an additional heatsink for it or not.

    I would like to hook two of these up to a 6volt battery (a 5 cell 700ma or 800ma pack) other than a switch what else would I need? Resistors?

    if I run the math (6 volts - 6 volts = 0 volts)
    then (0 volts / .350 amps) = 0hms <--- so does this mean that I don't need a resistor?

    700maH / 350mA = 2hrs <-- is this right for run time? cause it isn't very good :(

    There is an LED driver available that is supposed to give out a constant 350ma called a bucktoot (http://www.leddynamics.com/LuxDrive/datasheets/4023-BuckToot.pdf" [Broken]) but I am not sure if it is required. Will the battery not deliver the constant 350ma that the light asks for until the battery level drops below 350ma?

    Again thank you for your help. I will try to post some pictures of my current prototype using some off the shelf parts so you can see what the final product is supposed to look like.
    Last edited by a moderator: May 4, 2017
  5. Sep 28, 2009 #4
    Here (in thumbnail) is the cheapest low-voltage current regulator (limiter) I know. This one regulates current at about 20 milliamps. For applied voltages above 5 V, increase the value of R2.
    Bob S

    Attached Files:

  6. Sep 28, 2009 #5
    How about a single JFET variation, which eliminates needed BJT matching and possible thermal runaway? Example circuit: http://www.allaboutcircuits.com/vol_6/chpt_5/15.html" [Broken]
    Last edited by a moderator: May 4, 2017
  7. Sep 28, 2009 #6


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    That Buck-toot thing looks good. I'll check on those. I see they cost about US$10.

    Before reinventing the wheel, I wonder if you might like to have a look at what is available in the sporting goods stores in the US.
    I was in REI in Oakland, CA a few weeks ago and they had heaps of LED lamps on headbands and in flashlights.
    A lot of their devices used arrays of lower powered LEDs and this has the advantage that if one fails, it makes very little difference to the overall performance. If a single 1 watt device fails, you are in the dark.

    There is a big market for people who want to go out at night and do things like run through the bush and climb mountains.

    I have a 19 LED flashlight that draws 1 amp from 4.5 volts (three AAA cells). I can light up our entire backyard with this flashlight.

    if I run the math (6 volts - 6 volts = 0 volts)
    then (0 volts / .350 amps) = 0hms <--- so does this mean that I don't need a resistor?

    LEDs and most other diodes have a strange ability to destroy themselves. Below a certain voltage, they draw very little current and above it, they can, and will, draw enough current to destroy themselves. So, they have to have their current controlled somehow, rather than their voltage. If you get the current right, the voltage will be right as well.
    This probably sounds crazy, but it is how these things work.

    So, you might get away with putting two 3 V LEDs across a 6 V battery, but what if the battery is fully charged and giving 7 Volts out? You could get a huge flash and then nothing.

    Supplying this current efficiently is what that Buck-toot thing was for. If you just put in a resistor, you can easily use as much power in the resistor as in the LED. Switching regulators, like Buck-toot provide the current from a higher voltage by rapid switching so that little power is lost.
    In your example, you would use two bucktoots to two LEDs.

    No idea about heatsinks needed. 1 watt isn't much heat so quite a small heatsink might be all that is required.
  8. Sep 28, 2009 #7
    There are many LED array flashlights on the market. Why you want to make your own?

    ...now I see that vk6kro is way ahead of me on this one.
  9. Sep 29, 2009 #8
  10. Sep 29, 2009 #9
    thank you guys for all the help!

    There are several manufactures that make lighting for fire fighters helmets however they all are either seriously expensive (200-400 dollars) OR have some major down falls. When we are working in a non hazardous environment our face shields are up, when we are working a fire or cutting up a car for a rescue our face shields (plastic visor in the pics) are down. The biggest down fall I have found with most inexpensive products are they are bulky (heavy) and either have to mount on top of the helmet brim or under the helmet brim. If they are on top then when you shield is up the light is very diffused and doesn't work well. If they mount underneath then when your shield is down the dirt, dust and scratches on the visor lenses lights up and you can't see past your own helmet. The ones that mount on the front of the helmet don't really work for us either as we have velcro identifiers that we use on the front and the clearances when the shields flips down are very tight.

    If you look in the pics I used a commercially available helmet strap and modified two streamlight nano flashlights (1/2 diameter) filling the inside with epoxy (sealing against stray static charges) and hard wiring, to a bigger battery pack mounted underneath the strap and a sealed switch (unsealed switches can create a big spark hazard in an explosive atmosphere). This gives me one light on each side of the helmet and the way the lense sits it gives good light whether the shield is up or down.

    My biggest problem is soldering to the little tiny contacts inside the nano's. I am trying to come up with a drop in design or replacement back for them however, sourcing my own brighter LEDs and having custom reflectors/cases may be more reliable and easier to solder. As one of my lights has already stopped working... I think I got the LED too hot when I soldered it and am not sure how to do it in a reliable fashion every time when soldering inside a very small flashlight case.

    Attached Files:

  11. Sep 30, 2009 #10
    Normally, LEDs are not too easily damaged by heat and wouldn't need any special precautions (heat sinks/clamps on legs) while soldering them on a PCB, for example, unless one doing the soldering is really slow. I have no idea about high power / extra bright LEDs however.

    If heat really is a problem, you could try to wrap thick copper wire (assuming there's no space for clamping) above the joint to help transfer heat away from the LED itself. It won't do miracles, though, so getting the job done right is still essential.

    I myself have long periods of time when I'm not soldering anything, so when I'm going to do some "tricky" soldering (for example, in confined spaces) I first do an easy "practice run" (solder a couple DIPs etc) on something standard to get the feeling back how the solder & iron I'm using behaves so that I get consistent results.

    When it comes to confined spaces, nothing beats planning how to actually perform the soldering so that it goes smoothly. Doing this with a cold iron in hand and just getting the moves right first helps here. That is, if there's no way of opening up the case more for soldering - usually easier. Good pre-tinning helps too. There might be some utilities which help, but can't think of anything particular here (I rarely have to solder in confined spaces).

    Also, I'm not sure what your experience level with soldering is, so this might be a good time to suggest to search google about "soldering tips" - bound to give multiple links to check that you're actually doing the soldering right.

    Just make sure you're not doing cold solders, as they're bound to fail fast with the slightest vibration.

    All this you may or may not have known already - just wanted to give some basic tips to at least get started and maybe have some ideas where to go from here.
  12. Sep 30, 2009 #11
    How about making a kind of battery adaptor - something like a piece of 14mm dowel a screw in either end connected, with each screw acting as a teminal? This would allow replacements to be swapped in rapidly. There might be a commercial product available that does this but I can't turn one up.
  13. Sep 30, 2009 #12
    I had an apiphone at 6am this morning laying in bed and thought about using the parts from a fuse holder to make a batter adapter so I would have to solder on the actual LED. There is a flat contact that the fuse presses against that would work perfectly as the inner contact on the flashlight battery contact. At my local hardware store I found a plastic dowel that fits inside a metal expansion sleeve that would be perfect with the flat contact inside. Anyone have any idea what this flat metal connector is called and where to get them other than buying fuse holders and ripping them apart?

    I am hoping that the replacement nano streamlights will be in in the next couple of days and i can try this out. As far as hooking it all up, I am thinking about wiring them in parallel, with the switch on the positive leg. A couple of my guys want to see if we can wire up 4 of these so there are two stacked on each side of the helmet.

    If I understand correctly I should put a resistor on each LED even though they are wired in parallel so that they won't end up hogging the power or destroying themselves? I will be able to tell how many Ma each one of these draws once I get the replacements in. Then I can figure up the resistors for them.
  14. Sep 30, 2009 #13
    In short, yes.

    (In principle it would be possible to connect through a single resistor, but the LEDs have to match exactly (which they almost never do) and if one LED fails other is bound to go too, so I wouldn't recommend that)
  15. Sep 30, 2009 #14
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