Hitchcocks Forum

hi again another tech help needed I am thinking of adding led daylight running lights to my thunderbird 350 as I have no idea of the alternator output what would be a safe enough wattage for running lights along with my main headlamp

Hi, I am not sure how the electrics operate on your machine, but I can tell you that led's wont work on AC current, only DC. Regards, Paul.

Hi Paul...LED lights will work on AC...they will act as diodes and conduct on just one half of the phase so they will light up on this phase and remain unlit on the other half. As the devices are solid state there is no filament warm up time so the net result is they will look just as bright as long as the frequency is above the human eye response time or you will perceive a flickering. Many LED devices (calculators, TV screens etc.) strobe the display to save power yet the brightness looks the same...On the Bullet, if the flickering is too profound then all you need is a full wave (4-wire) bridge rectifier something like 1A will be sufficient although you might as well buy a 5A type as the cost is only about a pound...if this is wired in before the headlamp socket then the rectified voltage will drive the LED on both cycles and should a tiny amount of flicker still exist, then a small electrolytic capacitor (50V 470uF or similar)can be wired across the rectifiers output to smooth the voltage. If the bridge rectifier is used then polarities have to be observed and matched but otherwise the raw AC supply needs no special polarity check.....Les

Just read fishermanferg's question...He seems to be saying the LEDS will be used as supplement to the headlamp so it seems as if he will be driving the LEDS from the sidelight circuit which is battery fed and DC....Well if this is so no problem with AC (not as if there is one anyway!) the current consumption or watts drawn by the LED's is very small so no worry about total drain on system especially if the sidelights are disconnected as these draw more than the LED device will.......les

Hi Les, I thought I was speaking from experience, as recently I was asked to sort out a tail light unit on a small Yamaha, which would not work since the owner fitted led's. It had an AC lighting circuit and I soon discovered the wiring to the tail light was live. Putting 'ordinary' bulbs in fixed the 'problem' and the light worked once more, leading me to believe led's needed DC current, although I am no electrical expert - I am much more comfortable with the spanners! Regards, Paul.

Hi Paul...nice to speak with you again.... Yes strange the Yamaha, but I wonder if the small tail lamp was still driven by DC and the polarity was wrong for the LED bulb you tried?...If this wasn't the case, maybe the LED devices have a voltage control chip within them to control the drive voltage to them, but I've always assumed they have a simple series dropper resistor...reason why they need some voltage control is that the LEDS depending on either red or white need something like 1.5V or 3V maximum drive voltage and some current limiting device to stop them blowing. If they use a chip then the low voltage at low revs on an AC system might be too low to activate this so they remain unlit until this voltage is exceeded when it approaches the rated 12V rating for the device....so maybe a 6V type would have operated earlier or better?...If this is the case then using an LED in the headlight might cause the same problem, causing the headlight to extinguish below a certain rev range until nearer the full 12V was being generated by the alternator and received by the LED....Interesting....never used a white LED or red LED AUTO bulb so never looked into the characteristics for them...will try to find out more, after I get that damn sprocket off...All the best Paul.....Les

Just an extra thought..... but cannot find any tech info to back this up.....IF the LED bulbs DO have chips in them to control the voltage but they are not "reversed polarity" protected with a diode, then connecting to AC will almost certainly destroy them...IF ....repeat... IF this is the case, then simply use the small bridge rectifier and capacitor I mentioned to drive them, they should work normally then.....But you still might have the situation that they extinguish completely at lowish engine speeds....would be an easy experiment to carry out....Les

Light emitting Diodes do indeed rectify AC to DC, but you get half-wave rectification, half the wave is wasted. So unless the LED is over twice as bright as the incandescent bulb, no net gain. There is no real need to smooth a rectifier's output unless you are operating electronics (hifi amplifiers and such), most bike electronics that need smoothed DC have their own smoothing built in. Adding a full-wave rectifier just for a couple of LED DRL seems like a sledgehammer to crack a nut, expensive and complicated solution. If LED are supplied by DC and the polarity is wrong, they won't work. If they're supplied by AC, you only get half their potential output.

Once again jefrs...so many things you say are wrong...1) since when is 2 quid expensive?....2) since when is connecting a couple of wires up complicated? 3)The human brain perceives a flashing light once its above a certain frequency as the same brightness of a non flashing light so power is saved for the same brightness so no halving of brightness....4) How would you remedy the problem of driving an LED with AC then?....pleased don't cop out by saying "you wouldn't"....please explain YOUR much better method.....Thank you...Any further reply by me might have to be some time off I'm dead busy ....so must go now.....Les

This unfortunately is not as straightforward as one may think, Les H is in fact correct that an led will in most cases light up if supplied with an AC voltage, as he says they are in fact "Light Emitting Diodes" by the nature of being diodes they will conduct on one polarity and block the reverse polarity, the problem with them is that unlike most diode's they are very sensitive to voltage and will only light when a threshold voltage has been reached and will be destroyed very soon after if the voltage continues to rise, an AC waveform from an alternator is a sine wave and constantly ranges from 12v negative through zero volts to 12v positive ( I only use 12v as a figure this will vary quite a lot from one alternator to another).

Most LEDs work on around 1.7 to 2.2 volts connecting them directly to 12v will in most cases destroy them, for this reason and because vehicle voltages vary from around 12.6 volts with the engine off and 14.2 volts with it running vehicle LED lights have a built in electronic regulator, therefore as Paul says they will not work on AC.