Best way to get high current from dirtbike magneto?
Best way to get high current from dirtbike magneto?
(OP)
Hi, I have struggled with this problem for many years now and Im almost ready to conceed defeat...
I want to generate aprox 12 amps @ 12VDC average from my dirtbikes for a powerful headlight (3 50W halogens). At least 5 Amps at idle. I can easily generate say 10 amps at 12V even at idle by rewiring the stators to suit but the problem is regulating the voltage when the RPM goes up. At around max RPM I end up with over 150Volts. Other than it being lethal voltage/current, no bulb will work over such a broad voltage range. I have got around it in the past by using bypass power mosfets around a (special hi input voltage) 7812 voltage reg. Works, but the 7812 is not durable enough.
btw, yes I know there are aftermarket solutions but they are very expensive and still quite low output. Also I enjoy the whole DIY challenge. I dont want to run a battery so a (self regulating)alternator is out of the question.
There must be a better way. I have been thinking of ways to regulate by way of limiting the induction with rpm increase.
I have designed a very simple mechanism that increases the gap between the stator coils and the magnets in proportion to RPM rise (using centrifugal force). Can anyone tell me (before I build it) will increasing the gap with RPM rise lower the *VOLTAGE*? Im pretty sure it will lower the current, but I can increase wire size and add winds to compensate. Will this work, or is there a better way still?
Thanks.
I want to generate aprox 12 amps @ 12VDC average from my dirtbikes for a powerful headlight (3 50W halogens). At least 5 Amps at idle. I can easily generate say 10 amps at 12V even at idle by rewiring the stators to suit but the problem is regulating the voltage when the RPM goes up. At around max RPM I end up with over 150Volts. Other than it being lethal voltage/current, no bulb will work over such a broad voltage range. I have got around it in the past by using bypass power mosfets around a (special hi input voltage) 7812 voltage reg. Works, but the 7812 is not durable enough.
btw, yes I know there are aftermarket solutions but they are very expensive and still quite low output. Also I enjoy the whole DIY challenge. I dont want to run a battery so a (self regulating)alternator is out of the question.
There must be a better way. I have been thinking of ways to regulate by way of limiting the induction with rpm increase.
I have designed a very simple mechanism that increases the gap between the stator coils and the magnets in proportion to RPM rise (using centrifugal force). Can anyone tell me (before I build it) will increasing the gap with RPM rise lower the *VOLTAGE*? Im pretty sure it will lower the current, but I can increase wire size and add winds to compensate. Will this work, or is there a better way still?
Thanks.





RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
JTK
RE: Best way to get high current from dirtbike magneto?
A better approach is probably to rectify the voltage and feed it to a switcher.
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
Feed a switcher? yikes, talk about complicated. He wanted simple.
JTK
RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
Did you really do your math?
Ever heard about NS Simple Switcher? It is really simple - at least compared to the problem of getting rid of 1.5 kW dissipation in a bike. Don't forget that the heat sink gets covered with dirt - not an easy thing to handle...
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
I dont think there are zener diodes with zener voltages around 12V that will happily see a voltage swing from 12 to 150 volts. Same goes with switching regs. Please, Im happy to be proven wrong.
Yes, efficiency is not important. The energy will be wasted anyhow, such a shame I cant use another bulb as a ballast!
RE: Best way to get high current from dirtbike magneto?
Better ilumination and remember that any current that goes throught the lamps does not have to go through the zener. The normal voltage range is more like 13 to 14 volts. (For battary charging) The zener limits the voltage to 13 or 14 volts, that's the point. It loads the dynamo up so that increased speed causes increased current at the same voltage. The increased current causes an increased internal voltage drop in the dynamo. The excess energy is mostly wasted as heat in the dynamo windings. That's why the system is only used on small dynamos. Size the diode for full current so you don't over load the diode if the lamp fails or is removed.
Also, remember that the dynamo is probably generating AC that is then rectified. We are probably looking at an impedance that increases with frequency.
Any comments or suggestions on that point?
And on a completely different note;
In my bicycle years, I had a bicycle dynamo that had a very simple mechanical regulator. The magnet could slide on the shaft. a small arm (Radius maybe 5/8" or so) was pinned to the shaft so that it could move in an arc. There was a short link from the arm to the magnet. The magnet was held in the center of the windings by a spring. The reaction arm was as close to parallel to shaft as it could be. Centrifugal force would tend to move the reaction arm perpendicular with the shaft, restrained by the spring. As the magnet moved out of the center of the coils, the voltage was regulated.
It's not very high tech or electrical, but it might be the answer to your question. It may not balance perfectly but who would notice on a dirt bike at high RPMs. I would ballast it with a lamp and try it at different RPMs and measure the voltage. Adjust the spring force and/or the weight of the reaction arm as required until you get acceptable regulation. If you get it close, then throw the zener in the circuit for fine tuning.
Respectfully
RE: Best way to get high current from dirtbike magneto?
Hook up the three lamps.
Hook up a fourth that would drag things down to say 9 or 10V when they are all on.
Hook up a voltage controlled relay that closes contacts to the fourth lamp. Let it vibrate away controlling the power to the fourth bulb.
Keep in mind that until 15 years ago all vehicle alternators were controlled by vibrating relays.
A step up would be the same thing based on a solid state relay or a transistor.
Any "waste" energy goes to providing more light.
Comments?
RE: Best way to get high current from dirtbike magneto?
You said: "remember that any current that goes throught the lamps does not have to go through the zener." How so? Are you suggesting I could shunt the zener into the gate of a power transistor (or relay coil)which in turn feeds a dummy/ballast bulb? Will this still regulate? My understanding of electronics is limited. Excuse the dodgy drawing:
+ O------------------
| | |
| | | Z= Zener
Z / | T= Transistor
|------T | BB=Ballast Bulb
\ H H= Headlight
| |
BB |
| |
- o------------------
1. Will this regulate?
2. If so, I'll still need a zener that will go to 150V, damn.
itsmoked; hmmm, voltage controlled relay, I like it too. Never thought of that. Just have to be careful not to get contacts wet/dirty...not so easy on a dirtbike tho.
RE: Best way to get high current from dirtbike magneto?
A The magneto delivers a voltage that is proportional to speed.
B The frequency is also proportional to speed.
C The lamp is purely resistive.
If you chose a low voltage lamp and put an inductor in series, the current through the lamp will be determined mostly by the reactance of the inductor. Not solely, but mostly.
So, when speed goes up so does voltage - and frequency. If the lamp voltage were nil, the current would stay constant since V = k1 x frequency and X = k2 x frequency whence I = V/X = k1/k2 = constant. Now, the lamp voltage is not zero so you get a certain change in light flow. But a lot less than you would have without the inductor.
Regarding previous posting: What do you mean with "zener going to 150 V"? That is not what happens. The zener goes to whatever voltage it is designed for (say 12 V) whereupon it starts conducting. Current is determined by external voltage and resistance. Dissipation in zener is determined by current through it. But the zener diode per se never goes to 150 V.
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
Please, tell me Im wrong and it will in fact work as you say as this would be an EASY solution. If so, could someone please take a stab at a value for that inductor and I will begin experimenting!
Im guessing frequency would be roughly 1200Hz idle to 10000Hz peak and my voltage swing is from 12 volts to 150 volts. I want to regulate to 12 volts.
btw, what I meant was, I believe a zener diode that has a zener voltage (breakdown voltage) of 12 volts would not be able to withstand voltages as high as 150 volts. No?
RE: Best way to get high current from dirtbike magneto?
Good point. I think some us are assuming that the dynamo includes rectifiers so that the final current is DC, but it may be AC. My old bicycle dynamo was AC. With rectifier diodes, the inductor you suggest could still be used in series with the windings ahead of the rectifier.
I would imagine that the induction of the windings acts just as you suggest to somewhat limit the current at high RPM.
I think it's worth a try.
itsmoked
That's a good theory but there is a problem with scale. We may have a voltage swing of 10 to 1 or more.
With three lights in service the ballast light would give us a load ratio of 4 to 3. Probably not enough load. How about sections of the element out of an electric clothes drier for a ballast load?
In support of your theory, I once had a bike that used a variation of your theory. The head light was always on. It must have been used as a ballast, because whenever the headlight burned out, the alternator would overpower the regulator and start blowing out the other lamps on the bike.
Riding conservatively the other lamps may last 1/2 hour or so. At high RPM's the other lamps would be gone in seconds.
Re the vibrating relay.
You would be switching the load current rather than the field current and your ballast load may have to be several times that of the working load. Before altenators, generators used the same vibrating rely control of the generator field but the field current was several times higher. Yes, I'd try it if I had the parts to hand. If I had to buy parts,however, it may not be my first choice.
tabletop
Zener diodes are available in ratings up to at least 300 watts. For a bike regulator they are connected in parallel with the lights.
- 0----------------
I I
I I Z=Zener
Z H H=Headlight
I I
I I
+ 0-----------------
yours
RE: Best way to get high current from dirtbike magneto?
Yes waross. I think so too (that the generator's inherent inductance does what my external coil would do). Which leads to this question for tabletop: Do you measure the voltage without having the lamp(s) on? If that is so, you are reading the no-load voltage and that is usually very high compared to loaded in these little machines.
There is another question that I think need to be addressed: The frequency in an unusually high range. How did you measure 1200 Hz idling - and 10 kHz peak? Power frequencies are usually in a much lower range - I would say 25 - 500 Hz.
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
Are probably correct. That would correspond to a two pole alternator turning 1200 RPM and 10,000 RPM respectively.
With more than two poles, the frequency will be multiplied by the number of pole PAIRS. Consider also the gearing reatio between the motor and the alternator.
yours
RE: Best way to get high current from dirtbike magneto?
I think that is unloaded, hence the dynamic loaded voltage is much smaller. 8V to 2? V
warross; granted the alternator field is probably smaller though I would expect that if you wind the magneto correctly so that it has just enough windings to run 3 bulbs (12.5A @12V) that another +4A load should be enough to control the voltage. I thought regulator field currents hovered around 4A. Not that I'm a fan of mechanical relays. I would use a BJT or a FET strictly in switching mode.
Hey tabletop what is the "LOADED", with three bulbs, voltage swing of your present set up?
RE: Best way to get high current from dirtbike magneto?
I have only measured unloaded to get max of 150 volts (on my last bike). Unfortunately I cant measure loaded because the bike I have now doesnt even have a stator coil for headlights etc. Only one for the ignition. Easy enough to include more coils though. I can tell you though from past attempts, that at idle the lamps would burn slightly dull, just open the throttle and they'd be bright, abit more and...POP.
Yes y'all were spot on, that should have been aprox 20Hz to 167Hz (got my seconds and minutes confused)
I guess Im more after a concept of how best to achieve this as I often change bikes and may want to apply it to another. It seems than the zener in parallel with the lamps is the easiest concept.
waross, I cant find zeners at digikey or rs-components above 50 or so watts. There are 'transient voltage suppressors' in 500watts+ is this what you mean?htt
Thanks guys, I feel like Im finally getting somewhere.
skogsgurra- "You are wrong there tabletop. Voltage never kills anything - current does. " ...but I can put this same lamp across a 12v car battery with loads of amps and it wont die?
RE: Best way to get high current from dirtbike magneto?
Usually current causes heat and excess current causes damage and failure.
Higher voltage causes higher currents.
Current does the damage.- Voltage causes the higher Current.
I am sure that your familiar with Ohm's Law.
Current times Resistance equals Voltage.
or Voltage divided by Resistance equals Current.
Simple example: Connect a resistor of ten ohms across your alternator and at 1000 RPM you get 10 volts.
10 volts divided by 10 Ohms equals 1 amps.
Now wind it out to 10000 RPM and at 10 times the speed you get 100 volts.
100 volts divide by 10 ohms equals 10 amps.
In the first example the watts lost in the resistor are current squared times resistance or 10 Watts.
In the second example the watts lost in the resistor are current squared times resistance or 1000 Watts.
Now inductance;
The inductance of a coil increases proportionally to the frequency. If we use the formula to convert the Inductance to Inductance Reactance, we can measure or express it in Ohms and we can use use those Ohms in Ohm's Law.
Here comes the good part; One of the factors in the equation that converts Inductance to Inductive Reactance is frequency. As the frequency rises, the Inductive reactance rises. So we may have the following induction coil characteristics;
10 Ohms at 1000 RPM. 10 Volts, 1 Amp.
At 10000 rpm, the impedance increases to 100 Ohms (Inductive Reactance.)
So we have 100 Ohms and 100 Volts and 1 Amp.
Now what about the Watts loss in the coil. Well the current through Inductive Reactance does not cause heating. The current through Resistance causes heating.
Our induction coil will have some Resistance.
For this example let's assume a resistance of 1 Ohm.
In the first example, 1 Amp squared times 1 Ohm Resistance equals 1 Watt loss in the induction coil.
In the second example the coil still has 1 Ohm Resistance and the Current is still 1 Amp so the Watts loss in the coil is still 1 Watt.
Now, in real life it isn't quite so simple. Because you are losing some of the voltage across the Induction Coil, You will need more voltage from your alternator before the lamp is bright enough to use.
The circuit is not linear; We have an Inductance in series with a Resistance (The Lamps). The voltage across the inductance will be changing with frequency while we will be trying to hold the voltage across the lamps at a constant value.
I'm starting to really like you inductor suggestion skogsgurra.
Trying different values of inductive reaction will probably yield a better value to use in practice.
If you let me add my Zener, I think we have the solution. Intuitively the rating of the Zener will probably be much less with the help of your induction coil.
tabletop, I googled Zener Diodes and found a rating of 300 watts quite quickly. When I tried to find it again, I found myself in the middle of a bunch of surge limiting Zeners with ratings of 1500 Watts. I am going to have another look and try to find the best steady state ratings. I don't know if the previous rating was peak surge or steady state.
respectfully
RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
RE: Best way to get high current from dirtbike magneto?
I checked your stats and find that you are quite new to this site. That may explain your rather verbose posts. After some months, you will find that people do not read your posts in full - they are sometimes simply too long to be read. I suggest that you try to condense your text a bit. You will develop a sense for it as you continue writing your otherwise excellent postings. I hope that you can take this advice without being offended - I'm not so sure that I could...
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
No offence taken. I will take it as good advice from a friend.
Thanks for your patience.
Re the diodes.
Found 75-100 Watts (5000) Peak.
http:
yours
RE: Best way to get high current from dirtbike magneto?
Seems I have my 'concept' now: inductor in series and zener in parallel. Can anyone save me many hours by giving me a ballpark value for the inductor?
Thanks so much all.
RE: Best way to get high current from dirtbike magneto?
Remember that the zener doesn't have to be extremely powerful any more. I would gues something like 50 watts and good heat-sinking. The inductor keeps current down. Also remember that you need two zeners back-to-back. Zeners have a break-down voltage in one direction only. They behave as normal diodes in the other direction.
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
battery-based system. The battery does not need to be very
large; you could use Ni-Cad cells distributed about the
frame in convenient locations. The electronics are more
stable and much more available.
An added bonus is lighting at idle, or with the motor stopped.
I have found that extremely helpful in some situations
(fixing a flat, finding dropped tools, etc).
I hate shunt-regulators - if you lose the regulator, you lose
anything attached. With pass-regulators, all you lose is the
regulator.
You did not mention what type of bike/engine you were dealing
with, but bear in mind that the magneto on some models is
barely enough for the ignition; there is not enough extra
to supply an additional lighting system.
<als>
RE: Best way to get high current from dirtbike magneto?
Mike Halloran
Pembroke Pines, FL, USA
RE: Best way to get high current from dirtbike magneto?
A battery system doesnt compare to 200W of light. Tried it. Great as a backup to limp home but riding a dirtbike is difficult enough with GOOD visability. Oh and btw, I ENJOY "screwing around". The challenge of trying to tame all that wasted energy stimulates me.
Bikes (at the moment) are 250 RMX, 290 Sherco, KX100.
Mike, I need to look at this "cute" alternator. Do you know how many amps? Have inbuilt reg?
RE: Best way to get high current from dirtbike magneto?
You can also try changing the circuit to a simple voltage source by switching the load from the collector to the base of the transistor.
http:/
You can (likely should) put a seperate current regulator in series with each headlamp. This will lower the current/power requirements of each circuit and also provide redundancy in case of a lamp or circuit failure.
RE: Best way to get high current from dirtbike magneto?
To limit 20% over current at 150 Volts approximately 10 mH inductance must be added. So it will be connected in series with the 0.96 ohms of three 50 watts, 12 Volts bulbs in parallel.
The total impedance at 167Hz (neglecting the dynamo internal impedance)
Z = (0.96^2 + (2*3.1416*167*0.010)^2 )^0.5 = 10.54 Ohms
The current, I = E/Z = 150/10.54 = 14.23 Amperes
The bulbs power W = 0.96*14.23^2 = 194.4 Watts.
When the dynamo frequency is 20 HZ and delivers 12 Volts:
Z = (0.96^2 + (2*3.1416*20*0.010)^2 )^0.5 = 1.581 Ohms
And the current, I = 12V /1.581 = 7.6 amps
The bulbs power W = 0.96*7.6^2 = 55.5 watts (1/3 of nominal)
Not a real regulator, but avoids killing the bulbs.
RE: Best way to get high current from dirtbike magneto?
I like your numbers. Can I add a few more.
Looking in my old repair manual for a 1983-84 Honda Shadoo 500 I find these specs;
Nominal voltage of bulbs. 12 Volts
Output voltage of the alternator 14 to 15 volts. (The range of the voltage regulator.)
The old Honda went through a couple of bulbs a year, so 14 volts may be as high as we want to go.
Also when a lamp burns out, the current in the inductor will drop, and so also the voltage drop across the inductor. This will give us a higher voltage across the remaining lamps.
If you let me add my diode, oops, diodes, (thanks skogsgurra).
Then we can drop 5 or 6 volts from maximum voltage and protect the lights. Two 13 Volt diodes should give us 13.7 volts. 13.8 was a standard automotive voltage during my hot rod years.
If we use your figure of 14.23 amps, times 6 volts we get
86 Watts for the zener. That's if all three lights go out at once and you run full RPMs. Much less with all three lights.
Or you could wind your alternator Star and buy a Honda regulator.
No don't do that, this is more challenging.
yours
RE: Best way to get high current from dirtbike magneto?
The only problem I can see is low lamp voltage when idling. On the other hand, there are buck/boost regulators.
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
The same circuitry is used on motorcycle battery regulators, you might be able to just use one of those rather than build your own.
More details for those interested:
When the SCR's are triggered the magneto will output 10-20A across the small voltage drop of an "on" state SCR, so very little power is generated/dissipated. A bridge rectifier isolates the SCR circuit from the lamps. The SCR's gates are driven from zener diode/resistor combination powered from the rectified DC output of the bridge. Cap's are required across the SCR gates and on the output of the bridge rectifier to eliminate lamp flickering.
RE: Best way to get high current from dirtbike magneto?
Gunnar Englund
www.gke.org
RE: Best way to get high current from dirtbike magneto?
Thanks. My diagram for the old Honda regulator just shows the control circuit as a box labelled "IC" and the power diagram has a couple of obvious typos, but what I trust of the power diagram of the regulator is expained perfectly by your explanation. Thanks.
RE: Best way to get high current from dirtbike magneto?
A buck converter such as a switching transistor-inductor-shunt diode acts as a variable autotransformer for direct current which would decrease the current drawn from the magneto as the speed and output voltage go up. You would also need a rectifier and a big capacitor before the the switching transistor and also another big capacitor at the output.
However, by the time that you build all of that you might be getter off to get a small automotive alternator and belt that to your dirt bike. You might need to change to output sprocket or pulley to one that will take 2 chains or belts. There is nothing wrong with replacing the alternator pulley with a sprocket. 150 watts or so is a lot of power to expect from a cheap permanent magnet magneto.
RE: Best way to get high current from dirtbike magneto?
I apologize, but I dont know the actual pulley ratio. I just wouldnt want one coming apart between my legs while I'm going WOT up my favorite jump hill!
Just a thought.
Scott
In a hundred years, it isn't going to matter anyway.