Can I use a USB wall wart as a rectifier/regulator.

[ferulebezel]

I just bought a bike with a hub dynamo for the incandescent front light. It turns out that when they say "dynamo" they mean just that. They're not using it as some kind of old timey sounding term that sounds like "dynamic".

It is rated to put out 6vac at 2.5 watts. I want DC in the 5 voltish range for an LED front light and rear flasher.

I've found a number of schematics to make one, but with hand soldering discrete components it would be larger than I want.

It occurred to me that USB is 4.5vdc and pretty well regulated. I'm thinking that the transformer before the bridge in the wall wart USB power supplies brings it down to pretty close to 6vac. I could just take out the transformer and connect the bridge straight to the dynamo,

I've also considered putting a really big capacitor and a diode at the output to keep the lights on for a couple of minutes when stopped at a light.

Am I thinking more like MacGyver or Wile E. Coyote? Will this work or will it fail, leading me to ride all the way to Arizona and off a cliff because I can't see where I'm going?

 

[Harald Kapp]

I'm thinking that the transformer before the bridge in the wall wart USB power supplies brings it down to pretty close to 6vac

Unlikely. The USB wall warts I know all have a switch mode power supply internally which means they operate by a very different principle. Your dynamo wouldn't work with the innards of such a device.

Here is a simple circuit that can be built in a small space.

I've also considered putting a really big capacitor and a diode at the output

Use an Electric double-layer capacitor (also known as supercap, goldcap etc.)

Have a look at this site which discusses these issues in detail.

 

[ferulebezel]

OK. What about a PS/charger from an old phone. Truth be told, I'm just being lazy.

 

[Harald Kapp]

I'm lazy, too.
Have a look at this site which discusses these issues in detail.

 

[ferulebezel]

I did and that is more work than I want to do. I did find an instructable where a guy claims to have done it with a bridge and a car USB charger.

It's a few bucks and half an hour of work. I think I'll give that a shot.