Reducing 20v ac to 6v ac

[budgie955]

I have an old 175 trail bike , the 6v light run direct from ac generator, however its putting out 20v ac, and blowing the bulb...rather than having the stator rebuilt , I wish to put in a reducer to drop the voltage..I believe a resistor will suffice, but what size ?
The headlamp is 25watt and tail light is 5watt
Thanks

 

[Avo Mk8]

I have an old 175 trail bike , the 6v light run direct from ac generator, however its putting out 20v ac, and blowing the bulb...rather than having the stator rebuilt , I wish to put in a reducer to drop the voltage..I believe a resistor will suffice, but what size ?
The headlamp is 25watt and tail light is 5watt
Thanks

Doing it with a resistor means having to get rid of a lot of heat, more than twice the amount that the bulbs produce.
If your two bulbs can stay in parallel all the time, they would be equivalent to a 30W 6V lamp, which means 5A would flow.
The resistor would need to drop 20-6 = 14 Volts, which at 5 Amps equates to 70 Watts. The resistor value would be 14/5= 2.8 ohms. Nearest preferred resistor value is 2.7 ohm
So you would need a 2.7 ohm, I suggest 100W resistor. To get rid of the heat you would have to bolt it to a chunky bit of the bike, or a big heat sink bolted to the bike (and use thermal paste/heat sink compound)
I am dubious that the 'ac generator' you have would produce 20V at 5A. You might burn it out if it's not rated for 5A
Can you fit 24 Volt bulbs instead?

I'm not sure if this is practical, but it might be possible to put a bridge rectifier on the generator output, prob with a capacitor, and use the resultant dc to run a 'buck converter' to get the 6V at 5A. That would be much mote efficient.
Something like this, but it must be run from DC!

https://www.amazon.co.uk/Blackr-Converter-Voltage-Changer-Regulator/dp/B07LGZLBBV/ref=asc_df_B07LGZLBBV/?

 

[budgie955]

Doing it with a resistor means having to get rid of a lot of heat, more than twice the amount that the bulbs produce.
If your two bulbs can stay in parallel all the time, they would be equivalent to a 30W 6V lamp, which means 5A would flow.
The resistor would need to drop 20-6 = 14 Volts, which at 5 Amps equates to 70 Watts. The resistor value would be 14/5= 2.8 ohms. Nearest preferred resistor value is 2.7 ohm
So you would need a 2.7 ohm, I suggest 100W resistor. To get rid of the heat you would have to bolt it to a chunky bit of the bike, or a big heat sink bolted to the bike (and use thermal paste/heat sink compound)
I am dubious that the 'ac generator' you have would produce 20V at 5A. You might burn it out if it's not rated for 5A
Can you fit 24 Volt bulbs instead?

I'm not sure if this is practical, but it might be possible to put a bridge rectifier on the generator output, prob with a capacitor, and use the resultant dc to run a 'buck converter' to get the 6V at 5A. That would be much mote efficient.
Something like this, but it must be run from DC!

https://www.amazon.co.uk/Blackr-Converter-Voltage-Changer-Regulator/dp/B07LGZLBBV/ref=asc_df_B07LGZLBBV/?

View attachment 118638

Thanks for the info..I can mount it on an alloy plate to dissipate the heat....for the money it's worth trying as a rewind on the stator is over 100 pounds
Great advice there
Many thanks

 

[Avo Mk8]

Thanks for the info..I can mount it on an alloy plate to dissipate the heat....for the money it's worth trying as a rewind on the stator is over 100 pounds
Great advice there
Many thanks

But you need to be sure the alternator/generator you have can deliver 5 Amps continuous, otherwise you will be rewinding it anyway!
Here's another option that should convert the 20V ac to your required 6V (dc). This would take much less current from the alternator than using the resistor. Might be worth a try! You would need a multimeter to set the voltage.
An advantage would be it would give you the 6V with one bulb or two, rather than being essential that both bulbs are on. And with the resistor method, if the headlamp fails, the rear 5W bulb will blow immediately!

search for the below

 

[mainline]

I have an old 175 trail bike , the 6v light run direct from ac generator, however its putting out 20v ac, and blowing the bulb...rather than having the stator rebuilt , I wish to put in a reducer to drop the voltage..I believe a resistor will suffice, but what size ?
The headlamp is 25watt and tail light is 5watt
Thanks

Sounds as if the regulator is duff rather than the stator.

One of these should do it,

6 Volt Regulator & Rectifier Repair Kit Honda | Suzuki | Kawasaki | Yamaha

6 volt repair kit regulator and rectifier with opposite terminals for systems with separate regulator and two wire rectifier. DT, SP, RXS100, DR, XT500

rexs-speedshop.com

 

[budgie955]

Sounds as if the regulator is duff rather than the stator.

One of these should do it,

6 Volt Regulator & Rectifier Repair Kit Honda | Suzuki | Kawasaki | Yamaha

6 volt repair kit regulator and rectifier with opposite terminals for systems with separate regulator and two wire rectifier. DT, SP, RXS100, DR, XT500

rexs-speedshop.com

There is no regulator, the lights run ac from a single output

 

[budgie955]

Doing it with a resistor means having to get rid of a lot of heat, more than twice the amount that the bulbs produce.
If your two bulbs can stay in parallel all the time, they would be equivalent to a 30W 6V lamp, which means 5A would flow.
The resistor would need to drop 20-6 = 14 Volts, which at 5 Amps equates to 70 Watts. The resistor value would be 14/5= 2.8 ohms. Nearest preferred resistor value is 2.7 ohm
So you would need a 2.7 ohm, I suggest 100W resistor. To get rid of the heat you would have to bolt it to a chunky bit of the bike, or a big heat sink bolted to the bike (and use thermal paste/heat sink compound)
I am dubious that the 'ac generator' you have would produce 20V at 5A. You might burn it out if it's not rated for 5A
Can you fit 24 Volt bulbs instead?

I'm not sure if this is practical, but it might be possible to put a bridge rectifier on the generator output, prob with a capacitor, and use the resultant dc to run a 'buck converter' to get the 6V at 5A. That would be much mote efficient.
Something like this, but it must be run from DC!

https://www.amazon.co.uk/Blackr-Converter-Voltage-Changer-Regulator/dp/B07LGZLBBV/ref=asc_df_B07LGZLBBV/?

View attachment 118638

I've ordered the parts, but there is only 1 output from the ac generator, how would I wire in the bridge rectifier,? also there is no battery

Thanks

 

[Avo Mk8]

I've ordered the parts, but there is only 1 output from the ac generator,

When you say one output, do you mean one wire? If you're sure, then the other connection must be the metalwork of the generator, so assuming that's bolted soundly to the bike, take a wire from the chassis nearby.

how would I wire in the bridge rectifier,? also there is no battery

The module I mention (in post #4) has the bridge rectifier on board, and terminals for ac input, it obviously doesn't matter which way round your two wires go to the ac input terminals.

 

[Avo Mk8]

There is no regulator, the lights run ac from a single output

If that's the case, have you got the wrong voltage bulb ('cos it blows!), or is there a fault with the generator?
Or is it supposed to have a regulator which is missing?

 

[mainline]

There is no regulator, the lights run ac from a single output

The link I gave would do what you wanted, it's a regulator with a diode for battery charging if you had one.

6 volt AC regulator – for the headlamp and to stop bulbs blowing
Rectifier – Provides DC for battery charging

 

[budgie955]

If that's the case, have you got the wrong voltage bulb ('cos it blows!), or is there a fault with the generator?
Or is it supposed to have a regulator which is missing?

I'm guessing the generator has a fault, I'm trying to find a cheap way around the problem...its supposed to kick out 6v ac, 6volt bulbs are the correct fitment...going to try 24v bulbs,and see what happens

 

[mainline]

I'm guessing the generator has a fault, I'm trying to find a cheap way around the problem...its supposed to kick out 6v ac, 6volt bulbs are the correct fitment...going to try 24v bulbs,and see what happens

What will happen is you will have dim lights if any at idle.

 

[budgie955]

When you say one output, do you mean one wire? If you're sure, then the other connection must be the metalwork of the generator, so assuming that's bolted soundly to the bike, take a wire from the chassis nearby.

The module I mention has the bridge rectifier on board, and terminals for ac input, it obviously doesn't matter which way round your two wires go to the ac input terminals.

There are actually 2 outputs from the ac gen,one goes through a silicon rectifier and supplies the ignition circuit, the other is direct to the lighting circuit, the ignition works perfectly, the lighting circuit has the fault..
Thanks for the info

 

[budgie955]

Sounds as if the regulator is duff rather than the stator.

One of these should do it,

6 Volt Regulator & Rectifier Repair Kit Honda | Suzuki | Kawasaki | Yamaha

6 volt repair kit regulator and rectifier with opposite terminals for systems with separate regulator and two wire rectifier. DT, SP, RXS100, DR, XT500

rexs-speedshop.com

This is just the job...thanks.