Aznman

I am currently using a dynamo powered headlight. I have a bright battery taillight separate from the dynamo. The batteries are rechargeable, but I often forget to charge them.

I want to convert by taillight into a dynamo powered one. I pretty much know what to do except for the capcitor. I found an old capacitor in my large collection of spare parts.

Please look at the picture closely


I have some questions:
1. Has anybody ever open these types of capacitors up? I'm afraid of opening mine.

2. I know that this is AC powered.
How can I quickly convert this to DC?

3. This capacitor was designed for high voltage, so it is dangerous. How would I design it to only discharge 8 volts and 3 watts to power my 20 lumen taillight for about 1-5 minutes?

Thank you very much

unterhausen

capacitors only hold the voltage that they are charged with, so they are not inherently dangerous. I don't know if a motor starter cap is the best choice. You don't want to open it up, it will let the magic out. Search for standlight circuit. I don't know if znomit will comment, here is a post of his

fietsbob

that is the function of a full wave bridge rectifier.

Schematics :
https://www.google.com/search?q=full...w=1808&bih=916

CliffordK

There is nothing serviceable inside. No need to open it, unless it is broken, or non-functional.

Essentially a capacitor has two plates separated by a gap, and collects the charge between the plates. What you benefit by opening it?

Using diodes or a rectifier.
Capacitors can help get rid of ripple if it is a problem. However, some ripple may not be an issue for your lights.

It is easy enough to "short" a capacitor to remove any stored power. Some are wired with a small resistor between the two contacts to help them self-dischage.

You can limit it to 8V by only charging it with 8V. It does not magically get more voltage than you charge it with.

Your generator, however, may not give you a constant voltage output

Aznman

Thanks for the advice guys. I know that it is "amp that kills", but I heard that high voltage can be dangerous too. See Mythbusters shock treatment of Adam Savage. I thought that 2100 VAC is not healthy?

If I am completely wrong about this, why is there a label that says "Danger H.V." at the bottom?

znomit

Here's the stand light thread.
Standlight circuits (Will this circuit work?)

Whats the output from the headlight?

Aznman

About 100 lumens, so about 1 watts right?

wphamilton

discharge it by shorting the leads using something with an insulated handle.

1 microfarad isn't huge and probably wouldn't have enough current to deliver much of a shock. Very high voltage capacitors can charge electrostatically by themselves, or so I'm told, so I'd just discharge any big capacitor on general principle before handling it.

CliffordK

It is a combination of Volts + Amps that "kill".

For example, your standard car battery puts out 12V, and perhaps 1000A. More than enough amps to stop the heart, but under ordinary conditions, the skin provides adequate insulation to prevent getting shocked. So, while I've seen spectacular sparks with tools, I've never been zapped by a car battery.

Periodically I'll get a tingle from 110V, and believe it is probably the full force of 110V, but it is usually just enough to make me want to move my hands.

An electric fence, on the other hand hits a person with high voltage, low amps for a brief amount of time to give a strong, non-lethal jolt.

Even if the heart is stopped, it would typically resume once the power is removed unless there is some other underlying problem. However, there is also a risk of steady power causing contractions and making it difficult to release.

Anyway, caution is advised, but you should be fine as long as you're running down in the 12V range.

Aznman

Thanks guys.

ItsJustMe

That is exactly the wrong capacitor to use. I bet you got that from a microwave.

High voltage means it can HANDLE high voltage, not that it automatically has high voltage. it's not an amplifier. An extension cord can handle 110 volts, but you only need to be concerned about it if it's plugged in to 110 volts.

It says "danger high voltage" because it's DESIGNED to be part of a high voltage circuit. When in a high voltage circuit, it has high voltage in it. If you hook it up to 5 volts, it will have 5 volts in it.

High voltage capacitors are not magic voltage amplifiers. It simply means that they have thicker plates and are able to have more voltage applied to them without the insulation breaking down and arcing internally. The thicker insulation (or dielectric) also radically reduces the capacity of the device to store electricity. The capacity is related to the square root of the thickness of the plates. Go with thinner plates, you get far more capacity (in watt hours) but it can take less voltage before it breaks down and fails.

So what you want is a capacitor with a lot more capacity than this (this probably has maybe 1 microfarad, maybe enough to run a taillight for a tenth of a second or less). I'd guess that what you WANT is more like a few thousand microfarads or more. To get this in a reasonable package size, you want the voltage rating of the capacitor to be as low as possible - double what you expect to see in your circuit is a good starting point.

Also you need to put in diodes and current limiting so that the capacitor charges off the residual power and doesn't hog all the power and keep your light from working until it's charged.

Aznman

I have some more questions.

Instead of a capacitor, can I instead use my dynamo to charge two AAA rechargeable NiMH batteries ?

Of course, batteries have a life limit unlike capacitors (theoretically speaking if I understand things correctly). If the batteries die, I always carry an extra non-charged pair. I think it would be awesome, but I have a problem: although NiMH are rechargeable, can they be "charged" and "discharged" continuously like during the stoplights scenario? Would they deteriorate faster than if I use them normally (use until they run out of juice completely and then charging them for 9 full hours as instructed)?

Aznman

Thanks very much for this highly informative post. I will do as you say.

unterhausen

it really depends on the circuit you have hooked up to it. It's amazing what you can do with a diode, an inductor and a transistor. But in any case, it can't store more voltage than it sees across its terminals. The rating just tells you how much voltage it can handle without smoking. Which, in my experience, is mostly unpleasant because of the smell

noglider 

Charging your stuff with a dynamo is complicateder (to coin a word) than you might think. Chargers that hook up to bike dynamos are surprisingly expensive.