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  1. #1
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    Soft Start Circuit?

    Need some help. I'm seconds from pulling the trigger on a 6.4volt 13.2 Ah LiFePO4 water bottle battery. I'll be using it to power a TurboCat S25 system. So... a 10watt flood and a 15watt spot. Both lights can be turned on independently, however I will using the 10watt flood as my main light and the 15watt spot when I'm on high speed descents. I understand that halogen's require extra amps on startup. The battery pack is rated at 12 amps, and I'm worried that I might trip the PCB. I've done a little searching around for a soft start circuit, even seen a post referring to one, however I'm having trouble finding a simple design that I can build. Can someone point me in the right direction? Or will I even need one with my setup? Batteryspace is simply warning that I *might have a problem. From what I can tell the 10watt light should consume 1.67 amps, so the combined draw shouldn't be more than 4.175 amps, the question is just whether the startup power will cause problems. Help!


    Thanks! Logan

  2. #2
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    A soft start circuit that'll actually avoid the problem you're talking about is actually quite complicated. That's because the typical way to soft start a halogen light is using pulse width modulation (PWM). Which is basically turning the light on and off using electronics 1000's of times a second and ramping the ratio of on to off time up slowly. The problem is that the inrush current on a cold halogen bulb is extremely high, but for a very short period of time (typically a few milliseconds). So the PWM modulation frequency has to be extremely high to control the current during the very short timeframe of the inrush current. The only other choice is a linear instead of digital soft start circuit but that's typically even more complicated.

    The better solution is to use a well designed protection PCB that responds to average current over a window of say 10-50ms instead of one that responds to instantaneous current. Unfortunately, very few of the BatterySpace protection boards are designed that way.

    The simpler, though not very elegant solution, is to add a filter capacitor to the output of the protection PCB. This helps the problem because the capacitor stores energy that helps satisfy the large inrush current. Then the bulk of the large current spike comes from the capacitor instead of through the protection PCB. For a halogen bulb, you may need a fairly large capacitor and one that is good at providing large current very quickly. You may have to play with values, but something like a 50 to 100uf tantalum capacitor is typically enough, but you may have to go even larger.

    By the time you buy a battery and fool around with problems like this, you have to consider whether you'd be better off just buying a low cost LED system. The LED system will be many fold more efficient and brighter.

  3. #3
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    Thanks for the advice! At this point I've already ordered the new battery, and I bought the TurboCat lights for a very good price. I'll play with the current setup for now and research LED conversions using the TurboCat housings. After my trial ride last Saturday I'm more than happy with the brightness, I just needed longer runtime. With 13.2 Ah I'll be in great shape.

  4. #4
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    Yep - your pack is an 84watt-hour pack - so even with the 10 and 15W beams running, you should get ~3.3hours.

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