Dynamo to charge a battery, to power lights and a USB port

[JesusCB]

I have several questions about a project I am trying to make, I want to use a dynamo, to charge a battery,  use the energy in a usb port and 2 led lights (for a bike).


Parts I think I need:

• Bridge rectifier
• Bottle dynamo (12V 3W)
• Battery [Lead acid] (6V  4mah)
• Voltage regulator ( I think the best option is to use something based on the LM2596 or LM317)

What I know:

• They are called dynamos, but in reality they are alternators.
• I have used the LM317 regulator in the past, so I know (kind of) how to regulate the voltage with this IC.
• To charge the battery I can use constant voltage, for that specific battery I need 6V+1.5V=7.5V
• LDOs get hot depending on the voltage dropout and the current, and if I use this option I should use a heatsink
• I did some math and I found that If I use the LM317 I will have around 3-4W of heat that I need to dissipate.
• I will need to pedal a lot of time to charge 100% the battery so I will add an external AC wall adapter with 7.5V DC to charge the battery at home

What I dont know:

In the store, it says that it is a 12V dynamo, but that is RMS? or Peak to peak? (I think I will have to buy it and test  with an oscilloscope in the school)


Circuit

12V Dynamo ->Bridge rectifier -> 7.5V Regulator (LM317)  -> Battery -> (LM2596 Circuit with 2 USB outputs, and 2 lights).

Note: the LM2596 circuit has an output of 5V and 3A max (to charge my cellphone I only need .5A ) (I bought a cheap one from DealExtreme that I will test when I get it)


Do you have any tip or trick that could help me?

PS: I read 3 other questions related to dynamos, but I could not get an answer from those.

[Ian.M]

Permanent magnet alternators have very poor regulation with the output voltage more or less proportional to the speed.   The 12V rating is purely nominal and a guide to what voltage bulb is likely to work well with it at typical road speeds.

[JesusCB]

I did some test with a DC motor, I used it like a generator, and I got up to 30 V, I don´t want to waste my energy to get 30V and then regulate it to 5-9V.

Ian.M Thank you, I have been reading, and some people say that you get the 12V when you ride at 15-20 km/h.

I wanted to use diodes to lower the voltage but that depends on the current, and in my case the current is not always the same. (That is why I think the LM317 is perfect)

What do you guys think about my idea?

12V Dynamo ->Bridge rectifier -> 7.5V Regulator (LM317)  -> Battery -> (LM2596 Circuit with 2 USB outputs, and 2 lights).

[Ian.M]

That wastes a low of power.  You need a switching regulator (buck converter) to use the available output from the generator more efficiently.   Start with a  bridge rectifier into a 63V reservoir cap then a  LM2576HV module(to allow enough voltage headroom for a wild downhill ride without blowing it - readily available on EBAY) down to your battery charging voltage. 

Be aware that a 6V 4AH SLA gel battery has a fast charging voltage range of about 7.2 to 7.35V and once the charging current has dropped to 40mA, you need to switch to float charging mode, where you maintain a voltage of 6.8V as long as input power is available.   Do NOT restart fast charging until the battery has been discharged 10% or so (detect the terminal voltage under load).  Float charging can be resumed immediately input power is available again.

This involves a circuit to switch in an extra resistor into the LM2576's feedback network to allow two different voltages.  The easiest way is to switch it in in parallel to the lower arm of the feedback divider where it will act to boost the voltage.   The circuit is going to need some intelligence and if you have PIC, AVR or Arduino programming experience, here's where to use it.

For the rest of it, once you have a stable battery supply, its no big deal to use another switcher to regulate to whatever voltage(s) your lights and USB charging socket need.

[JesusCB]

That wastes a low of power.  You need a switching regulator (buck converter) to use the available output from the generator more efficiently.   Start with a  bridge rectifier into a 63V reservoir cap then a  LM2576HV module(to allow enough voltage headroom for a wild downhill ride without blowing it - readily available on EBAY) down to your battery charging voltage. 

Be aware that a 6V 4AH SLA gel battery has a fast charging voltage range of about 7.2 to 7.35V and once the charging current has dropped to 40mA, you need to switch to float charging mode, where you maintain a voltage of 6.8V as long as input power is available.   Do NOT restart fast charging until the battery has been discharged 10% or so (detect the terminal voltage under load).  Float charging can be resumed immediately input power is available again.

This involves a circuit to switch in an extra resistor into the LM2576's feedback network to allow two different voltages.  The easiest way is to switch it in in parallel to the lower arm of the feedback divider where it will act to boost the voltage.   The circuit is going to need some intelligence and if you have PIC, AVR or Arduino programming experience, here's where to use it.

For the rest of it, once you have a stable battery supply, its no big deal to use another switcher to regulate to whatever voltage(s) your lights and USB charging socket need.

I have read that those modules have a lot of voltage ripple, will that affect anything? I think that adding some capacitors would lower the ripple.

For the usb output I will use a "ZnDiy-BRY DIY DC 6~35V to 5V 3A Double USB Voltage Step Down Regulator Module " as you state they are more efficient, let me search more about "float charging"

Thank you

EDIT: I have used an Arduino UNO, but maybe something with the Attiny would fit better. I need to read a lot!!!

[Ian.M]

The battery wont mind the ripple and will smooth it more than any practically sized bulk capacitor could.  You only really need to be careful with the USB regulators where choosing a module with a precise low ripple 5.2V output (as some devices don't charge well at 5.0V)  and good over-current protection would be advisable.

[PeterFW]

A stepper motor salvaged from anny sort of old tech can be used as a generator for zero cost.

[JesusCB]

A stepper motor salvaged from anny sort of old tech can be used as a generator for zero cost.

I tried with a DC motor and the output is 34V, I would waste a lot of energy

[Ian.M]

The whole point of using a switched mode converter is that it transforms input I*V to output I*V with low or very low power loss.  As long as the worst case maximum input voltage is less than the converter's maximum input voltage, all is good.   If you use the LM2576HV module its rated for 60V max, so 34V is OK.

[JesusCB]

The whole point of using a switched mode converter is that it transforms input I*V to output I*V with low or very low power loss.  As long as the worst case maximum input voltage is less than the converter's maximum input voltage, all is good.   If you use the LM2576HV module its rated for 60V max, so 34

Thank you Ian, I did an adjustable regulator with the LM2575, I tested it with an input of 19V DC and it works fine.

I will test my circuit with an oscilloscope next week.

[Ian.M]

The last sentence of my previous post got chopped off, don't know how don't know when.   I was going to say that you should design with a generous margin, i.e. I wouldn't push a 60V max input buck converter past 50V and would prefer 45V in.  You need to find out the maximum input voltage which will be obviously dependent on the generator used and how fast it can be spun, which you should be able to calculate from your maximum road speed and the gearing of the drive-train used.   

I wouldn't push an ordinary LM2575 much past 30V in so if you want to use it with the motor as a generator you tested at 34V, get the HV version.

The input voltage will drop under load of course, but you always need to have enough margin to cope with the load being switched off, and enough bulk capacitance at the input to damp any transient spikes to under the converter's max input rating.  A polyfuse between the generator and the input cap + a beefy 55V Zener shunting the cap to limit the peak voltage wouldn't be a bad idea if you cycle in areas with steep hils.