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Inline Current Limiting Devices for 12-24VDC Solar / Battery Power Systems

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spork891

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Hello,

I am certifying a device for operation on a battery based power system, and I'm running into trouble during safety evaluation, because UL stipulates that the available current to my device must be less than or equal to 250W at the operating voltage, not taking into account any fuses or circuit breakers. Essentially, I need to find an external inline device that can be used to limit the available current from a 12 or 24VDC system under all circumstances, to less than 10A. My required current is only 1A, so anything above 2A is fine.

Anyone know of any off the shelf devices that could accomplish this? Thanks!
 

KlausST

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Hi,

A passive solution could be a NTC.

An active solution a "hot swap circuit", or other power distribution devices, or a protected high side fet, or low side fet. Some smart device...

Klaus
 

spork891

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Thanks. Yeah, some kind of thermistor could work... Also, I'm new to thermistors, but it seems like they trip really slowly. I'll probably need an active device of some sort, but I haven't run across anything. It's weird to me that devices like this aren't common in marine applications, considering the available current from deep cycle batteries, but I can't find anything at Defender or West Marine.
 

mtwieg

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Hello,

I am certifying a device for operation on a battery based power system, and I'm running into trouble during safety evaluation, because UL stipulates that the available current to my device must be less than or equal to 250W at the operating voltage, not taking into account any fuses or circuit breakers.
Are you sure you're explaining this correctly? Do you mean that the power drawn by your device must be less than 250W, or that the device must be capable of operating on less than 250W? Because that sounds like something that can be guaranteed by design, without any extra current limiting.
 

schmitt trigger

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Are you sure you're explaining this correctly? Do you mean that the power drawn by your device must be less than 250W, or that the device must be capable of operating on less than 250W? Because that sounds like something that can be guaranteed by design, without any extra current limiting.

Without having the actual UL spec, my understanding would be that the source, under any nominal operating voltage range should not provide a V x I product higher than 250 watt.
Since voltage varies, clearly at the highest rated operating voltage, will set the maximum available current.
 

spork891

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Without having the actual UL spec, my understanding would be that the source, under any nominal operating voltage range should not provide a V x I product higher than 250 watt.
Since voltage varies, clearly at the highest rated operating voltage, will set the maximum available current.

Thanks. Yes this is correct, the reason for this is that UL considers instantaneous available power of greater than 250VA (W) to be a fire hazard, regardless of circuit breakers or fuses, which have a delay before tripping. Because our system is designed for battery systems, our safety evaluator believes that we need to somehow limit the available current, in spite of external and internal circuit breakers, so now I'm stuck trying to find some device that does current limiting that isn't a typical circuit protection device, which seems crazy to me, but they issue the certs...
 

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