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>From: "DYER, BILL" <BILL.DYER@mi*.co*.uk*> >I use two UK Q60s head mounted. I have owned them for about 3 years ... >I do have problems with the bulbs popping every 20 hours or so. This is >because I object to paying 15 GBP for Alkaline batteries with about 3 hours >burn time. I have adapted the torches to use NiCds which means a higher >current thro' the bulb due to the batteries' low internal resistence. >The bulbs 12 GBP each, but this is still cheaper than Alkaline batteries. > >Does anyone know of a source of NiCd compatible bulbs for Q60s? Underwater >Kinetics are not interested and act like they have shares in a disposable >battery company. > NiCad batteries (and lead-acid batteries) have low internal resistances. They can deliver large currents without much internal voltage loss. This means that more energy is delivered to the bulb during use. The cause of the bulbs "popping" is *not* because NiCads deliver more current during use (assuming the voltage on the bulb is within +/- 10% of the rated voltage) but that there is nothing in the circuit to limit the current in to the cold bulb when the light is switched on. A cold bulb, which have resistances about 12 to 15 times less than its "hot" resistance, is subjected to a large current (12 to 15 times the steady state current), referred to as "inrush current" in the literature" when the light is switched from off to on. High inrush currents drastically reduce the average life of light bulbs. A tell-tale sign of an excessive inrush current is the observation that at most bulbs blow when the light is turned on. I am not aware of NiCad batteries with high internal resistances. I can think of numerous reasons for why you will not find such beasts either. There are however solutions to your problem: a) use a bulb rated for less wattage. b) don't turn your light off/on unless when you have to. or c) build a current inrush limiter around a standard power MOSFET. d) build a pulse width modulated (PWM) driver for the bulb. Solution (c) is simple, fail safe and compact. It will fit inside many (?) of the divelights available. It works by slowly (4 seconds) ramping the voltage on the bulb up to the full battery voltage. It requires a serious N-channel power MOSFET, a long-life capacitor, two diodes and ohhh four resistors. However, unlike (d), the voltage rating of the bulb must be matched to that of the batteries. (d) requires more components than (c) and it needs more room, it is more complicated and less predictable with respect to failure modes but has a huge advantage in that you run a say 6V from a say 10V source ( 8 NiCads or 8 alkalines at about 1 Amps of current). In the case of alkalines the PWM regulator will keep the brightness of the bulb constant even as the voltage out from the alkalines drop. By keeping the bulb brighter, less energy is wasted in the infra-red and we get more useful light per colomb of charge in the batteries. I can supply further details on solution (c) and can refer you (assuming the person is still on the net) to plans for the circuit mentioned under (d). john 015
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