Choosing capacitor

I have a unusual projection mind. The basics are that I’ll have a dc motor act as a generator. The motor will be rotated in one direction and then the rotation will be reversed. There is no way of calculating the length of time or the actual power output per minute. The motor is 12 volt. Not sure of amperage. Playing with a multimeter I found that that in one direction it produces a positive out put and the other will show a negative on the multimeter. To stop this difference I thought a bridge rectifier will produce a current in one direction only but I still have no way of guaranteeing a continuous regular current output. Thinking that a capacitor should fix that problem but how do I decide the correct capacitor. Aiming to pull 12 volts out of the system while testing the apparatus. Also what’s the best way to test that actual output.

Hi Rodney

Quite normal.

Correct. Power bridge rectifiers have all 4 diodes in the one package. Connect the generator to the pins marked AC or with s Tilde ~. Then positive and negative will come out of the pins marked so. Does not matter which way around the generator is connected.

You first have to spin the generator fast enough to generate some volts.

A capacitor won’t do anything to regulate current. It may however clean up any hash and spikes.

The generator will only output the current the load requires. Just because a generator might be rated at 20A if the load only requires 10A that is the current that will flow.

The generator (motor) may output up to 14 or more volts. If for some reason you can’t accommodate this and can only have 12V you will have to regulate the VOLTAGE, probably electronically You just have to make sure that you don’t exceed the generator safe current limit. You can limit current but it is not as simple as it looks. If you need to a DC-DC converter that regulates voltage and limits current to preset levels would be easiest. Just how much current do you anticipate needing.

Incandescent lamps. Go down to your nearest auto store and get yourself some lamp sockets and a few 21W incandescent globes and wire the sockets in parallel. Keep plugging in globes until you get the current you need. It will be about 1.75A per globe.
Cheers Bob


As Robert said, a capacitor will help clean up hash and spikes, but is limited otherwise.

A large electrolytic capacitor after the bridge rectifier might also help to smooth brief dropouts, up to (say) 10 or 20ms. During a dropout the capacitor’s voltage will decay, with a rate dependent on the current being drained out of it. To give you an idea of how fast it will decay, the basic formula is:
I (in amps) = C (in farads) x (dV/dt) where dv/dt is the rate of voltage decay in volts per second.

More simply, if you know the load current I:
Voltage Decay Slope in volts per second = I / C, where I is in amps and C is in Farads. For example a 1000uF capacitor is 0.001 Farads in this equation.

This will allow you to predict roughly how long a capacitor will smooth over a brief interruption of say 10 or 20ms.

For longer interruptions you would need to consider charging a rechargeable battery - which may need some extra circuitry for battery management. If so, I would suggest considering Lead Acid Gel Cell batteries. Lead Acid Gel Cell batteries are relatively straightforward to use, and they don’t have the fire-hazard risks that Lithium Ion batteries have if under-charged and over-charged.


Hi All
I think at this stage we need to know exactly what type and size of this motor. I and probably Edward have assumed it is quite large and the current is in 10s of amps. Then it could well be a little hobby motor in which case my load using lamps will not be much good,
Info please Rodney.
Cheers Bob


Thanks for the replies. Yes it’s only a hobby motor. And smoothing spikes to get a smooth current flow is the correct interpretation of using the capacitor. I had planned of using a battery as storage but as output can’t be guaranteed to be 12 volts thought a capacitor might be the way of building a voltage that can charge a battery at 12 volts. I’d think that time between direction changes and therefore stop and full rotation speed will be more then 10-20 ms so maybe the capacitor is not needed and just use the lead acid battery which still has me confused when it comes to defining its output as the storage capacity removes the load directly from the generator. Unless output can be defined as time taken to recharge the battery from say 10 volts to 12 volts but then how do we define wattage when amperage is in amp hour when defining a battery. If a 12 volt battery is rated for 100 amp hour is it a 1200 watt battery and therefore at 10 volts will only have storage of 1000 watts. Is it as simple as saying that if the unit takes 1 hour to charge a 100 amp hour battery from 10 volts to 12 volts then the units output is 200 watts.

A battery is the only way you will get smooth output from a generator that changes direction.

A lead-acid battery doesn’t have to be 12 volts. See, for example:

However, if the output from the generator can vary you will need a regulator (in addition to the bridge rectifier) to ensure you don’t exceed the charging parameters for the battery you choose. That will be the case for any style of battery.

There are many parameters the define the battery characteristics, other than voltage. Capacity is Amp hours and defines the time over which a fully charged battery can supply a defined current until it reaches the nominal discharged level. Watts is related to an instantaneous measure of power and will involve things like the battery voltage, the load and the internal resistance. It cannot be compared to capacity without also considering time.

An alternative to your proposed setup is mechanical - the DC motor drives a large flywheel though a gearbox that is connected to a generator. The gearbox converts the two-way rotation into a single direction, while the flywheel takes up the variations in speed as the DC motor changes direction.
Mechanism for converting two-way to one-way rotation 2 - YouTube

A flywheel and gearbox as in the video is a good solution. Possibly a bit too involved at this stage. The way in which it’s driven ( gets its energy/ movement ) is only a concept and looking to keep things simple but it would be nice to see that the concept can do more then just capture the energy and create movement. It would be nice to be able to say it’s generating a certain amount of watts and if we change a part of the concept it now produces +/- amount of watts. I’m more into physics then the electric side of things and getting anyone to work with is impossible unless it’s government funded or something you can buy so I have to understand the electrics to build it myself at this stage. Appreciate all of your input. I had thought of two one way bearings to drive a flywheel arranged so that each direction delivers power to the fly wheel and that may be the next part once it can show potential.

Hi Rodney

A Capacitor can only get to the voltage that is charging it. No more.

While we are on this subject do you know what the maximum voltage this motor cum generator will produce. The output from your bridge rectifier will be generator volts MINUS 2 diode voltage drops, that is generator volts MINUS 1.2 -1.4V.

A 12V lead acid battery will need 14.4V to fly charge it.

That is 1200 Watt HOURS, as in Amp HOURS only converted to watts. (Volts X Amps)

Does not quite work like that. Anyway you should only discharge a lead acid battery down to 10.8 or 11V or accumulative damage will happen thus shortening the batty life appreciably.

You are tying yourself into a bit of a knot here with battery ratings and what they mean. A 100AHr battery is only that at a discharge rate of 100/20 or in other words any lead acid battery is rated at a C/20 discharge rate where C == battery capacity in AHr. The available capacity decreases as the battery discharge rate increases. This de-rating is quite dramatic when you get to high discharge rates. So much so that at extremes say 100A which is C/1, this time to 10.8V is only a few minutes instead of the expected 1Hr. There are curves published regarding this. If this interests you I would suggest you get online and do some reading on lead acid (and other chemistries) charge and discharge rates and their effects.

You are probably right there. I have only see a flywheel use where a back up supply absolutely has to be “no break” The flywheel is used to keep things going until the back up (usually a diesel engine) gets going. The flywheel weighed several tons and actually started the diesel via a magnetic clutch which engaged on loss of mains power. Believe me that diesel got spinning in a hurry. Tremendous forces involved here and the whole thing worked smoothly and very well designed.

I don’t know what you are trying todo here, project or experiment, but there is always going to be an appreciable pause while changing direction. With this in mind a battery would be the best solution but to float charge you need something like 13.8V which mesne something like 15 or 15.2V at the generator using a diode bridge.
Cheers Bob
PS: Just noticed this

You can’t have negative watts!!!

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Can’t have negative watts only meant that it produced 100 watts and now the concept has been tweaked it produces + 20 watts or it produces less 20 watts giving an idea of what works and what doesn’t. I’m going to have to wait and see what peak voltage is before deciding on the battery as everything is unknown at this stage. Not needing the capacitor makes the build a bit more straight forward at this point though.

The motor is 12 volt and stall amperage is 23 amp with max power amperage at 13.44. Not going to get near that output though. I also have a 12 volt 500 watt starter motor that could be used although I’d need to machine the housing and fit proper bearings. The hobby motor will do for a start. Also have a permanent magnet motor laying around but it’s hard to turn even by hand without any load attached so that might come much further down the track.

Hi Rodney

That is a pretty good sort of “hobby” motor. That is the term I associate with those small motors you use to drive small robots etc. All pretty relative I suppose. That 13.44A I would consider as maximum output as a generator for any design criteria.
Is this hobby motor not a permanent magnet type? I would think it would be. If not it will have a field coil which has to be excited and may not work too well as a generator.

There is one thing to think of if you are going to dabble with higher currents. A motor is designed to be a motor, a generator is designed to be a generator. While a motor will work as a generator and vice versa there is a difference.
A generator brushes are not quite in sync with the position of the rotor in relation to the magnetic field (either permanent magnets or field coils). They lead or lag slightly, I don’t remember which. This exact position is selected for minimum arcing at the commutator contacts. In fact some of the higher current generators had the back plate (which holds the brushes) adjustable over a small arc to facilitate adjustment. I think some old automotive generators had this facility. If the generator is reversed in rotation the brushes are no longer in the optimal position and the commutator and brush connection will arc severely which not only shortens the life of these components but is extremely noisy and generally “dirty”.

Now a motor used as a generator may not have the brushes in the optimal spot but the best scenario will be in sync with rotor position. I think that would be the case as DC motors are designed to reverse by reversing voltage.

For the reasons outlined above your choice of motors used as generators is probably a good one as you want to be able to rotate in either direction but they could be a bit noisy at higher current.

I have had personal experience with reversing the rotation of DC generators. We had an automotive generator mounted up and driven by a small petrol engine as a small emergency plant. This set up was meant to float charge HF transceiver batteries. Due to mechanical requirements this generator ran backwards. Turned out to be so noisy the radios could not be operated with this generator running. Had to arrange for the generator to rotate in the correct direction.

You should be able to turn this OK unless it has a gear box attached. If not could be a short circuit or some other load on the connections which would make turning difficult or pretty much impossible.
Cheers Bob

The permanent magnet motor came off a cheap mini lathe. Variable speed drive. It shorted out so had it rewound but then I destroyed the variable speed electronics twice ( it was definitely my fault ) The lathe was not what I expected and I couldn’t be bothered spending anymore money on it so I keep it around for parts. I have hooked a drill up to it which makes the drill work hard to generate a current out of it connected to a 12 volt 100 watt light.

Hi Rodney

That has not answered this question

It probably would if you still had the defunct speed control connected, even so 100W might be a fair load for a drill, depending on what size the drill is. That is about 8A. if you expect to get 100W out you will need to put something more than 100W in.
Cheers Bob

Motor wasn’t connected to speed controller. Hobby motor is usual hobby motor. Magnets around the outside and rotating wound core with commutator

Thought it would be magnet. A big sigh of relief, If it had been a field coil motor that would have been another ball game getting it to be a generator.
Cheers Bob

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Reading through your posts and the various replies, I would suggest getting a controller that a wind generator is connected to.
This will fast track traversing through many of the issues without having to reinvent the wheel, as it sounds like you are just after a solution.

A wind generator controller will provide you with an input within the likely tolerances your motor/generator is able to deal with, output (at your required voltage), Battery output (for energy storage, charge and discharge).

Years ago, we used to disect fisher and pickel smart drive units (re-jig the stators for the required voltage 48V, 24V, 12V. effectively we wired such to be 3 phase which made the stator easier to turn as not all the voltage peaks happened concurrently. Depending on the model of stator they ranged up to 600W. Stators were fixed and the magnets ran around the outside of the stators.
If you rebuild the magnet ring outside, you would be able to replace each rare earth magnet with higher flux ones. which should enable you to produce even more electricity, although upsizing some of the wires coming from the stators might be required.

Specs are easy to find online, as are rewiring tutorials.

In the wild (during conventional use), they are designed to spin one way then spin the other way (as washing machines do).

As for fly-wheels because of the way smart-drive units are designed and built, it might work out easier for facilitate. Solutions for converting 2 directional to 1 directional are out there, and I would highly recommend looking into this as the fly-wheel (mass of the motor spinning) would be required to come to a complete stop before reversing direction (velocity and moments of force come to mind from my first year Engineering math), thus lag and energy loss will result from spinning down stopping and reversing to spin up again.

I believe this could be a solution in need of your problem - even if it happens to replace your initial proto-type.

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Hi Rodney,

This sounds like a cool and unconventional project but I’d agree with @David61301 's approach and avoid as much as possible reinventing the wheel. If there is an existing device out there that already does what you’re attempting then you can take advantage of the hours of optimisation and design that’s already been done with the off-the-shelf generator and redirect your energy and effort into finding a way to integrate it into your project.
DIY projects break the mold a little bit as sometimes you want to build it yourself but for something as complicated as generator design I’d stand on the shoulders of others as much as I could. Integrating an existing generator into your design will still of course come with its own set of DIY challenges.

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