Motorcycle Headlights Question

Thread Starter

ChrisTsall

Joined May 16, 2018
40
Hello everyone,

I got an old motorcycle (Sachs 125cc) and I need to fix my front headlights, because their lighting isn't powerful enough. I am not using a battery for them and they only work with the dynamo. As a result when I hit the gas the headlights shine enough and I want to keep that light consistently. The problem is that I can' keep that light steady because, I don't hit the gas for ever, as a result the light's level is going down. I have attached a photo with the dynamo and the yellow cable is the one that goes to a switch and from the switch to the front headlights. Is there any module that I can attach in series to that yellow cable in order to keep the voltage constant at 6 volts ? (I am not using a battery for the front lights)

Thank you so much for your time, appreciate your help.

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Thread Starter

ChrisTsall

Joined May 16, 2018
40
You cannot raise the power coming from the dynamo. What voltage do you read at at idle with the headlight off and on?

A more efficient (LED) headlight might be your best bet.

Bob
Hello sir,

dynamo outputs 0 to 6 peak voltage. I want to keep 6 voltage steady... so with what you are saying I guess its not possible with the dynamo and I am going to need a battery.

Thanks for your help!
 

crutschow

Joined Mar 14, 2008
29,466
If the dynamo can output enough current you might be able to use a DC-DC boost switching converter to maintain a steady 6V at the bulbs.
How much current do the headlights take?
 

Thread Starter

ChrisTsall

Joined May 16, 2018
40
If the dynamo can output enough current you might be able to use a DC-DC boost switching converter to maintain a steady 6V at the bulbs.
How much current do the headlights take?
I haven't measured but I can use up to 35W bulb. So a DC-DC boost switching could work?
Thanks!
 

BobTPH

Joined Jun 5, 2013
4,738
Only if the dynamo can produce extra current. I expect it cannot produce 35W at idle. And if it did, the light would probably burn out when racing the motor.

Bob
 

BobTPH

Joined Jun 5, 2013
4,738
My post is about what I think is happening without the boost converter. I don’t think it produces 35W at idle. I think it is a simple design that puts the full output of the dynamo into the light at whatever speed it is running. The dynamo seems to have a separate winding for each thing it powers.

Bob
 

PeteHL

Joined Dec 17, 2014
390
The least expensive solution to your problem I think might be converting to a LED headlight (suggested by Bob TPH) and using a gel cell battery to power the headlight. Possibly it wouldn't be too difficult to make a battery charger drawing voltage from the dynamo of your motorcycle. Alternatively you could periodically charge the battery from a charger running off of house current. A LED is many times more efficient at converting electrical power to light than incandescent. So converting to a LED headlight would substantially reduce the size of battery needed to produce the light intensity that you desire.
 

MisterBill2

Joined Jan 23, 2018
11,556
The solution to the variable light that is seldom enough is a battery of adequate capacity, and then getting a greater dynamooutput voltage to be able to recharge the battery. Not simple, but effective.
 

Tonyr1084

Joined Sep 24, 2015
6,861
35 watts at full engine speed means the dynamo is producing 5.8 amps of electrical current. That's not a small dynamo. If you convert your headlights to LED type, depending on their amperage draw, lets assume it's 80% more efficient, meaning it's achieving the same level of light output at 20%, then instead of drawing 5.8 amps the LED should draw 1.2 amps. Under that circumstance you might find a buck-boost converter that can deliver close to the desired results. But it all comes down to how much power the dynamo is producing at an idle. You're going to have to take measurements.

Measure the headlight voltage output circuit at idle and at full speed WITHOUT the headlight being on, then measure it again WITH the headlight on. Give us the numbers and one of the wizards here will be able to decipher the rest of the capabilities. It's beyond me - so I can't help more. But assuming 35 watts at full (6V) power should indicate a 5.8 amp dynamo output.
 

MisterBill2

Joined Jan 23, 2018
11,556
Now another question. Every one of the wires has a voltage and resistance measurement, but we are not given the reference point. So is each resistance and voltage relative to some common frame connection? Or in reference to a common wire that is not shown in the picture? One terminal measurements are of only marginal use in most instances. Are all of the connections in use? It might be possible to use power from more than one winding to power the light, but certainly it would not be a simple parallel connection. These individual windings may be on separate dynamo poles,delivering voltage at separate angles of flywheel rotation.
And so it might even be useful to use a diode and capacitor circuit to capture the peak voltage of the yellow wire and have the capacitor store that peak. With that action it might power an LED headlight very well.
 

crutschow

Joined Mar 14, 2008
29,466
My suggestion was for a DC circuit, but this appears to be an AC system, so the LEDs would likely need a diode to block the reverse voltage.
Alternately, isolating the LEDs and powering them from a bridge rectifier would be more efficient.
 

MisterBill2

Joined Jan 23, 2018
11,556
Indeed, probably it is an AC system, and probably one side of the light feed circuit is frame ground, as is one side of the magneto circuit, since we se no common wire. So a bridge will not be usable. Likewise a voltage doubler would not be usable. A half wave rectifier will work as probably te pulses have a higher peak voltage.
 

PeteHL

Joined Dec 17, 2014
390
It is all a matter of the amount of time, expertise, and money that the thread starter has. A regulator for the LED headlight must be a good one, as a LED is very quickly destroyed by over-current. Another factor is the length of time of the typical trip on the bike. If the cost of a battery that wouldn't discharge too much during the trip is not too high, then recharging the battery at the end of the trip with a commercial charger powered by house current might be the best solution.

Certainly powering a LED headlight by the dynamo of the bike is the more elegant solution.
 

MisterBill2

Joined Jan 23, 2018
11,556
No need for a regulator, just use the diode and the capacitor, and then a suitable resistor to set the current. LEDs are durable withing their ratingsand so a properly selected resistor will be all that is needed. If excess voltage is an issue, then a zener diode as is already used in English bikes is an adequate method of voltage limiting.
 

Thread Starter

ChrisTsall

Joined May 16, 2018
40
My post is about what I think is happening without the boost converter. I don’t think it produces 35W at idle. I think it is a simple design that puts the full output of the dynamo into the light at whatever speed it is running. The dynamo seems to have a separate winding for each thing it powers.

Bob
That's the logic of the circuit, I am pretty sure!
 

Thread Starter

ChrisTsall

Joined May 16, 2018
40
The least expensive solution to your problem I think might be converting to a LED headlight (suggested by Bob TPH) and using a gel cell battery to power the headlight. Possibly it wouldn't be too difficult to make a battery charger drawing voltage from the dynamo of your motorcycle. Alternatively you could periodically charge the battery from a charger running off of house current. A LED is many times more efficient at converting electrical power to light than incandescent. So converting to a LED headlight would substantially reduce the size of battery needed to produce the light intensity that you desire.
Appreciate your help.
 

Thread Starter

ChrisTsall

Joined May 16, 2018
40
No need for a regulator, just use the diode and the capacitor, and then a suitable resistor to set the current. LEDs are durable withing their ratingsand so a properly selected resistor will be all that is needed. If excess voltage is an issue, then a zener diode as is already used in English bikes is an adequate method of voltage limiting.
The circuit that you are talking about is going to be in series with the yellow cable which goes to the light bulb ? Thank you so much !
 
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