Equipment Making continuous lighting [SURVEY]

The problem with using a driver from DealExtreme is that I won't get any credit for using a pre-bought one. Output control is usually via PWM, which may or may not be at too low a frequency for photo and videography.

I'll probably stick with NiMh batteries, but charging is either slow or complicated. Applying 1.5V per cell isn't enough - you need to either regulate current or cut off at a certain temperature or time. Stopping according to voltage is possible, but they are only charged when the voltage drops slightly (a very tiny amount), and that's difficult to measure and probably not reliable.

 

I see your problem with credit for using a driver especially pwm but surely your not going to get much credit for using a resistor? How about a constant current driver from a LM517, efficiency isn't brilliant but its not pwm though still has real control over current unlike a resistor.

Depends how long you need the batteries to last you can abuse NIMH for a few months before they really start to lose capacity its not like this is a commercial product, if it was Lithium cells and a good charge regulator are the way to go. But diy + lithium cells is asking to burn your house down. If you can't manage a voltage cut off, its difficult to do diy, then charging at 1/10c is fail safe other wise you can charge them at up to 1c and just keep an eye on the time. A final option is a hobby charger, this is designed to charge battery packs of virtually any size and handles most modern chemistries but might fall foul of your lecturer.

Any way good luck.

 

Efficiency is a bit of a problem - at the moment the array only puts out about as much light as a 100W incandescent bulb, and the batteries will only last for 5 hours. I've had a look at the LM517 and I don't really understand how to do it. I suspect it'll help if I look at examples instead of spec sheets.

I'm thinking of using battery holders to make the battery pack so the batteries can be removed and charged quickly, or left in the light to charge slowly. I think voltage or temperature cutoffs are a bit beyond my electronics knowledge at the moment, so charging at 1/10C is probably the best and neatest solution. I have two problems with doing that: Firstly, is C the capacity of an individual battery in the batter pack or the whole pack? Secondly, I can't find a high current 7.2V voltage regulator. All the ones I've found top out at 0.5-1.5A. The LM317 looks ideal except for that problem. Could I use multiple regulators in parallel to provide enough current or would that cause problems?

 

Ok, I realised an error in my calculations that meant the batteries would actually last for less than half an hour. In response, I've upped the current in each LED to its maximum and reduced the number of LEDs, and that's allowed me to make the batteries last for about an hour (with a small fan included in the calculations).

My batteries are a pretty simple solution - 2 parallel sets of 8xAA batteries. That gives me 9.6V, 4000mAh and a max current of 6A, which is all plenty other than the capacity. Usefully, that battery pack size means I can charge at 12V, which is a nice even number. Does anyone know if 12V laptop-style chargers give a constant enough voltage for charging batteries?

I also sorted the voltage regulator problem - An LM338T is a high-current version of the LM317, so that's perfect. It can supply up to 5A, and I'll be drawing 4.5A, so it'll definitely need the heatsink the manufacturer recommends.

I'm thinking of adding a switch to turn off half the LEDs - most of the time it won't be used, but it could be a useful feature. It doesn't require any extra effort on my part either other than adding the switch to the circuit, which is nice.

So would you say that having a light which has a small built in battery and a larger external battery pack is a good compromise? It looks like the only cost-effective solution for me.