I'm replacing the front turn signals on my motorbike with LEDs. The LED light is rated at 0.4w at 12 volts, and the globe it is replacing is 21w.
When I connect the new LED light to the original wiring, the turn signals (on the dash, front LED and back globe) all flash too fast. I am wanting to slow these down and was hoping someone could provide some advice on what type of resistor I need to apply to the circuit to increase the resistance to the same as the original 21w globe.
I'm not sure if I've provided enough information for this calculation, so if more is needed, please let me know.
Since power = volts * volts / resistance then resistance = volts * volts / power, so
R = 12V * 12V / 20.6W (note that 20.6W = 21W - 0.4W) R = 7 ohms
You can probably get away with using a resistor rated at only 10W since the blink rate will make the effective power closer to only 10 watts (or so). Typically, you don't want to run a resistor at its max power rating, but in this case it's not a critical application, and if you select a wirewound resistor it will (most likely) never fail even at its rated power. Also, you'll probably never exceed its rated max temperature either.
Having said all that, it seems like a real waste of power just to be able to use LED lamps. I've resisted putting these types of lamps on my bikes' blinkers because it seems to defeat the advantage of using LEDs.
Someone must have come up with a flasher replacement that works properly with the lighter load drawn by LEDs. Have you looked for this type of device (I haven't). Let us know if you find anything.
Assuming you mean 2 globes at 21 watts each, one forward facing, the second rearward. Lets round that down to 40 watts.
Power in watts is volts x current
40W/12V= 3.33A
The LED load is insignificant at 33mA so I am ignoring it.
Voltage is resistance x current
12V/3.33A= 3.6 ohms but it will have to be quite a resistor at 40 watts plus a safety margin! Since they are operating intermittently, you can get by with a some what lower rating however I think you'll do better to find the minimum load at which the flasher operates or replace the flasher with an electronic version.
I've spoken to the local auto electrician and he has advised me that a resistor will not work in this situation. Since the flasher cans require power usage to create/break the circuit (creating the flash), using a resistor will only reduce the power usage even more and therefore speed up the flash even more. I hadn't thought of it this way previously but it seems to make sense. He has advised me to replace the flasher can with an electronic one, and this should apparently solve any problems.
What the electrician is saying is true if you were to put the resistor in series with the LED pack.
What I think you need to do is to put a resitor in parallel with the LED pack.
The resistor will look like a bulb as far as the flasher is concerned so the flasher will work normally.
The LED pack in parallel will simple light whenever the resistor is switched.
Resistance of bulb is given by : POWER = Volt x Volt / Resistance
So is 0.15 ohms.
So if you were to put a 0.15W resistor (rated at 21W) in parallel with the LED they I beleive the LEDs will flash normally.
Note trying to keep simple - I know I have considered the parallel resistance but in reality the power in resistor (21W) is so big compared with the LED (0.4W) that it makes little difference to the calculation.
Obviously, this is a waste of power (21W rather than 0.4W) but without changing the flasher circuit I can't see how you can avoid this. Besides the indicators are not on for long.
Also, 21W is the power when the flasher is on. Because it actually, flashes on and off you may be able to get away with a lower rated resistor.
Allot of people suggested using a resistor to increase the amount of current drawn by the circuit. You can do that, but it might be easier and cheaper to just use an 1156 bulb (or something similar) instead of the resistor. Just wire a bulb in "parallel" with the LEDs somewhere along the wire feeding each turn signal. You can wrap them in foil and hide them inside the headlight bucket or under the seat, wherever you can find a spot.
OTOH, you could fabricate a new flasher module that doesn't depend upon the load to determine the flash rate. You may be able to find something already being manufactured that plugs in place of your stock module. Have you checked?
Of course fabricating a flasher that does not depend on the load would be a contravention of the rules in most jurisdictions that require the flash rate to change to indicate the failure of a bulb.
I never knew that to be a requirement anywhere in the US. A couple of years back I did a little research on DOT regulations regarding turn signal flash rates, but I don't recall seeing anything indicating that a failed bulb should cause the remaining bulbs to flash faster. Only that bulbs should flash between 60 - 120 flashes per minute. Could you cite a reference?
I have a bike and put leds in the front turn signals and tail light. You can just put power resistor in parallel with the LEDs to simulate the full load to the flasher. Cheap and dirty.
My goal was to save power the alternator only puts out 10 amps on my bike and 5 goes to the headlight, the rest supports the ignition and running lights. Using LEDs cut about 2.5 amps load to 600 milliamps total - turn and tail light.
The auto parts place wanted $17+ for the electronic flasher. I opened the box and through the smoked plastic cover there was a large electrolytic, small TO92 device, a few two wire parts like resistors or diodes, and a large relay with very fine wire.
I built my own for ~$2. One bipolar transistor, one zener, one mosfet, small electrolytic a few resistors - no relays.
Works like a champ and can switch between ~200 milliamps to 20 amps, between 11 to 15 volts supply, with very little variation in rate. I potted it in epoxy in a small plastic case.
By now you've probably solved your problem. But if you want to roll your own I can post the schematic on the binary group or email it.
My design is for a two wire flasher - should work in any application. My bike was wired for a two or three wire flasher. Two wire design was fun because the flasher is essentially shorting out its own power supply when the lamps are on.
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There are various standards applying to vehicle turn flashers and lamps. The CEC catalog refers to the appropriate standards applying to each of their flasher products. SAE J590 appears to be the original flasher standard.
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A list of standards applying to lighting on passenger vehicles is here
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so you can Google for those which you think might apply.
I notice all the common automotive flashers they are selling meet the SAE 590 standard whether heavy duty, or led or whatever, including the one that is supposed to be able to flash one to twenty lamps.
I'm guessing if you have 20 lamps on the same flasher and one burns out, there will be no change in the flash rate. Although, it could mean that if all the lamps burn out you'd be alerted, presumably the small indicator lamp in the dash won't be enough to keep it toggling - and some of those light during the off period of the turn signal.
The common thread on the product list is the 60 -120 flashes per minute (no mention of on versus off period - but that may be in the standard)
Scrolling down from the multi-led lamp flashers they show some fixed load flashers that are supposed to alter the flash rate when a lamp burns out - they also meet the 590 standard
Found it for $59; decided I didn't want it enough.
Both of these meet the same SAE J590 standard - one is designed to alert the operator of a lamp outage the other isn't. Lamp outage must Not be a requirement for turn signal flashers.
EF43 20 Lamp 3 Terminal Electronic Flasher
FMVSS 108 SAE J945, J590, and 1690 Class A
Heavy Duty Design Rated for 100 hours continuous operation.
3 Terminal Double Flash Rate Design for Lamp Outage
Flash Rate Electronically Timed
Fits all Standard Sockets found in newer import cars & trucks
11-15 volt, Fixed Load 2 lamps for turn signal / 4 lamps for hazard warning
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Thanks, after digging around some, I was able to find that FMVSS do aparently require some kind of turn signal "lamp outage" indication on at least some vehicles. Seems silly considering that there is apparently no warning required for failed brake lamps! Of course outage sensors aren't required on vehicles over 80" wide either, allot of common sense in that one too, huh? ;-)
I had primarily been looking for flash-rate and duty-cycle information before and didn't even think about that aspect of it. No biggy though, I can deal with it if necessary. Automotive high side drivers are chock full of handy features like that. :-)
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