Hey I just have a question about electrical circuit. This is for a headlight switch that controls high and low beam on a snowmobile. The system uses 12VAC to power and switches the high and low beam. The ground is always used and the button switches between high and low beam wires. I need to convert this 12AC to 12VDC in order to run an LED headlight.

I tried the diagram below and what happens is that DC current is produced but it is produced to both the high and low wires at double a voltage at the same time (24 VDC). Not 12VDC to either high/ground or low/ground.

When I just had one rectifier to the highbeam it worked fine at 12 V DC but I wanted to be able to have a high and low beam. Do you know if I need to diode or something to make 12 V DC at the high and ground or 12 V DC at the low and ground?

 

A basic problem with converting to DC is that one terminal of the alternator is almost certainly grounded to the vehicle chassis and so is one terminal of each lamp. Consider this circuit :-

For AC, the high and low beam non-grounded terminal would be connected to A1 and A2 respectively. The lamp holder would provide the ground connection.
For DC, using a full-wave rectifier, each lamp would connect between B and C. Note that C is no longer at ground potential, so the usual lamp holder would short out D4.
Will all your LED lamps have holders which are isolated from chassis ground?
Isn't Hi/Lo switching on a LED headlight normally done using PWM (pulse-width-modulation)?

 

A basic problem with converting to DC is that one terminal of the alternator is almost certainly grounded to the vehicle chassis and so is one terminal of each lamp. Consider this circuit :-
View attachment 110803
For AC, the high and low beam non-grounded terminal would be connected to A1 and A2 respectively. The lamp holder would provide the ground connection.
For DC, using a full-wave rectifier, each lamp would connect between B and C. Note that C is no longer at ground potential, so the usual lamp holder would short out D4.
Will all your LED lamps have holders which are isolated from chassis ground?
Isn't Hi/Lo switching on a LED headlight normally done using PWM (pulse-width-modulation)?

Hi. Thanks for your reply. I should have said that I bought a harness that plugs into the existing head light plug and switches to high and low with the existing switch.
The problem is that this light relies on DC power when my snowmobile produces AC power (no battery).

 

Can you post a link to the harness spec? I see a power input plug on the right, but no sign of how you connect the Hi/Lo switch?

 

Can you post a link to the harness spec? I see a power input plug on the right, but no sign of how you connect the Hi/Lo switch?

This the harness I built with the 2 rectifiers and I picture of how the wiring to the bulb/harness works.

 

Try 1/2 wave.
A diode in each wire. H and L.
Might end up with too much voltage full wave.

 

Try 1/2 wave.
A diode in each wire. H and L.
Might end up with too much voltage full wave.

Ok thanks. Would that be the same as keeping my existing setup with the 2 rectifiers and putting 4 diodes going to the rectifiers?

 

No, just to diodes.
Shouldn't hurt to try. Use standard headlamp to test.
Use a single diode from each bridge.
ie. either ac to +.

 

No, just to diodes.
Shouldn't hurt to try. Use standard headlamp to test.
Use a single diode from each bridge.
ie. either ac to +.
View attachment 110857

Thanks I will give that a try.

 

I already merged some of your two separated posts to be one with other member's replied and your new post.
When you click the 'reply' button then you can see the left side and right have 'QUOTE', you just write down your new reply on the next line as below.

'['QUOTE="Pedroz, post: 1032313, member: 369455" ']'Thanks I will give that a try.'['/QUOTE']'
You just write down your new reply from this line ......

 

View attachment 110773
Hey I just have a question about electrical circuit. This is for a headlight switch that controls high and low beam on a snowmobile. The system uses 12VAC to power and switches the high and low beam. The ground is always used and the button switches between high and low beam wires. I need to convert this 12AC to 12VDC in order to run an LED headlight.

I tried the diagram below and what happens is that DC current is produced but it is produced to both the high and low wires at double a voltage at the same time (24 VDC). Not 12VDC to either high/ground or low/ground.

When I just had one rectifier to the highbeam it worked fine at 12 V DC but I wanted to be able to have a high and low beam. Do you know if I need to diode or something to make 12 V DC at the high and ground or 12 V DC at the low and ground?

You can get "corn cob" LED H4 replacements, but they need DC, and I'm not sure if you can get 24V ones.

It would be easier to sort out the AC before it reaches the dip switch.

The standard corn cob H4 is only 7W, so it might be worth looking for a battery you can take the supply from. Using the AC feed means rectifiers, smoothing capacitors (or a battery) and regulators.

The old Brit motorcycles used a 100W 15V Zener - if you need 24V; just hook 2 in series - but each needs a finned heat sink in the airflow.

 

You can get "corn cob" LED H4 replacements, but they need DC, and I'm not sure if you can get 24V ones.

It would be easier to sort out the AC before it reaches the dip switch.

The standard corn cob H4 is only 7W, so it might be worth looking for a battery you can take the supply from. Using the AC feed means rectifiers, smoothing capacitors (or a battery) and regulators.

The old Brit motorcycles used a 100W 15V Zener - if you need 24V; just hook 2 in series - but each needs a finned heat sink in the airflow.

The led I got works from 8 - 24VDC. I will see if I can find a wiring schematic to see about getting DC to the switch. Thanks

 

The led I got works from 8 - 24VDC. I will see if I can find a wiring schematic to see about getting DC to the switch. Thanks

If its the AC lighting system typical of small motorcycles - there may not be much in the way of regulation! Off load it could probably deliver a dangerous shock at optimum RPM. Any regulation is likely to be crude - many motorcycles just switch in a shunt resistor when the main beam is off.

Given the example of a "corn cob" LED H4 taking only 7W - you're going to have to dump some power to regulate the voltage.

Simplest is to clamp the output from the bridge rectifier with a 100W 15V Zener - but probably not cheap. The 8 - 24V range opens options, you might be able to get a salvage power Zener without getting too spendy.

 

If its the AC lighting system typical of small motorcycles - there may not be much in the way of regulation! Off load it could probably deliver a dangerous shock at optimum RPM. Any regulation is likely to be crude - many motorcycles just switch in a shunt resistor when the main beam is off.

Given the example of a "corn cob" LED H4 taking only 7W - you're going to have to dump some power to regulate the voltage.

Simplest is to clamp the output from the bridge rectifier with a 100W 15V Zener - but probably not cheap. The 8 - 24V range opens options, you might be able to get a salvage power Zener without getting too spendy.

I tried a diode and the high and low lead at the bulb harness and I only got 6V output.

3 wires go to the headlight switch, so not sure how I will rectify the power to DC before it got there.

I guess I will just have to live with either high or low like I had it earlier (non-switchable).
I appreciate all the help and suggestions out there! Thanks

 

Hey, I’ve dealt with this exact issue on several sleds that run AC power to the headlights. You’re right — when both the high and low beam wires carry 12VAC, a standard rectifier setup can cause backfeed or weird voltage doubling if not isolated correctly.


To safely run an LED headlight on a snowmobile with AC power (especially when you want both high and low beam switching), you’ll need a plug-in rectifier at the headlight plug that:

• Converts 12VAC to 12VDC
• Has internal isolation so high/low don’t energize each other
• Includes a capacitor or mini PCB to stabilize the output and prevent flickering

I wouldn’t recommend trying to DIY this with just diodes unless you’re isolating each beam wire and building your own buffered circuit — it gets messy fast. You could try using two rectifiers (one per beam wire) with proper grounding and then feed both into a switching DC load, but again, that’s complex and not super trail-reliable.


I’ve seen plug-and-play options that solve this by handling the power conversion right at the bulb plug, no wiring needed.


If you’re unsure whether your sled runs AC or DC to the lights, here’s a guide that breaks it down with test steps and visuals:


https://ledpowersport.com/blogs/how...ile-has-ac-or-dc-power-before-installing-leds


Let me know what sled you’re working on — I might be able to help more based on the wiring (Cats and Polaris can be trickier than Ski-Doo). Or if you need direction on what a plug n play Snowmobile Headlight rectifier looks like, just reach out.

 

Since the only thing being changed is putting in an LED headlight that is polarity sensitive, why not simply add a series diode at the headlight to only allow the correct polarity?? Then if you need to switch back, the change is really simple. The benefit is no changing any of the wiring, adding connections that must be adequately protected and live with all of the vibration!! One simple adequately rated diode in series with each LED headlight.