DO IT MYSELF Tailight Wiring

[MinasIthil]

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

 

[Steve K]

....I pretty much know what to do except for the capcitor. I found an old capacitor in my large collection of spare parts.

I'll bite... what are you trying to do? i.e. when you say "I pretty much know what to do...", what exactly do you know?

As a quick overview of powering a battery powered light from a dynamo, the first step will be rectifying the dynamo's AC power, which produces a DC voltage with a lot of ripple. A filter capacitor will likely be needed at the output of the rectifier to avoid having the voltage drop to zero during each AC period. Assuming that the light is powered by 3 volts (two AA or AAA batteries), you'll need a way to convert the dynamo voltage (which is roughly 6V) to the 3V required by the light. There are a variety of ways to do this, but all of them (that I know of) require some knowledge of electronics, the ability to read a datasheet, knowledge of resistors, etc. Your post suggests that you do not possess this expertise.

Some taillights should be compatible with this technique. Others won't be.. these are the lights that use a momentary switch to turn the light on. They would require a large filter capacitor that can keep the light powered for as long as the bike will be stopped.

Overall, converting a battery powered light to be powered by the dynamo is not trivial. It would be much easier to replace batteries in the taillight (especially if using rechargeable NiMH batteries) or to buy a taillight designed for use with dynamos.

 

[AnAppleSnail]

Why does Steve-K think you can't read datasheets yet? This capacitor will not work for your application. It's 0.98 micro-farads at 21000VAC. You want low-voltage, DC parts, with rectified AC input, to have a good dynamo experience.

 

[Steve K]

the comment about datasheets was in regard to voltage regulators.

still, the OP clearly doesn't understand the basics of electronics as evidenced by the discussion of taking a capacitor apart. The need to put together a voltage regulator, while a pretty basic design effort, requires knowledge and skills that the OP does not possess. The best case scenario is to find a module that can accept AC and produce the needed DC output. I don't personally know of such a part, but it might exist.

 

[MinasIthil]

I'll bite... what are you trying to do? i.e. when you say "I pretty much know what to do...", what exactly do you know?

As a quick overview of powering a battery powered light from a dynamo, the first step will be rectifying the dynamo's AC power, which produces a DC voltage with a lot of ripple. A filter capacitor will likely be needed at the output of the rectifier to avoid having the voltage drop to zero during each AC period. Assuming that the light is powered by 3 volts (two AA or AAA batteries), you'll need a way to convert the dynamo voltage (which is roughly 6V) to the 3V required by the light. There are a variety of ways to do this, but all of them (that I know of) require some knowledge of electronics, the ability to read a datasheet, knowledge of resistors, etc. Your post suggests that you do not possess this expertise.

Some taillights should be compatible with this technique. Others won't be.. these are the lights that use a momentary switch to turn the light on. They would require a large filter capacitor that can keep the light powered for as long as the bike will be stopped.

Overall, converting a battery powered light to be powered by the dynamo is not trivial. It would be much easier to replace batteries in the taillight (especially if using rechargeable NiMH batteries) or to buy a taillight designed for use with dynamos.

Thank you for your advices and your concern. Indeed, I know very little about electronics. I have several more questions.

Is my dynamo by itself acceptable (without any rectifier, etc.), given that it was designed to directly charge batteries with just a USB cord?

If so, then can I use it to charge my two AAA rechargeable NiMH batteries instead of a capacitor? I think I can, but I have a problem: although NiMH are rechargeables, can they be "charged" and "discharged" continuously like in the stoplights scenario?

 

[Steve K]

The dynamo is designed to produce AC power that is suitable for a standard headlight; usually in compliance with the German standard StVZO. It is commonly used to drive a 12 ohm load at 0.5A rms. It is not designed to charge batteries via USB, but some companies have developed circuits that allow it to do this.

I don't know how much power your taillight uses. If it is low enough, then a circuit could be designed to allow it to be powered from the headlight and probably charge the NiMH batteries in the process. It's not rocket science. I'm not aware of a commercially available circuit that does this, but I don't keep on top of that sort of thing... I tend to just design my own circuits.

 

[MinasIthil]

The dynamo is designed to produce AC power that is suitable for a standard headlight; usually in compliance with the German standard StVZO. It is commonly used to drive a 12 ohm load at 0.5A rms. It is not designed to charge batteries via USB, but some companies have developed circuits that allow it to do this.

I don't know how much power your taillight uses. If it is low enough, then a circuit could be designed to allow it to be powered from the headlight and probably charge the NiMH batteries in the process. It's not rocket science. I'm not aware of a commercially available circuit that does this, but I don't keep on top of that sort of thing... I tend to just design my own circuits.

Alright thanks

 

[Steve K]

if it's any help, I have two bikes equipped with dynamo hubs that power home built headlights and taillights. However, I keep battery powered taillights on the bike too, because you never know when there might be a problem with the dynamo wiring or the dynamo taillight. It hasn't happened yet, but lights are critical to safety, so I always have a back-up system ready. Yeah.... I keep a small battery powered headlight in the pannier too.

If you have a problem keeping the battery charged, I'd recommend a second taillight on the bike and a regular schedule of recharging the battery.