Looking for a 12v full wave rectifier

[Pontiac787]

Hi All,

Can anyone recommend a full wave rectifier that will convert 12-20v AC to similar voltages in DC? There are a number listed on Amazon and eBay but they appear to 1/2 wave despite what the titles say. I'm going to be powering an LED head and taillight from a stand-alone magneto on a 2-stroke motorized bicycle engine.

I've found this guy. Rectifier Any reason it wouldn't work?

 

[rlitman]

Hi All,

Can anyone recommend a full wave rectifier that will convert 12-20v AC to similar voltages in DC? There are a number listed on Amazon and eBay but they appear to 1/2 wave despite what the titles say. I'm going to be powering an LED head and taillight from a stand-alone magneto on a 2-stroke motorized bicycle engine.

I've found this guy. Rectifier Any reason it wouldn't work?

I'm sure it will work, but it's incredibly bulky for what you need. Why do you think the rectifiers you find are 1/2 wave? If it has four leads/terminals, it is full wave (single phase). A 1/2 wave rectifier is just a diode.

Forget the voltage for a moment. What sort of current draw are you expecting for your lights?

My guess is that even this is overkill for you:
https://www.amazon.com/dp/B079KDL8Y5/?tag=atomicindus08-20

 

[exmaxima1]

 

[exmaxima1]

...............
My guess is that even this is overkill for you:
https://www.amazon.com/dp/B079KDL8Y5/?tag=atomicindus08-20

+1
Any of those metal-cased 35-50 amp bridge rectifiers would be ideal. They are easy to mount, require very little (or no) heatsinking for this application, and accept faston connectors. Go for it!

 

[dogdog]

Hi All,

Can anyone recommend a full wave rectifier that will convert 12-20v AC to similar voltages in DC? There are a number listed on Amazon and eBay but they appear to 1/2 wave despite what the titles say. I'm going to be powering an LED head and taillight from a stand-alone magneto on a 2-stroke motorized bicycle engine.

I've found this guy. Rectifier Any reason it wouldn't work?

Would be interesting and see how this is going...

My guess, you'll probably needed a more higher voltage spec "full-WaVe rectifier" magneto outputs very high voltages, but very low current. unless you are mounting a second magneto, not sure if it has enough AMP to drive a decent bright LED. You'll probably needed the proper caps too plus some sort of right ranged zener diode as well to trim the voltage.


maybe figure out how to mount one of those bicycle dynamo / generator on the belt instead ?

would be interested in the results though.

 

[Sevenhills1952]

LED is a diode. It will run on AC.
But you need to know 2 things:
1) voltage and current requirement of LED light(s).
2) magneto output on engine voltage and current. If you're talking about ignition, that's thousands of volts and it won't work.
Lots of other choices though. If lights run on, say, 12v...I'd switch connect them to a rechargeable battery (should last for your riding time). Or just get a bicycle generator that runs on the tire.


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[Captain Spaulding]

Like Sevenhills said, you can run an LED from AC. You can’t get 12VDC from 12VAC. Normally you want 18VAC to get 12VDC, but it depends on frequency and peak voltage in this application.

 

[dogdog]

Like Sevenhills said, you can run an LED from AC. You can’t get 12VDC from 12VAC. Normally you want 18VAC to get 12VDC, but it depends on frequency and peak voltage in this application.

 

[Pontiac787]

Thanks all. To clarify, the unit is purpose built to provide about 12v-18v depending on engine RPM. The person selling the units said the LEDs will be brighter if run on DC. The LEDs require 12v.

I’ll probably go with something like this for simplicity.

 

[rlitman]

+1
Any of those metal-cased 35-50 amp bridge rectifiers would be ideal. They are easy to mount, require very little (or no) heatsinking for this application, and accept faston connectors. Go for it!

That's what I was thinking. The metal case should be more than enough of a heatsink for LED lighting. While a rectifier a fraction of the size might be sufficient, heat comes into play with smaller options.

Would be interesting and see how this is going...

My guess, you'll probably needed a more higher voltage spec "full-WaVe rectifier" magneto outputs very high voltages, but very low current. unless you are mounting a second magneto, not sure if it has enough AMP to drive a decent bright LED. You'll probably needed the proper caps too plus some sort of right ranged zener diode as well to trim the voltage.


maybe figure out how to mount one of those bicycle dynamo / generator on the belt instead ?

would be interested in the results though.

We're talking about 1000V and higher rated rectifiers. So long as the load is connected, the voltage from the magneto will not get high enough to damage them. Yes, a 12V diode may be toasted by a magneto.

Thanks all. To clarify, the unit is purpose built to provide about 12v-18v depending on engine RPM. The person selling the units said the LEDs will be brighter if run on DC. The LEDs require 12v.

I’ll probably go with something like this for simplicity.

I'm not sure what that unit does, but the description makes it clear that a battery is absolutely required in the circuit. I don't think that's a good idea.

Just find an LED module that accepts a wide range of input voltages. Such a unit probably rectifies already too, but you can add the rectifier if it does not.

 

[Sevenhills1952]

That one says ac input voltage 20 to 60v. Your original post said 12 to 20vac, so LED would only come on at high rpm (20v).
Try your LED on ac...see how it does without diodes.
12vac through full wave bridge output would be 12 X 1.414 --- (2x0.7) assuming silicon diodes. About 15.6vdc
20vac= 27vdc which is too high.

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[dogdog]

..........

We're talking about 1000V and higher rated rectifiers. So long as the load is connected, the voltage from the magneto will not get high enough to damage them. Yes, a 12V diode may be toasted by a magneto.

.................

not sure what is the peak voltage generated by the small engine magneto when the magnet induce a manget field and break down flux.. but the amp is in sub milliamps. supposedly very high flux break down voltage. I got shock by it before. didn't kill me. Let's say it ain't no 12V. at least that is the last time I learned about magnetos...

something about the PIV of a diode says hmmm.. only takes fraction of seconds.

https://www.maximintegrated.com/en/glossary/definitions.mvp/term/Peak Inverse Voltage/gpk/1049

 

[dogdog]

Thanks all. To clarify, the unit is purpose built to provide about 12v-18v depending on engine RPM. The person selling the units said the LEDs will be brighter if run on DC. The LEDs require 12v.

I’ll probably go with something like this for simplicity.

Please Note: This regulator rectifier has to work together with a 12V battery. If without 12v battery, the output voltage will be much higher.

That person is probably selling you a expoxy glued rectifier that is spec for high voltages as said probably with a zener diode inside. the Battery is used to absorb the spikes and peaks, ... most people don't have a high voltage probe handy to measure what the flux break down voltage is on a magneto... yea that is the term ... I am suspecting it is more than 10,000 volts but very very very low amps...not high enough voltage, it probably won't jump that air gap on your spark plug.

so what does that mean,, that 100A rating is a waste. IMO..
$10, I would just get a production already made for it... unless you just want to build one.

 

[PWC Repair]

Most of the bike, atv, and jet ski rectifiers are also voltage regulators. Typically the stators are wound 3 phase A/C and generate 20-50 vac at around 1000 rpm. The regulator/rectifiers then output a pretty steady 13-14.5 VDC at a couple of amps. I have a few Kawi and Seadoo units around....you pay the shipping. What I'm not sure of is 1/2 or full wave.

 

[rlitman]

not sure what is the peak voltage generated by the small engine magneto when the magnet induce a manget field and break down flux.. but the amp is in sub milliamps. supposedly very high flux break down voltage. I got shock by it before. didn't kill me. Let's say it ain't no 12V. at least that is the last time I learned about magnetos...

something about the PIV of a diode says hmmm.. only takes fraction of seconds.

https://www.maximintegrated.com/en/glossary/definitions.mvp/term/Peak Inverse Voltage/gpk/1049

Re-read my post. You're talking about micro-amps of surge. And my point was that the voltage will not be able to climb significantly, SO LONG AS A LOAD IS CONNECTED.

Anyway, you're right, it ain't no 12V, and without a load, maybe it is a couple of kV, but with just about any load, it's not going to hurt a 1kV rated diode.

 

[rlitman]

Most of the bike, atv, and jet ski rectifiers are also voltage regulators. Typically the stators are wound 3 phase A/C and generate 20-50 vac at around 1000 rpm. The regulator/rectifiers then output a pretty steady 13-14.5 VDC at a couple of amps. I have a few Kawi and Seadoo units around....you pay the shipping. What I'm not sure of is 1/2 or full wave.

3-phase rectifiers are also full wave. You just add another pole for every phase, so a 3-phase full wave rectifier has 6 terminals instead of the 4 for a single-phase one.

 

[dogdog]

Re-read my post. You're talking about micro-amps of surge. And my point was that the voltage will not be able to climb significantly, SO LONG AS A LOAD IS CONNECTED.

Anyway, you're right, it ain't no 12V, and without a load, maybe it is a couple of kV, but with just about any load, it's not going to hurt a 1kV rated diode.

I am not sure about the load thing.. maybe. Almost makes me wanted to connect my semikron to test that theory.. but nope... not worth the $80.

https://www.galco.com/buy/Semikron/SKB52/14

 

[Pontiac787]

I'm going to try a simple bridge rectifier like rlitman suggested. It looks like the easiest option.

 

[dogdog]

I'm going to try a simple bridge rectifier like rlitman suggested. It looks like the easiest option.

The one in your original post is exactly the same full wave bridge rectifier as the one rlitman posted... difference is the specs and casing. Don't let the drawing fool you.

me... I would go with the original one it has higher voltage specs, higher amp specs and better heat sink. better terminals using screws instead of blades.

The point was none of those have high enough PRV/PIV in the diode to handle the output of the magneto voltage... what his argument was that because the amps are so low that as long as there is a load it won't cause issues...

 

[Pontiac787]

Makes sense. Just to clarify this is the stator I’m planning on using to power the lights.

Mhttps://www.ebay.com/itm/12-VOLTS-MINI-GEN-MAX-For-Motorized-Bicycle-Lighting/400108733087?hash=item5d2856c29f:g:eWMAAOxykYtSITj0

 

[dogdog]

1.5AMP... I think you are better off and smaller package with a smaller sized rectifier instead... 100A is a total waste in this... that coil says output is 6-14-ish volts.. and 1.5AMP...

even the 50AMP is way way over kill... besides that you will probably have no place to mount any of those...

are you sure it is needed for LED lighten ? This have already mentioned to you LED it self is a diode....

I would have go with some thing from your post#9....

 

[rlitman]

The one in your original post is exactly the same full wave bridge rectifier as the one rlitman posted... difference is the specs and casing. Don't let the drawing fool you...

The OP's link is for a 100A version with screw terminals. Mine was for a 50A version.
Internally, they're going to be surprisingly similar. The biggest difference is in the casing, and that's why I was suggesting the 50A version, because it's well under 1/4 the size of the 100A version.

1.5AMP... I think you are better off and smaller package with a smaller sized rectifier instead... 100A is a total waste in this... that coil says output is 6-14-ish volts.. and 1.5AMP...

even the 50AMP is way way over kill... besides that you will probably have no place to mount any of those...

are you sure it is needed for LED lighten ? This have already mentioned to you LED it self is a diode....

I would have go with some thing from your post#9....

Ok, so here was my line of reasoning (right or wrong as it was). Based on the head/tail lamp LED idea, my first guess was that 5A was a reasonable limit. There are plenty of 5A rectifiers that will fit on Lincoln's head on a penny. The problem is heat. Those tiny rectifiers may need a heatsink the size of a quarter to work. The 50A one I linked is about as small as you can find that's wrapped in enough metal to not require any heat sink at all at the expected load, since the can it's potted in is already about the size of the heatsink I was assuming would be needed. Yeah, it's ludicrous overkill, but it also keeps things simple.

At 1.5A, we could search for something smaller, but there's a diminishing return if we still need to use a heatsink.

 

[dogdog]

the size between the two at least according to the listing specs.

4.25 x 3.54 x 1.34 inches; 1.45 Ounces

3.1" x 1.6" x 1.2" (L*W*T); Mounted Hole Dia. : 6mm / 0.24"

5 x 3.2 x 0.8 inches

 

[iron block]

This is an interesting project. But there is one potential trap to be avoided.

If you have a metal frame bike, and if the LED and the magneto both use the frame for the "ground" connection, then read on.

The output of the magneto has only one wire (the red one). So the other end of that circuit must be tied to frame ground inside the magneto.

That means that one of your "AC" inputs on the bridge rectifier will go to the red wire, the other to frame ground. S far, so good.

The "+" output of the rectifier will go the LED positive input. No problem. But the "-" output of the rectifier has to be kept away from frame ground. If it goes to ground the rectifier will become half-wave. And worse, on the negative cycle the input will be shorted to ground through one of the diodes. Not great for the lifetime of the mag nor the rectifier.

So you need LED lights that bring out both plus and minus leads. And the minus lead cannot be tied to frame ground anywhere -- including inside the light fixture. If they tie the minus lead to ground, you will have the problem described.

 

[Pontiac787]

Thanks Iron block. I’ll make sure the lights are isolated from the frame. Both the headlight and taillight have positive and negative leads.

 

[iron block]

Thanks Iron block. If I don’t connect the neg. side of the LED to ground where do I connect it?

The terminals on the rectifier block will be:

• Two inputs, labeled either "AC" or "~",
• the "+" output, and
• the "-" output

The LED's negative lead would go to the "-" output terminal.

 

[TractorJeff]

Back tot eh other post of the diode or negative post of battery?
Negative post can not be tied to bike frame.
Disclaimer:
If I understand what is going on?

 

[ttpete]

Most small flywheel magnetos of that type have a separate lighting coil that might put out up to 20 volts AC. All you need is a small full-wave bridge rectifier and a 14 volt Zener diode to control voltage. If the voltage goes above 14, the diode will dump part of the output to the ground side to maintain proper voltage. If wanted, a small AGM battery can be added.

 

[Sevenhills1952]

Most small flywheel magnetos of that type have a separate lighting coil that might put out up to 20 volts AC. All you need is a small full-wave bridge rectifier and a 14 volt Zener diode to control voltage. If the voltage goes above 14, the diode will dump part of the output to the ground side to maintain proper voltage. If wanted, a small AGM battery can be added.

No current limiting on the Zener, it will take a dump.

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[ttpete]

No current limiting on the Zener, it will take a dump.

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Only until the voltage is within limits. Older British motorcycles used this setup for years. Typically, the Zener was rated for the output limit of the alternator and in a heat sink exposed to the air. The positive wire from the battery was fused at 30 amps for protection from a dead short.

 

[Captain Spaulding]

Did you have a question or do you not understand why you can’t?

12VAC is about 17V peak, but there are still periods of 0V. You have to filter and then regulate to get ,12VAC, and there isn’t enough headspace to produce 12V until the AC is about 18V.

 

[ttpete]

Did you have a question or do you not understand why you can’t?

12VAC is about 17V peak, but there are still periods of 0V. You have to filter and then regulate to get ,12VAC, and there isn’t enough headspace to produce 12V until the AC is about 18V.

If you're using full-wave rectification and charging a battery, filtration isn't a problem because the battery acts as a big capacitor to smooth out the pulsating DC. Without a battery, the use of a 5000 mfd electrolytic capacitor across the output will smooth the DC and prevent light flicker.

 

[dogdog]

Did you have a question or do you not understand why you can’t?

12VAC is about 17V peak, but there are still periods of 0V. You have to filter and then regulate to get ,12VAC, and there isn’t enough headspace to produce 12V until the AC is about 18V.

If you're using full-wave rectification and charging a battery, filtration isn't a problem because the battery acts as a big capacitor to smooth out the pulsating DC. Without a battery, the use of a 5000 mfd electrolytic capacitor across the output will smooth the DC and prevent light flicker.

he is talking about Vrms and Vpp.... most multimeter meter measures Vrms already. unless you are measuring with a scope I think. what was ambiguous was the OP later clarify that it was a separate magneto "like" generator installed right next to the magneto used for sparks. Filtration will be difficult since the calculation for the filter cap is dependent on frequency and vpp of the rectified dc voltage. and frequency is dependent on engine RPM in this case. I am even wondering if a Full wave rectifier is even needed on these type of setup...

 

[rlitman]

...I am even wondering if a Full wave rectifier is even needed on these type of setup...

I'm not positive either, but it wouldn't hurt, much, except for some voltage drop. You could test it with and without and compare. Some LED units full-wave rectify already, while some do not. If not, and if the waveform is truly AC, then you can expect only half brightness without full-wave rectification, since the LED will go dark half the time.

 

[ttpete]

he is talking about Vrms and Vpp.... most multimeter meter measures Vrms already. unless you are measuring with a scope I think. what was ambiguous was the OP later clarify that it was a separate magneto "like" generator installed right next to the magneto used for sparks. Filtration will be difficult since the calculation for the filter cap is dependent on frequency and vpp of the rectified dc voltage. and frequency is dependent on engine RPM in this case. I am even wondering if a Full wave rectifier is even needed on these type of setup...

That's not that critical in this application. All that's necessary is to charge a battery. Pulsating DC works fine, and frequency can be anything as long as you're shoving amps into the battery. A motor vehicle alternator's output is full-wave rectified pulsating DC, the same as our motorcycle system, and it works fine as well.

 

[dogdog]

Never owned a motorbike with this type of setup let along with LED lights... that might or might not be "sensitive" to voltage spikes... in this case magnetic flux break down voltage..

Can't tell you. but one thing is for sure.. if OP uses full wave rectifier, it's 1.4v drop... and .7 if it is just a diode.

I do remember there was a thread something similar 3 years back on ATV I think (here)... with similar question to run LED lights.

 

[ttpete]

Never owned a motorbike with this type of setup let along with LED lights... that might or might not be "sensitive" to voltage spikes... in this case magnetic flux break down voltage..

Can't tell you. but one thing is for sure.. if OP uses full wave rectifier, it's 1.4v drop... and .7 if it is just a diode.

I do remember there was a thread something similar 3 years back on ATV I think (here)... with similar question to run LED lights.

Half wave rectification makes half the wattage, and that's why full wave is used. That can be taken care of by increasing the voltage rating of the Zener.

The battery damps out any voltage spikes.

 

[dogdog]

Half wave rectification makes half the wattage, and that's why full wave is used. That can be taken care of by increasing the voltage rating of the Zener.

The battery damps out any voltage spikes.

LOL if your generator only generates half wave, do you needed full wave rectification?

 

[Sevenhills1952]

To me it's all getting complex.
1) how much current does this 12v LED head & tail light draw?
2) how many hours do you plan on running it at night?
I have an LED head/tail lamp I bought years ago for a standard bicycle. It has a small rechargeable battery pack that you can mount anywhere. It runs for hours...then at home plug in a charger. It's simple.
I have a bicycle I put a light on and a 2 stroke motor. That light runs on a small generator that contacts the tire. It's simple.
Both work fine...k.i.s.s.

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