Showmans engine generator

A permanent-magnet alternator is a fairly robust device. If you can find one with eg six windings around the stator, it may be possible to detach the windings electrically and use them in opposite pairs, with a switch to bring two, four or six into use. This would allow you to vary the output to match the load. Motorcycle 12v zeners are available, and will cope with about 100 Watts, enough to run the LEDs on a full-sized engine ...

Look closely at the workings of the small brushless motors used in models (not just aeroplanes) and you may well find simpler single-phase versions. In this design, all the magnets line up with a coil at the same time, whereas the 3 phase versions do it in sequence.

If you could find someone local who played with motorcycle alternators in the sixties and 70s, make friends.

Cheers, Tim

Scrumpy ,

Would the Stuart Models dynamo be suitable , albeit expensive ?

Trevor

Schottky diodes are worth using.

Possibly why half-wave rectification was used?

I have a number of generators and alternators, this is a 6 pole, shaded pole motor, modified with a permanant magnet claw type armature. The shading rings have been cut, but this makes little, if any difference. It uses 2 Ferrite magnets from old speakers, another source of these magnets is old Micro Wave ovens.

Ian S C

Edited By Ian S C on 11/10/2016 10:06:16

Just to muddy the water last night a friend called with a permanent magnet motor from a small wind turbine on the can is printed 36w-12 v-24 v is this a better option for me as the size is perfect if so what regulator would I need as I have bought the LEDs these are Vaol-10 gwy4 rated 20 ma /3.5 forward voltage what ever that is there would be 50/60 of them over to you people as I find this all a black art wrong age thanks in anticipation Derek

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First step is to undertand what that spec. means:

Each LED will draw 20 ma, when supplied with the specified 3.5 volts.

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You can connect them in series, or parallel, or any mix thereof ... the voltage and current for the full set will depend how they're interconnected.

 12 parallel groups of 'four in series' running on 12 volts would seem a good start.

MichaelG.

Edited By Michael Gilligan on 11/10/2016 10:56:31

LEDs behave differently from incandescent bulbs - a small change in the voltage across them means a large change in the current going through them. It is always necessary to control the current somehow, usually by making the supply voltage a good deal higher than the voltage across the LED(s) and adding a resistor in series. For example to run a single white LED (3.5V forward voltage) at 20mA from a 12V supply one needs a resistor to lose 8.5V. Ideally that would be 425 ohms, which unhelpfully isn't a standard value but 470 ohms is standard and would only reduce the current slightly. Two LEDs in series would have 7V across them leaving 5V for the resistor (250 ohms ideally this time), but you have to be aware that the smaller the voltage across the resistor the more sensitive the arrangement is to variations in supply voltage and LED voltage, which can change with temperature. Series and parallel wiring works but you always have to consider how the current is being controlled.

In view of your lack of experience with LEDs and their oddities, can I suggest a way forward?

You could settle on a voltage standard for which there are many regulators around - 12volts - as used on all cars and almost all motorcycles for lighting etc. This would also make two other things easier - the supply of a range of suitable LED bulbs, without head scratching over voltages and resistors etc; and the possibility of adding a battery supply so that when the engine is not running you can display the lamps.

The wind turbine seems ideal for this sort of use, and regulation could rely on a motorcycle-type Zener diode. These will cope with an alternator output up to 150 Watts, given a sensible heat-sink (or 'radiator, in unhelpful car-type terms). This is well over the output of the 36W wind device, but a regulator which will cope with more than you have got is going to last for ever.

You will need to play with the generator driving a small pulley on the generator with a fairly big one to suit the engine, and this will tell you if these sizes are going to give you sensible output (ie that you don't need another gearbox etc).

It seems a shame to abandon the LEDs you have bought, but you should be able to get some joy by connecting a few of those you have, in series, and driving them with a car battery. If they are 3.5V each, then four in series should work, but try six and five, too. Then when you are happy that one 'set' does what you want, further identical sets can be added in parallel. And if these terms are new to you. come back to us.

And don't forget that the generator will produce AC, alternating current, but the LEDs need DC, direct current. This means you need a 'rectifier' - again a simple matter of a few quid (or lots more from a motorcycle spares shop). The wind generator may well have a rectifier within it, so check this detail. It certainly would if it was set up to charge a battery on a boat, for example, but you might not have got the complete kit.

I am tempted to say 'Bring it all round and I will try to get something going for you' but I don't suppose Ludlow is just up the road, is it?

Hope this helps

Cheers, Tim

PS I suspect that the generator shown by Ian S C is going to be too big for you (in electrical terms). It looks like a full size motorcycle stator (the outside ring of coils) and so could be expected to give 200W or thereabouts, at about 2000rpm upwards. The rotor is a home made mod, replacing an electromagnet - so it will need a big regulator too, as well as a powerful rectifier. It will also be capable of powering a boxful of LEDs.

Cheers, Tim

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Quite so ... That''s why I suggested "a good start" ... for the subsequent design process.

Constant current regulators are a better arrangement than dropper resistors; but whatever supply is decided upon, the basic choice of how many LEDs are in circuit is a good place to start.

MichaelG.

There's a relevant app note from Infineon here that outlines some of the design considerations. Nothing clever about the constant current driver they are offering, as it's just a linear circuit incorporating a transistor, 2 diodes and a couple resistors.

You could easily knock something up yourself with guidance from here. Otherwise, they are ready to go, offering typically 10-60mA and up to 40V. Obviously you'd need a small PCB with heatsink or heat spreader.

It's pretty easy to wire a low drop out voltage regulator as a current limiter.

Google will provide circuits & formulas

Choose the regulator for Vref as small as possible.

Neil

Edited By Neil Wyatt on 11/10/2016 16:31:44

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Neil & Muzzer

I was simply doing Scrumpy the courtesy of taking this in gentle steps:

Thanks for posting the Infineon note, Muzzer ... It's the obvious choice, but I had mislaid it.

MichaelG.

I should probably have mentioned constant current drive (like Michael G I was starting from basics) but now others have dealt with it. It is certainly the most efficient approach if you have a high supply voltage to take advantage of.

In fact the efficiency will be pretty miserable, especially at high input voltage, as pointed out earlier in the thread but it's probably the simplest solution. For efficiency you'd want to use a switching converter but that would add to complexity. Linear have quite a range.

Something like the LTM8042 boost regulator can operate over the range 3 - 30V with efficiency over 90%, although the LT3519 is a more versatile beast specified for automotive applications.

I've not used any of these but there are a range of options, depending what you are looking for.

I would like to thank every one for all there help and advise especially Tim sadly i live on the Cornwall Devon border with all the information I have my brain is trying to evaluate it all

My first decision is which of the two Outrunner motors I have I use along with the LEDs so only a regulator required but what along with other components

Do I use the permanent magnet motor from the wind turbine along with the LEDs again only a regulator required but which one along with other components

I would like to use the main body of the engine as a heat sink if one required

But I feel at this moment its away to a dark room with a drink Derek

Hi Muzzer

What I meant was that constant current drive is the most efficient approach if you have a number of LEDs to drive and you have a high enough supply voltage (e.g. 12V) that you can connect a number of them in series, leaving just a few volts across the regulator. Under these circumstances I'm not sure that a boost converter with its own losses followed by a constant current regulator would end up being any more efficient, and it would be more complicated. However I haven't done any real world calculations on this arrangement.

There was a comment about why would a single diode be used for half wave rectification.

The answer, I think, is that the particular generator used was voltage challenged and the 1.2V drop from a full bridge rectifier could not be tolerated while a 0.6V drop was just acceptable.

Re the series connection of LEDs. I think you'll find that you can daisy chain LEDs of the same colour but the different colours will need different currents to deliver equivalent brightnesses, Hence a separate circuit for each colour of LED.

The alternator in my photo has an over all size of 4" diameter x 4" length. My motor/alternator was I think from an older model ceiling fan. Derek, the motor used as an alternator on a wind turbine would probably chosen to generate at reasonably low revs, probably less than 500 rpm if it was direct drive from the turbine.

Ian S C