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Black buck 5.5A reverse polarity fuse location

barthchris

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Long-time no see fellas! Getting back in the light amplifier hobby after some time off. I used DTRs excellent service to obtain a num44 V2 and a blackbuck 5.5A driver. I had it working but was testing with two 18650s outside a host and made the rookie mistake of connecting power in reverse polarity(facepalm)! In my defense, I was in the dark oogling at the bright but horribly divergent beam.

I understand there is a ceramic fuse which may save the day. I have SMD rework equipment and good soldering skills, I could check for continuity but I'm multimeterless ATM due to moving. Never let the wife pack tools!!!!!

Anybody give me the location of the fuse. 65557
 



Alaskan

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I had never heard about a fuse on this driver but looking on DTR's web page, I see it does. I would expect it to be the first SMD device inline with one of the two plus or minus input wires, but maybe not. DTR often answers emails the same day.

That white SMD sure looks like a fuse to me, I don't know what else that could be. The convention is to put the fuse on the plus line most of the time, that white SMD is close to the positive or red wire, is it soldered to it, or to a trace which goes directly to it? If so, I'm 99% certain that is the fuse.

 
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paul1598419

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I was unaware of the fuse as well. You know you can add a silicon rectifier to the input to eliminate reverse polarity problems. I have never had this issue, though.
 

barthchris

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Thank you! Thats the one I suspected. Guess Ill just jump it until or if I ever get around to finding a replacement. Fish out of water with no meter, guess I could rig something up with a button cell and a led.
 
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RedCowboy

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I was unaware of the fuse as well. You know you can add a silicon rectifier to the input to eliminate reverse polarity problems. I have never had this issue, though.
Why use a fuse in place of a diode to achieve reverse polarity protection ?
The driver would have to take a reverse current until the fuse blew if it was meant to serve as reverse polarity protection, it would use a diode and a fuse, so it likely has a diode as well/already.
 

Alaskan

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Only one reason I can think of why use a fuse instead of a diode is to lower the voltage the device can otherwise operate from. That, and a diode will make the regulator less efficient, the voltage drop across the diode X the current through it is lost power. I had a coworker who was a top notch supertech in electronics who had a hole in his knowledge regarding diodes, for some reason he thought there was no power loss when using them that way. They heat up when current goes through them due to the voltage drop, i.e. a silicon diode drops approx. .7 volts. At one amp, that's 700 mw of lost power. I prefer to use Germanium diodes over silicon for such use, as they have about half that amount of voltage drop.
 
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RedCowboy

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Yes .7 -1.0v but if the diode is across the input in reverse then when a backwards current is applied the diode takes the power and blows the fuse, a fuse alone would not be a good idea.
 

Alaskan

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Oh, reverse biased insertion across the input instead of in series with it, yep, in that case, no additional loss, it blows a fuse, but then you must have a fuse in addition to that.
 

RedCowboy

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Yes I was thinking if paul added a diode that it would not be needed and as you said it would raise the Vin as well, I suppose you could add a diode and use a higher Vin to prevent blowing the fuse, I was thinking about how it was made and that a fuse alone would not be a good idea.
 

Alaskan

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Well, for myself, I might opt to just design a driver with an inline diode, no fuse to blow, but at a cost to efficiency and raising minimum Vin.
 

RedCowboy

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Yes a lot of the Chinese drivers do that, the designer was likely going for a lower Vin as you said where I would make it as foolproof as possible for American users.
I wonder what the cost would be for a tiny self resetting breaker ?
 

Alaskan

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I wonder, if you can develop it, you might be creating something which would be a money maker as I've never seen something small enough to integrate onto a circuit board small enough to fit inside an Axis module. I suppose if we can imagine it, someone makes it, or can?
 

RedCowboy

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I'm sure there is much made that I have not yet seen, probably such a thing is obsolete already. :)
Just a thought, rather than a silicon why not a schottky diode if used in series ?
 
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paul1598419

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I immediately thought, after my post, that a Schottky diode might be a better option. Much lower Vf and higher current.
 

Cyparagon

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Simple rectifiers are stupid for high power applications, as that's a waste of 4W of power, and now you need to heat sink something else. This is "when all you have is a hammer, everything looks like a nail" thinking. Expand your electronics knowledge before suggesting such 1970s solutions, please. At least google reverse polarity protection - there are vastly superior methods these days. Even something this simple is way better:

 

paul1598419

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Selecting the right Schottky diode will reduce your loss by about a third. Another way to do the is to use a P channel MOSFET. Connect the drain to the positive terminal and the source to the positive input of your driver. If the SG breakdown voltage is high enough, you can just connect the gate to the negative terminal of the battery or driver input. Very easy and your losses will be very small in comparison to any diode. If you choose a MOSFET with too low a SG breakdown voltage, you can still get by with the addition of a Zener diode and a gate resistor.
 




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