Magneto powered moped voltages - Page 1
Forum section: electronics / beginners

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 05, 2023, 03:04:10 pm
content: I only come here when I need something. I never give anything back. That's because I don't know anything. Having said that, will you help me once again? I have a 1957 Sears moped exactly like the one my parents gave me that year for my 13th birthday. I'm now 77y/o. I'm restoring the moped (actually two of them). The electrics are magneto only. There is no battery. The magneto provides spark for the plug. The headlight and taillight are 6VAC from different coils of the same mag. The headlight was working. I haven't yet found a bulb for the taillight. My headlight blew out yesterday. I put my meter on the wire feeding it, and set my meter to AC volts. I expected to see something around 6 volts and thought it would be more or less stable. Instead, the meter displayed voltages all over the place without regard to RPM. They ranged from less than a volt to more than 12 volts which, I suppose, accounts for the blown headlight. I'm wondering if the absence of the taillight would blow the headlight. And I'm wondering if it's normal to get these kinds of meter readings from a magneto. I suspect the solution would be to put some kind of voltage-liming resistor in line, but I'm still curious to know why I can't measure ac voltage with my meter. I have an oscilloscope, brand new, but don't know how to use it. Given all that-- what can you tell me? I appreciate any advice.

author: floobydust
author_rank: Super Contributor
timestamp: May 05, 2023, 04:28:20 pm
content: This is how they work - the output is unregulated and rises with RPM or decreased load.So a burnt out tail light causes output voltage to soar, same with running at high RPM. Everyone in India and Bangladesh encounters this problem.One common fix is (back-back) power zener diodes to clamp the output voltage to say 8VAC max. but this wastes the magneto's energy as heat in the diodes and windings, and it loads down the engine and wastes fuel. It would save the other lamp from over-voltage though.Otherwise you need a bunch of electronics. A diode bridge rectifier is no good due to the losses of 2-3V giving dim lighting at idle.I think you need to go mosfet rectification and a bunch of other circuitry to regulate to say 6VAC and maybe power a USB charger at the same time.No one has come up with a circuit to regulate a 6V magneto.Measuring the ACV, it's not a sine-wave it is a blip blip from the two magnets that varies with RPM. So some voltmeters would read that wonky.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 05, 2023, 05:04:32 pm
content: Wow! That's a lot of info, mostly over my head. But, you mentionedrectification. I thought rectification changed AC to DC so that the 6VAC you mentioned would be 6 (or something) DC. But maybe it's enough for me to know that the absence of the taillight bulb would cause the "voltage to soar" as you stated. So I'll try sourcing some 6v bulbs (not easy in this world), put in both the headlight and taillight bulbs, and hope that the voltage produced while running both will be low enough so they don't blow. I don't remember as a kid having a problem with replacing blown bulbs. And the electrics on my mopeds are all original. Hmm...

author: langwadt
author_rank: Super Contributor
timestamp: May 05, 2023, 05:32:08 pm
content: Quote from: floobydust on May 05, 2023, 04:28:20 pma shunt regulator it very common and the energy wasted is minuscule compared to the engine output

author: floobydust
author_rank: Super Contributor
timestamp: May 05, 2023, 06:01:03 pm
content: It looks like it uses lamps with a BA15S single bayonet base. Guessing a 6V 5-10W tail light #63 or #81 and a 21W headlight #1129, say around 20W load total. A 50cc engine 1-3HP so yes, the lighting load is not much. I do see the primitive shunt regulators cooking the generator coils though.Yes the incandescent lighting is AC powered here. But if there is a battery, converting to LED lighting, voltage regulator, then a rectifier is needed to make 6V DC power. A rectifier has a power loss (voltage drop) that is a problem at 6V so I mention there is no electronic solution out there.I would say it's not magically making more voltage than usual, it might have lower wattage (dimmer) lamps installed?

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 05, 2023, 07:42:26 pm
content: "It looks like it uses lamps with a BA15S single bayonet base. Guessing a 6V 5-10W tail light #63 or #81 and a 21W headlight #1129, say around 20W load total. A 50cc engine 1-3HP..."Wow again! How did you know that?

author: floobydust
author_rank: Super Contributor
timestamp: May 05, 2023, 08:22:10 pm
content: Look, there's some old dogs here, we know moped light bulbs lol. The BA15S socket was very popular for decades.You can pick the part number off the brass base on the old ones you have to confirm the wattage etc. It should be in the Sears Allstate manual?

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 05, 2023, 08:28:13 pm
content: Yes. I have manuals for both bikes. I wasn't confused about which bulbs to buy. I was wondering why I couldn't read the voltage on my meter and what might have caused my one and only bulb to blow and how I might prevent it. But mostly, I wish I knew some way to read the voltage. From there, I might be able to do some experiments and solve my problem. The numbers go all over the place WITHOUT regard to RPM. And, I might need to read VAC on some future project.

author: Kim Christensen
author_rank: Super Contributor
timestamp: May 06, 2023, 12:34:44 am
content: You might have better luck trying to read the average AC voltage with an analog meter rather than a digital one.Or.... If your digital meter supports it, turn off auto-ranging, and set the range to something like 20Vac max. The varying voltage is probably confusing the auto-ranging function in your meter.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 06, 2023, 01:23:50 am
content: Ahh! Sounds like a good idea. I'll try that. Thanks.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 06, 2023, 09:39:25 pm
content: I wonder if your moped is a Sears Allstate? And if so I think it's a Puch. Perhaps searching light bulbs for old Puch moped?

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 07, 2023, 01:13:17 am
content: It is a Puch. Thank you.

author: james_s
author_rank: Super Contributor
timestamp: May 07, 2023, 04:05:27 am
content: Quote from: langwadt on May 05, 2023, 05:32:08 pmAlso the energy is only wasted when one or both of the bulbs are burned out, with good bulbs the bulbs will load down the output and the shunt regulator will just sit there.

author: floobydust
author_rank: Super Contributor
timestamp: May 08, 2023, 04:42:28 am
content: I'm not sure the difference between idle and high RPM voltage-output would normally require no clamp at all. Lamps I'm guessing OK with +/-20% in voltage, the headlight #1129 bayonet BA15S is rated 6.4V 2.63 Amps 200 hours 21CP 265lm. Apparently that light bulb goes back to the Ford Model A in 1928. Wow.Sizing the generator coil for reasonable lighting at idle verses high speed not overvolting the lamps too much- it would be difficult not knowing/having consistent magnet stength.I found an #11296-12VAC LED bulbbut no spec on its minimum operating voltage. Then the tail light would get roasted so it would have to go LED, if OP converted.

author: james_s
author_rank: Super Contributor
timestamp: May 08, 2023, 06:03:25 am
content: Well if the voltage rises then so would the current and the bulbs would consume more power. They would be more efficient at producing light but at the cost of greatly reduced lifespan. I'm not convinced a shunt would cause a significant increase in engine load vs just letting it overdrive the lamps. If one wanted to get fancy, a full wave rectifier followed by a filter capacitor and a buck-boost SEPIC converter could maximize efficiency while delivering a constant light output and the whole thing could be small enough to tuck away discretely and cost under $20.

author: G-son
author_rank: Regular Contributor
timestamp: May 08, 2023, 08:10:03 am
content: These generators give a somewhat stable max current, so if you short it the current will not change as much as you think it would with changes in RPM - it's far from rock steady but kind of, sort of stable. The voltage on the other hand will pretty much follow the RPM if no current is flowing, making the voltage change very much without lamps connected. I've seen almost 100V out of 1980's era japanese 6V mopeds at high rpm.On top of this you have a points ignition right next to the generator coil, giving you interference that absolutely will have the ability to confuse a digital multimeter. So not getting any sensible measurements makes perfect sense, there's nothing stable about these systems in any way.They work good enough unregulated at 6V because 6V is so low, you are nearly at the short circuit current and as I said, that current is fairly stable. But this is dependent on having the right load, if you draw less current the voltage goes up, possibly blowing the bulbs. (A common mistake is to put 12V lamps in if the 6V ones keep blowing - that reduces the current used,increasingthe instability. Consuming a few watts extra solves the issue.)If these generators are regulated, a shunt regulator is the common choice. As the generator is shorted the voltage is practically zero, making the lost power small.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 08, 2023, 07:06:51 pm
content: In this particular application (my mopeds), faint lighting at idle is not an issue. What isclamping?

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 08, 2023, 07:11:25 pm
content: "... you have a points ignition right next to the generator coil, giving you interference that absolutely will have the ability to confuse a digital multimeter." Thank you for that tidbit. I'm glad to know that the crazy readings don't reflect my understanding of how to use a meter for a simple VAC check. I was beginning to wonder.

author: james_s
author_rank: Super Contributor
timestamp: May 08, 2023, 08:23:16 pm
content: Quote from: billbyrd1945 on May 08, 2023, 07:06:51 pmClamping is exactly what it sounds like, it's a shunt regulator that is in parallel with the load and prevents the voltage from exceeding a set value by shunting current around the load. It's effectively an automatic load that increases as needed in order to keep the voltage from rising past a point, like a mechanical stop that prevents a mechanism from moving too far. In its simplest form, a zener diode functions as a clamp.

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 10, 2023, 02:45:02 pm
content: Easy answer is to just keep a pair of good quality bulbs in it, as Floobydust explained, it is a workable system when complete.An analog meter may average out the chaos into something more interpretable, but the ultimate test of any electronic circuit is whether or not it performs as expected in actual operation. If you really want to get numbers, try measuring voltage with the proper bulbs loading the stator coils down, or check service manual for expected coil resistance readings and check that, but I'd just eyeball it. If it lights both bulbs at idle and they brighten up reasonably at cruise revs, you're good to go.

author: billbyrd1945
author_rank: Regular Contributor
timestamp: May 10, 2023, 10:34:37 pm
content: I agree with all that and ultimately decided that was good enough. The only problem with the resolution to keep two good bulbs installed is that blowing one blows them both (potentially). I can live with that, just a neighborhood moped.

author: james_s
author_rank: Super Contributor
timestamp: May 10, 2023, 11:39:32 pm
content: Keep some spare bulbs stashed somewhere on the bike, or add a shunt regulator that is set to to a voltage near the high end of what is acceptable. Incandescent bulbs suffer a very significant reduction in life if the voltage is excessive, something like 50% life penalty for 5% over voltage I think.