I need help with a shake flashlight project. The basic idea is that you shake a flashlight, charge up some capacitors, and light up some white and RGB LEDs. You can control the brightness and RGB LED color with some potentiometers and switches. They are all essentially in parallel (so the fV is not a big problem). The point is that if you are stuck in the woods or something, you don't have to worry about your batteries running out, and you just have to shake it a little bit for some constant multi-color light.

I have a neodymium magnet inside a thin plastic tube with foam taped to the ends (to protect the magnet). I wound some very thin 30 gauge wire around it for an agonizingly long time. I probably wound over a thousand turns. This is the generator part. It generates about 3-4 volts peak-peak AC (this is an estimate based on led brightness).



I didn't have any shottky diodes lying around, so I used some other rectifying diodes instead which have a forward voltage drop of .3-.6 volts (not 100% sure). This worked well, but given that it is not a square waveform (and probably sinusoidal), I need capacitors to get a good DC waveform after it is rectified. I decided to go overkill and use some 5.5 V 7.5 F super-capacitors with an ESR of 90 mOhms (in parallel). I wanted them to not only get stable DC but also act like rechargeable batteries.

This is where I am having problems. They are causing the LEDs to light up much dimmer (by consuming all the current) and not discharging (or even charging) properly. When they discharge, nothing seems to really happen They are also discharging into the rectifying diode and the the shake generator.

I would be OK getting some more capacitors and other components, but I do not want to have to have a bunch of complicated ICs. I need a practical circuit for converting the AC to stable DC, the charging the supercaps, and then having the supercaps ONLY discharge into the potentiometer controlled LEDs. The discharging can be controlled with a mechanical switch if it means I don't have to deal with complicated transistor/relay circuits.

 

First suggested improvement: get some schottky diodes. You will lose a lot of the generated power in silicon diodes in such a low voltage system.

 

I agree. You may be losing as much as half your power in the rectifier.

I'm a little surprised that your capacitor is so small. I would have expected that much capacitance at 5.5V to occupy a much larger package. I haven't kept up with these things and may be all wet, but it's worth double-checking the specs.

It may have a pretty high leakage current that is overwhelming the results. I know they don't make good battery replacements for this reason, but I wouldn't expect it to be as bad as your observations.

 

Could you suggest some good, inexpensive shottky diodes? Ideally they would also be from a place where there are minimal shipping costs.

With leakage currents, wouldn't you lose about 50% after maybe a few hours to a few days, not after a few seconds?

 

from a place where there are minimal shipping costs.

As you don't say where you are in the world, that might be a little difficult.

 

I am from the US. Not Russia.

So could you suggest some good schottky diodes?

 

It would sure be helpful to have a schematic of how you have connected these all together. If the cap is discharging into the coil, then that might indicate that your basic schematic is bad.

Also, have you considered how much shaking you are going to have to do to charge a cap that large up to a high enough voltage to get any brightness out of your LEDs?

Is there a reason you don't want to just buy a tiny little crank-powered flashlight? I have one that hangs on my keychain. Turn the crank for fifteen seconds or so and have quite a bit of light available for quite a while. Plus, when fully charged, the light will last for hours and the battery holds a charge for months.

 

So could you suggest some good schottky diodes?

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1N5817 is your standard run-of-the-mill Schottky diode

 

Here's one: https://www.mouser.co.uk/productdet...=sGAEpiMZZMtQ8nqTKtFS/D9SVzsgHTKGsrEMHLFTAoc=

 

Here's one: https://www.mouser.co.uk/productdetail/stmicroelectronics/1n5817?qs=sGAEpiMZZMtQ8nqTKtFS/D9SVzsgHTKGsrEMHLFTAoc=

Thanks. I'll consider getting that.

It would sure be helpful to have a schematic of how you have connected these all together. If the cap is discharging into the coil, then that might indicate that your basic schematic is bad.

Also, have you considered how much shaking you are going to have to do to charge a cap that large up to a high enough voltage to get any brightness out of your LEDs?

Is there a reason you don't want to just buy a tiny little crank-powered flashlight? I have one that hangs on my keychain. Turn the crank for fifteen seconds or so and have quite a bit of light available for quite a while. Plus, when fully charged, the light will last for hours and the battery holds a charge for months.

I want it to be more DIY, shake-able so it will charge more automatically, and AC is better because you can do more with it. Buying it defeats the purpose.

Also, this is the schematic:

So what capacitors should I ideally use to get good DC and how do I incorporate the supercapaciators into it?

 

Try leaving the LED load off and measure the cap voltage while you shake. Plot voltage against number of shakes (or time) and see what's going on. It would help to have another set of hands to record the data.

And I'm sorry, but I can't keep the Shake Weight vision out of my head, which reminds me of <SNIP>. I was going to link the videos but thought better of it. Follow your own rabbit hole.

 

Ironically, batteries for my multimeter will not arrive until a week from now, so I will have to wait for that. Should I shake the thing, disconnect the cap, and measure the cap voltage, or is there a better way to measure the voltage? Also, what do I do about the whole situation with the cap discharging into the rectifying diodes and inductor? And, should I add some normal capacitors in series that will charge and discharge much quicker?

 

If your diodes are working and connected correctly the capacitor should not discharge back through them in any reasonable time.

 

Ironically, batteries for my multimeter will not arrive until a week from now, so I will have to wait for that. Should I shake the thing, disconnect the cap, and measure the cap voltage, or is there a better way to measure the voltage? Also, what do I do about the whole situation with the cap discharging into the rectifying diodes and inductor? And, should I add some normal capacitors in series that will charge and discharge much quicker?

That wouldn't work. Putting capacitors in series or parallel simply changes the combined capacitance in this particular case.

You don't need a bridge rectifier. Just use a single Schottky diode with a low reverse leakage current such as BAT20.

 

Your LED load effectively 'clamps' the super cap voltage at about 3.2 volts, (assuming you have a white led connected directly) any increase in voltage shunts the current into the LED immediately, almost none is stored in the cap.

For this scheme to work, you need an LED circuit that draws a constant power over a wide range of input voltage, so the excess power goes into the cap, increasing it's voltage.

 

You can leave your voltmeter attached while measuring the effectiveness of the generator, and Sensacell suggests, you will have to keep the LED disconnected during charging and turn on the LED later.

A bridge is usually used to the energy from shakes in both directions can be used to charge the capacitors. You can experiment with the types of rectification but using a bridge would decrease changing time significantly in my own opinion.

There is a very good chance that if you shake long and fast enough that you will charge the capacitor to a voltage that can damage the LED when you connect it to your 90 milliohm capacitor bank. A small series resistor -20 to 30 ohms (just guessing) could save the life of your LED on those occasions.

 

That wouldn't work. Putting capacitors in series or parallel simply changes the combined capacitance in this particular case.

You don't need a bridge rectifier. Just use a single Schottky diode with a low reverse leakage current such as BAT20.

Isn't a full bridge rectifier much more efficient and better?

 

Isn't a full bridge rectifier much more efficient and better?

A full wave bridge will reduce your supply voltage by two diode voltage drops instead of one.

 

A full wave bridge will reduce your supply voltage by two diode voltage drops instead of one.

But that voltage will be available twice as often delivering more power.

 

Also, if you were charging a 7.5F 5.5V cap at 3.5V and ~150 mA, how long would it take to charge it to 3.5V?

I think that unless the diodes are really bad, a FULL BRIDGE RECTIFIER would be best for the quickest charging. And the rectifying diodes I have are not too terrible. I am probably going to go with one.