How to Power DC LED Lights on Yamaha YZ250 With Only AC Output and No Battery?

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#1 21683681 26 Dec 2019 05:50

Silverfox Racing Silverfox Racing

Anonymous

Post #1
21683681 26 Dec 2019 05:50

Hi All and merry xmas. Be gentle as I am a knob at electronics!!!! I am tring to rig up LED lights on an MX Yamaha YZ250 motorcycle. There is no DC current only AC which goes to the coil ignition, I am assuming it puts out around 1-500 volts generally but has very little current. I tried tapping into this power source and fitting a bridge rectifier. I can get the LED lights to work, but loose spark to the engine when it is hooked up. I am assuming it is the earth wire causing the issue as when I dissconnect that, spark returns to the engine and the led stop working. What do I need, or how do I get the lights to work? I really do not want to fit a battery. Anybody point me to what I need and how to hook it up. I have seen it done before just not sure how they did it. Thanks
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#2 21683682 27 Dec 2019 11:52

David Ashton David Ashton

Anonymous

Post #2
21683682 27 Dec 2019 11:52

The voltage going to the coil ignition will be high voltage and no use to you. Your LEDs would load it down too much hence you lose spark. Here is a wiring diagram:
You might get some better voltage out of the Brown and Red wires coming out of the magneto, they will probably also be AC but probably at a voltage that's easier to use. Can you measure it? It might be other colours but you'll have to poke around with a meter. As a rule, the thicker the wire the more power you'll be able to get out of it.You would need to rectify and smooth it and probably regulate it to 12V or so to use it for your LEDS. Does the bike have any lights? if so find out where they get their power from and use that.
#3 21683683 27 Dec 2019 13:59

Silverfox Racing Silverfox Racing

Anonymous

Post #3
21683683 27 Dec 2019 13:59

I have a white, brown, and black wires coming out of the magneto. 87 YZ250. T model. No lights on this model bike.I have hooked up a bridge rectifier to the Brown wire out of the magneto. If I T off the wire into the B/R and put the other side to earth I lose spark to the engine. but the lights work. If I Disconnect the earth, and I lose the LED's I have now spark. I then went in and out of the B/R with the brown wire and get spark and lights!!! Pulsating lights but that does not bother me. Will I lose any spark power this way ? as in will the bridge rectifier restrict it in any way? Thanks
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#4 21683684 28 Dec 2019 07:14

David Ashton David Ashton

Anonymous

Post #4
21683684 28 Dec 2019 07:14

Well that's usual - no two models are the same! You have done well with your experimenting though and I'd say if you can get your LEDs to light without losing spark then you shouldn't have any problems - LEDs don't draw much power. if you put a Capacitor (1000 uF or so) across the +/- outputs of your bridge rectifier you should eliminate the pulsing - but first check the voltage you are getting and get a capacitor rated at least 2 x that voltage, also check that the resulting voltage is not too high for your LEDs.
#5 21683685 28 Dec 2019 16:09

Silverfox Racing Silverfox Racing

Anonymous

Post #5
21683685 28 Dec 2019 16:09

Cheers. are any bridge Rectifiers better than others? Need it to be reliable
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#6 21683686 29 Dec 2019 09:59

David Ashton David Ashton

Anonymous

Post #6
21683686 29 Dec 2019 09:59

Well I'm assuming that you will not be drawing more than 1A for your LEDs - if so any of the common small round or square bridge rectifiers (as below) will be fine, they are all rated at least 1 Amp. If you have a lot of LEDs or a bright one, get one that's rated more than that, say 2A. 2A at 12v is 24W which is a lot for a LED.
#7 21683687 29 Dec 2019 12:22

Silverfox Racing Silverfox Racing

Anonymous

Post #7
21683687 29 Dec 2019 12:22

Using this atm, https://www.ebay.com.au/itm/2X-Bridge-Rectifier-50A-1000V-GBPC5010-AC-to-DC-G9W7/283625138725?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649
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#8 21683688 30 Dec 2019 03:58

David Ashton David Ashton

Anonymous

Post #8
21683688 30 Dec 2019 03:58

That's way overkill, it can handle 50 Amps but you wouldn't need more than 1 or 2 Amps. But if it works for you, no problem. If you need something smaller for space reasons, go for a smaller type as above.
#9 21683689 02 Jan 2020 17:03

Silverfox Racing Silverfox Racing

Anonymous

Post #9
21683689 02 Jan 2020 17:03

Cheers for your help David
#10 21683690 10 Jan 2020 04:50

Eric Richards Eric Richards

Anonymous

Post #10
21683690 10 Jan 2020 04:50

When you say "I have seen it done before" what was the source of that information ? if it came from youtube, a lot of those are a joke. Like one I saw they got a emergency plug in LED that has a non-replaceable internal battery inside manufactured for emergency lighting, when they short out the LED terminals with a coil of wire the LED lights up, it makes out the power is coming out of the coil of wire shorting the LED terminals.
#11 21683691 10 Jan 2020 18:05

Silverfox Racing Silverfox Racing

Anonymous

Post #11
21683691 10 Jan 2020 18:05

Hi. You can buy a Kit to do it, but did not want to spend the $300. I have it working now so all is good. Only thing is they run all the time the engine is running as you cannot break the circuit or earth anything, as you lose spark to the engine. Got an idea in mind to fix that though. Cheers
#12 21683692 16 Jan 2020 12:14

Emma wilson Emma wilson

Anonymous

Post #12
21683692 16 Jan 2020 12:14

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Topic summary

✨ A Yamaha YZ250 motocross bike (1987 T model) lacks a battery and provides only AC output from the magneto for ignition and lighting. The user attempted to power DC LED lights by tapping the AC output and using a bridge rectifier, but connecting the LED circuit caused loss of engine spark, likely due to grounding issues. The ignition coil output is high voltage but unsuitable for powering LEDs directly as it draws excessive current and disrupts spark. Alternative magneto wires (brown, red, white) may provide lower voltage AC more suitable for rectification and LED use. Adding a bridge rectifier with a smoothing capacitor (e.g., 1000 µF rated for twice the voltage) can convert and stabilize the AC to DC for LEDs, reducing flicker. Bridge rectifiers rated for at least 1–2 A are sufficient for typical LED loads; the user’s 50 A rectifier is overkill but functional. The LED circuit must not interfere with the ignition earth to maintain spark. Commercial kits exist but are costly. The LEDs run continuously with the engine as breaking the circuit or earth disrupts ignition. The solution involves careful selection of magneto output wires, proper rectification, smoothing, and ensuring the LED load does not affect ignition grounding.

FAQ

TL;DR: Power YZ250 LEDs by tapping the stator’s lighting lead, then rectify, smooth, and regulate to ~12 V; “2A at 12V is 24W.” [Elektroda, Anonymous, post #21683686]

Why it matters: Riders without a battery need a simple, reliable way to run DC LED lights from an AC-only dirt bike.

Quick Facts

Use the magneto’s brown/red leads for usable AC, then rectify, smooth, and regulate to ~12 V DC. [Elektroda, Anonymous, post #21683682]
Add ~1000 µF across bridge outputs; pick a capacitor rated at least 2× your measured voltage. [Elektroda, Anonymous, post #21683684]
Typical LED setups draw ≤1–2 A; size the bridge rectifier accordingly. [Elektroda, Anonymous, post #21683686]
A 50 A GBPC5010 bridge is overkill for LEDs but will still work if space allows. [Elektroda, Anonymous, post #21683688]
Grounding the rectifier to the ignition path can kill spark; keep the circuits isolated. [Elektroda, Anonymous, post #21683683]

Can I power DC LED lights on a Yamaha YZ250 that only outputs AC?

Yes. Tap the stator’s lighting lead (brown/red), feed a bridge rectifier, smooth with a capacitor, and regulate to about 12 V for LEDs. Avoid the high-voltage ignition wire. [Elektroda, Anonymous, post #21683682]

Why did I lose spark when I grounded the rectifier?

You tied the LED circuit into the ignition path. Grounding or loading that path removed spark. Keep the lighting tap isolated from the ignition and route it only through your rectifier and DC side. “Edge case: earthing the tap killed spark in testing.” [Elektroda, Anonymous, post #21683683]

Which wires should I tap from the magneto/stator?

Use the thicker lighting leads, commonly brown/red on this bike. They carry usable AC power. Confirm with a multimeter before wiring. If your loom colors differ, identify by measurement, not color alone. [Elektroda, Anonymous, post #21683682]

Do I need a battery to run LED lights on this YZ250?

No. A forum user reported successful LED operation without a battery by rectifying the stator output. The lights ran whenever the engine ran. [Elektroda, Anonymous, post #21683691]

How do I cut the LED flicker after rectifying AC?

Place about a 1000 µF electrolytic across the bridge rectifier’s + and − outputs. Choose a voltage rating at least 2× your measured DC. Verify the resulting voltage suits your LEDs. [Elektroda, Anonymous, post #21683684]

What size bridge rectifier should I choose?

For typical bike LEDs, select a 1–2 A bridge. As noted, 2 A at 12 V equals 24 W, which is plenty for many LED setups. “2A at 12V is 24W.” [Elektroda, Anonymous, post #21683686]

Is a 50 A GBPC5010 bridge rectifier okay to use?

Yes, it works but is excessive for LED loads. It’s physically larger and unnecessary unless you need its form factor. A smaller 1–2 A unit is adequate. [Elektroda, Anonymous, post #21683688]

Will adding a bridge rectifier reduce engine spark strength?

Not when wired off the lighting lead and isolated from ignition. A user ran LEDs with spark intact. “LEDs don’t draw much power.” [Elektroda, Anonymous, post #21683684]

My LEDs stay on whenever the engine runs. Can I switch them off?

In the reported build, breaking the tapped line or earthing it killed spark, so the lights stayed on. Plan switching on the DC side only, leaving the stator-rectifier path intact. [Elektroda, Anonymous, post #21683691]

How do I wire this in three steps?

Identify the lighting lead (brown/red) from the magneto with a meter.
Feed that AC into a bridge rectifier and add smoothing.
Regulate to ~12 V DC and connect your LED light. [Elektroda, Anonymous, post #21683682]

What capacitor voltage rating should I use?

Measure your DC after rectification, then choose a capacitor rated at least double that voltage. This provides headroom and reliability under rev changes. [Elektroda, Anonymous, post #21683684]

How much power can I expect for LEDs from this setup?

Plan around 1–2 A at ~12 V for common LED lights. That’s up to about 24 W available for lighting on many builds. Size components to that current. [Elektroda, Anonymous, post #21683686]

Are YouTube hacks showing coil-powered LEDs trustworthy?

Be cautious. Some videos hide internal batteries in emergency lights, misleading viewers about coil output. Verify with proper wiring and measurement. [Elektroda, Anonymous, post #21683690]

Can I buy a ready-made kit instead of DIY?

Yes. Kits exist around this mod, but one user avoided a roughly $300 option and succeeded DIY with rectification and smoothing. [Elektroda, Anonymous, post #21683691]

How do I find the most powerful stator lead without a diagram?

Use a multimeter to identify AC outputs and note that thicker wires generally carry more power. Confirm voltage before rectifying and regulating. [Elektroda, Anonymous, post #21683682]