555-based dynamo powered flasher circuit

[minisystem]

After months of fiddling, I built a prototype of my flashing dynamo circuit on a PCB. Build photos:

It works! The standlight is usefully bright for just over 2 minutes in solid mode and about 4+ minutes in flashing mode.

The only major issue I need to work through is with the 6.8V Zener diode that regulates the input voltage. With the LEDs connected, the dynamo voltage is Vf of the LEDs (around 5V), which is below the zener voltage, meaning the Zener isn't shunting any current. When in flashing mode, the dynamo voltage jumps during the off cycle (no load) and the Zener shunts all of the current. I'm using a 5W zener. If I understand correctly, the Zener's current will be the saturation current of the dynamo (0.5-0.6A) and the power will be that current multiplied by the zener voltage, so (6.8V x 0.6A =) 4.08W. Is that correct?

It's shunting this current a little less than half the time. Still, it gets too hot to touch. After several minutes it has yet to fail, but it makes me a little nervous. Not sure what to do with that dynamo output during the off cycle. A more elegant solution would be to capture it and pump it back into the LEDs in a burst, but I dont' know how to do that. If the zener does fail I'll have to find another way of dissipating or recycling the dynamo power during the off period of the flash. Any ideas?

 

[ianfield]

In last summer Elektor magazine bumper issue; I had published a design for a "no IC cycle light flasher".

It was based on a single transistor blocking oscillator driving an overwind to produce a suitably large voltage, that was rectified and used to charge a 47uF/63V electrolytic cap, a cheapo 24 LED worklight (£1.99p in QD stores) with 24 parallel connected white LEDs, and the battery compartment stripped out provides the load.

A DB3 diac (liberated from a scrap compact florescent) waits for the 47u cap to charge up to about 32V then dumps the charge into the bank of LEDs - the flashes are seriously intense!!!

The original had about 12-0-12T for the blocking osc running on 1.5V and 60T for the overwind, but you can increase the collector winding to suit a 6V supply. To do this you can no longer feed the base via a resistor from the base winding (you have to put a DC blocking cap in series now) you will also have to add a startup resistor from B+ to base (maybe 100 - 270k ish).

Probably a bit bright for a rear cycle light! And rather fiddly changing 24 close packed 5mm LEDs for red one's - I'd suggest gluing a red lens from a commercial rear light (this may be neccessary to satisfy plod)