ac to dc

twhitehousescat

brain fade has struck me


older motorcycle (moped) that uses a coil and rotating magnet to produce 6v AC to feed h/light and tail light ,

so far work done:


coil rewound and is capable of 6v at tickover rising to about 12v at full revs ALTERNATING CURRANT , a RG 6/12 ac regulator has been fitted , this as much the same as a zenner diode would be on a DC circuit, and basically "clamps" the voltage at a max of 6v AC

I now have a steady 6v ac , that does not increase with revs

the h.light on this bike is a massive 10w @ 6v and the tail light is 5w @ 6v there is no brake light or indicators fitted


I would like to install a brake light , , but I would like to use a modern led stop tail bulb , those will only work on DC

a guy here shows a simple circit , using a 4 terminal bridge diode , look here https://youtu.be/3qHJFvjwkI4

so my question , after explaining above is

6v ac into a bridge rectifier (plus extra diode and capacitor) , what voltage will the output be in DC

System

There is 0.7V drop across a silicone diode. At any one point during the rectification in a full wave rectifier (like the one in the video if it uses 4 diodes) two diodes are in play so you'll have a voltage drop of 1.4V. A half wave rectifier would only have a 0.7V drop however you'd notice a flickering in the light relative to the speed of the engine as when the AC input is negative into the rectifier there would be no voltage flowing at all unlike the full wave rectifier.

Also you have to know whether when measuring the AC voltage whether you're measuring peak or RMS. RMS is "the average" of an AC waveform and is 0.707 x the peak voltage. If your meter was measuring peak AC voltage so the peak voltage was 6V so the RMS was 4.24V and you're using a full wave rectifier you could actually see as little as 2.8VDC out of the rectifier.

Throw in an extra diode and you add an additional 0.7V drop. I personally can't see the advantage of doing so.

Depending on the value of the capacitor, if you use one of sufficient value you could get away with using a half wave rectifier and you'd only get flickering for a fraction of a second until the capacitor was charged if you had the lights turned on when you started the bike - if you turned them on after it had been running for a few seconds there shouldn't be any flicker.

twhitehousescat

ok , the AC regulator (rg6/12) holds the voltage at 6 v , no less , no more

coffeehound

Would it matter if the brake light flashed? Some high end motors have deliberately flashing brakes don’t they? The LED is a diode so may be happy to do its own half-wave rectifying; i’m not sure what electronics they include now. Failing that, a germanium rectifier would drop less volts than silicon.

twhitehousescat

coffeehound wrote: »

Would it matter if the brake light flashed? Some high end motors have deliberately flashing brakes don’t they? The LED is a diode so may be happy to do its own half-wave rectifying; i’m not sure what electronics they include now. Failing that, a germanium rectifier would drop less volts than silicon.

that might be acceptable , however a flashing tail light and a h/light like a strobe are not acceptable

from the first answer it woulds seem that my 6 v would reduce by about 1 - 1.5 v , this id not acceptable

Heedtheadvice

You really have asked the wrong question IMHO !
What you ought to as is 'given the required working voltage of the led lights how can that be achieved?' !
Led lights can be basic LEDs with a low voltage (around 1 to 2 volts range);
or those designed to work at specific supply voltages such as 6v or @12v (nominal) for vehicles i.e. with a built in resistor to reduce the voltage from supply level to LED level...or a built in driver circuit.


So, given the spec for the led lights we can design a circuit to produce that specified voltage. Current drawn by the lights will nearly always be small and can be ignored at this stage!


What voltage requirement are those you wish to fit?


A note on recifiers to convert the ac to dc.

There are several methods to do this and sometimes the full bridge rectifier is the best way ahead (even accounting for the two diode losses) as they make use of positive and negative halves of the ac cycle to give a higher average dc voltage.
There are also diodes that produce a smaller voltage drop if that really becomes significant and other ways to increase the voltage.

A_Nice_Englishman

You can get 6v ac LED stop lamp bulbs apparently eg.

https://www.classiccarleds.co.uk/collections/6-volt-collection/products/2-x-6v-6-volt-red-led-brake-light-glb382-ba15s-21w-positive-earth-negative-earth

System

coffeehound wrote: »

Would it matter if the brake light flashed?

Yes, its illegal. All lights other than direction indicators must be a steady light.

Some high end motors have deliberately flashing brakes don’t they?

Only to indicate emergency braking.

A_Nice_Englishman wrote: »

You can get 6v ac LED stop lamp bulbs apparently eg.

https://www.classiccarleds.co.uk/collections/6-volt-collection/products/2-x-6v-6-volt-red-led-brake-light-glb382-ba15s-21w-positive-earth-negative-earth

Sounds the job. All the required circuitry will be built into the bulb assembly.

that

Random collection of thoughts

it is the peak voltage you have to worry about.

Does the led panel have its own voltage controlling circuit - never seen a naked 6v led

Most generators/alternator/magneto are 3 phase, three times more peaks, much smoother voltage source, more efficient, far less flicker

the generator often goes to the voltage controller which powers the battery, most electrics go via the battery - it acts as a reservoir?

Even when the bike is not running (stalled) the break light still works - mine did, thanks to the battery

Shottkey diodes give a much lower volt drop

All leds generally work better with constant current, rather than a fixed or constant voltage

The question is the cable too small to pull my load? Without knowing the specs of the display, it is hard to know what to feed it. Suspect it will have its own power controlling circuit to make up for voltage variations?

10w + 5w + 10w stop light + ?w indicator lights + coil. how much power can that generator deliver?

coffeehound

^ it’s a moped, so no battery or DC

twhitehousescat

that wrote: »

Random collection of thoughts

it is the peak voltage you have to worry about.

Does the led panel have its own voltage controlling circuit - never seen a naked 6v led

Most generators/alternator/magneto are 3 phase, three times more peaks, much smoother voltage source, more efficient, far less flicker

the generator often goes to the voltage controller which powers the battery, most electrics go via the battery - it acts as a reservoir?

Even when the bike is not running (stalled) the break light still works - mine did, thanks to the battery

Shottkey diodes give a much lower volt drop

All leds generally work better with constant current, rather than a fixed or constant voltage

The question is the cable too small to pull my load? Without knowing the specs of the display, it is hard to know what to feed it. Suspect it will have its own power controlling circuit to make up for voltage variations?

10w + 5w + 10w stop light + ?w indicator lights + coil. how much power can that generator deliver?

Does the led panel have its own voltage controlling circuit - never seen a naked 6v led

yes the bulb(led) is designed to operate at 6v

Most generators/alternator/magneto are 3 phase, three times more peaks, much smoother voltage source, more efficient, far less flicker


umm its a simple moped with a single coil , one end idd earthed to frame , the other goes to a switch then a single fil h/light bulb and a tail bulb

the generator often goes to the voltage controller which powers the battery, most electrics go via the battery - it acts as a reservoir?

no its a single coil with no regulation , there is NO DC on the bike NO battery


10w + 5w + 10w stop light + ?w indicator lights + coil. how much power can that generator deliver?

there are no indicator lights , in fact no speedo , the coil is fed from another internal coil and has no beatings on the calculations , the coil (on a good day ) will give approx 15w , therefore removing the 5 w filiment bulb (tail light) and replacing it with a combined stop /tail in LED would be good