Bright Lights All Night?
(3 approaches to brighter lighting)
 High-Wattage bulbs and the LVR2
  Twin Headlights with a generator
   Three Micros with Lithium power

Bikecurrent Archive



How - or why - does the LVR2 work?
by Francis Cooke

The LVR2 can be used with any battery, primary or rechargable, up to a maximum of 15 Volts. (There is another, bulkier, version of the circuit that can handle higher voltages.) But the effect is most dramatically illustrated when it is used with Alkaline primary cells.

Imagine first of all a simple 6V cycle lighting setup. A pack of 4 C cells in series would give the required 6V, driving a 5W or 10W bulb. The current drain on each cell would be .83A, or 1.67A for the 10W, and this is simply far too high. The voltage delivered by the pack drops very rapidly as a result, and the light becomes unusable in a very short time.
We can improve matters somewhat by making up an 8-cell pack using two of these 4-packs in parallel. This still gives 6V, but the current drain on each cell is halved. There are twice as many cells, twice the expense, twice the weight, so you would expect double the run time. In fact its better than this, the run time is almost trebled, due entirely to the benefit of halving the current drain on each cell.
Even so, the end result is disappointing. The fresh cells, with an individual cell voltage of 1.6V, deliver 6.4V to the bulb for a fine bright light, but this lasts literally only minutes. Cell voltage is rapidly dragged down to around 1.2V, giving a total of 4.8V, and the light starts to look distinctly ordinary. By the time cell voltage drops below 1V, the light is getting quite brown and useless.

Now let us reconfigure those 8 cells and run them in series, giving a starting voltage of about 13V. The current drain per cell is exactly the same as in the previous example, and the profile of declining voltage per cell over time is exactly the same. Assuming for the moment that the LVR2 is 100% efficient, the pack will take exactly the same amount of time to die.
Of course the 13V starting voltage would destroy a 6V bulb, so the LVR2 is employed to chop this down to 6V.
As before, after a very short time cell voltage is dragged down to around 1.2V. However, this time, because the cells are in series, the total voltage is still 9.6V - still too high for a 6V bulb, and so still regulated down to 6V by the LVR2. The light is still burning exactly as brightly as it did at the start.
Even when the battery nears exhaustion, with voltage down around .8V per cell, total voltage is still over 6V, still subject to regulation, the light is still exactly as bright as it always was.

The cells are now almost sucked dry, yet are still providing a bright light - a remarkable use of Alkaline cells. In fact the LVR2 is reckoned to be over 99.5% efficient, so the losses introduced by adding this extra circuitry are quite minimal and can be disregarded. Only Willie Hunt's circuit achieves this efficiency.

What happens next depends a bit on how the LVR2 has been set up, but the best setup for Alkaline cells simply causes the LVR2 to become 'transparent' when the input finally drops below 6V, so that during the final stage of the battery discharge (actually lasting about 90 minutes with 8 C cells and a 5W bulb), you get the old familiar steadily declining light as battery voltage drops through 5V to 4V and below.

Note that, if used with SLA or other rechargable technology, the battery should not be allowed to get to this stage, which would damage it irredeemably, and the normal configuration of the LVR2 includes a 'low voltage warning' facility to prevent this from happening.

The LVR2 is a public domain circuit. However, you would be mad to attempt to build your own, as Willie Hunt can supply it ready built precisely to your requirements. Willie's circuit board is a masterpiece of miniaturisation and if requested he can also weatherproof it for you.

The maximum input voltage is 15V - so it would be possible to use 9 Alkaline cells in series for maximum runtime. I prefer to use 8 because it is physically much easier to make an 8-pack, using two 4-cell battery holders bolted back-to-back. A C-cell pack built like this is smaller than a can of Coke - although it weighs nearly twice as much! Adding the 9th cell would increase the runtime at full brightness, from just over 6 hours to just under 7, with a 5W bulb.

Output voltage can be set to any value above 3V. I set it to precisely 6V, but of course it could be set anywhere in the range 5V to 6.5V, for a trade-off between brightness and runtime. At 5V the 8-cell pack would give over 7 hours. Its best to discuss this with Willie when ordering the unit, in fact simply describe to him your precise lighting setup.

The LVR2 features a 'low battery warning' which consists of a slight flickering of the light when the input voltage drops below a certain level. This is an essential feature for use with rechargable packs, but is mildly irritating when sucking a pack of Alkalines dry. This feature can be disabled by changing the value of one of the resistors on the circuit board - it is possible, but fiddly, to do this yourself, but again its much better to ask Willie to do this at time of building.

The LVR2 also features a sexy 'slow start' which magically works even when you switch on using the normal switch on the lamp head - the light fades up over about half a second. This valuable feature reduces thermal shock and extends the life of the bulb.

For more information see Willie Hunt's WWW pages
Lighting Voltage Regulator

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