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Breaktru Forum  |  eCigarette Forum  |  Modding  |  Topic: DNA30 and fuses
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Author Topic: DNA30 and fuses  (Read 11252 times)

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Offline fullthrown

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DNA30 and fuses
« on: March 22, 2014, 03:59:03 PM »
Hi guys, i'm planning a DNA30 mod with two 18650 batteries in parallel (pretty much based off of Mamu's arlo tutorial). I have everything i need in front of me, however i only have 3- 3.2 amp resettable fuses. I also have 2- 5 amp resettable fuses. I was thinking of running a 5 amp fuse on each positive contact for a total of 10 amps, instead of the 4x3.2 fuses in parallel for 12.8 amps. Will this work? Even at the lowest atomizer resistance (.5) and the highest output (30 watts), it would be pulling around 8 amps, which is under the 10 amp trip. Right? Any input is greatly appreciated! thanks.

Online Breaktru

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Re: DNA30 and fuses
« Reply #1 on: March 22, 2014, 04:22:55 PM »
What is the internal resistance of the 5A PTC. If it's not too high then yes. Put one for each battery.

Offline fullthrown

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Re: DNA30 and fuses
« Reply #2 on: March 22, 2014, 07:08:37 PM »
.025 - .048 ohms. Would there be any chance of the fuses tripping at 10 amps if i chose to go up to 30 watts at that atomizer resistance (which i wouldn't), but just out of curiosity?

Online Breaktru

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Re: DNA30 and fuses
« Reply #3 on: March 22, 2014, 08:12:26 PM »
The fuse you show is not considered 5A. It's hold current is 2.5A. It trips at 5A. Meaning that past 2.5A, resistance starts to climb (not linear) and at 5A only a very small leakage current is flowing.
This fuse won't cut it.  :thumbsdwn2:

0.048 ohms or 48mOhms can introduce a voltage drop. That's why we try to use the least resistance such as paralleling them to half the resistance. SMD fuses seem to have lower resistance.

Another thing. Input current max is 12A on the DNA30 and 10A output current max.
I would recommend using 2x 6A hold current fuses for each battery = 12A
« Last Edit: March 22, 2014, 08:17:28 PM by Breaktru »

Offline fullthrown

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Re: DNA30 and fuses
« Reply #4 on: March 22, 2014, 08:34:28 PM »
 :wallbash: Hmm, where would i go about acquiring one of these? Im looking on digikey and mouser right now and cant seem to find one. I want to make sure im getting the right fuse this time, i just bought 5 fuses in total that wont work :(

Offline fullthrown

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Re: DNA30 and fuses
« Reply #5 on: March 22, 2014, 08:55:51 PM »
http://www.mouser.com/ProductDetail/TE-Connectivity-Raychem/MINISMDC260F-16-2/?qs=k1OHqHwubKfnLivJ0Ae6uA== Is this ok? I would have to run them in parallel but 50 mOhms with 5 amps trip sounds good to me, i think. 2 of these in parallel would be 25 mOhms correct? But the hold current seems kind of low, even if i ran 2 parallel on each wire:/

Offline fullthrown

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Re: DNA30 and fuses
« Reply #6 on: March 22, 2014, 09:43:41 PM »
Or these: http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=146787556&uq=635311175900684358 ..seeing as though mouser has a 9 week factory lead time. Sorry for attacking you with questions!

Online Breaktru

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Re: DNA30 and fuses
« Reply #7 on: March 22, 2014, 11:04:13 PM »
http://www.mouser.com/ProductDetail/TE-Connectivity-Raychem/MINISMDC260F-16-2/?qs=k1OHqHwubKfnLivJ0Ae6uA== Is this ok? I would have to run them in parallel but 50 mOhms with 5 amps trip sounds good to me, i think. 2 of these in parallel would be 25 mOhms correct? But the hold current seems kind of low, even if i ran 2 parallel on each wire:/

The amps are good but 50mOhms is still a bit high. 2x PTCs for each battery, 25mOhms in each battery.

These specs are perfect but the physical size is rather large for These - use one in each battery.
This one is smaller and is 19mOhms and you can use 1 for each battery --> HERE

The reason I am suggesting the 6A x 2 = 12A and not the 5A x 2 = 10A is because you mentioned running at 30W with a 0.5 ohm coil.
If you are not pushing the DNA to it's max then yes you can go with the lower hold rating.

Offline fullthrown

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Re: DNA30 and fuses
« Reply #8 on: March 22, 2014, 11:29:48 PM »
Hm, so what would the ideal mOhms rating be for each battery in a perfect scenario? The minimum hold for the 5A fuses are 25 mOhms, and the not so good resistance is the max- 48 mOhms. What would cause the fuse to go to that max resistance? And how would it affect everything else? I wont be pushing the DNA to its max, this is more about the knowledge and understanding of what would/ could happen if i used the 5 amp on each battery, running 30w @ .5 ohm atty resistance. The only thing that i'm confused about is why would the 5 amp fuses (equalling 10A) trip at 7.7A (30watts @ .5ohms) (just a scenario). Sorry, i'm just trying to get a better understanding for this instead of just following a set of instructions, i want to learn about this stuff! :thumbsup:

Offline CraigHB

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Re: DNA30 and fuses
« Reply #9 on: March 23, 2014, 02:26:55 AM »
Hm, so what would the ideal mOhms rating be for each battery in a perfect scenario?

Hehe, zero.  But that's not possible so the lower the better.

You can calculate power loss and decide for yourself.  If a PTC fuse specifies resistance say between 10 and 30 mOhms, then it will typically be 20 mOhms at room temperature.  I've found that PTC fuses tend to be right in the middle of the range.  For a maximal expected input current 10 Amps total from the battery, each fuse would have to bear 5A.  Power loss would be 5A squared times 20 mOhms or 1/2 Watt for each fuse.  Total power loss is the sum of both fuses which is 1 Watt.

If input current is 10A and battery voltage is a nominal 3.7V, total input power is 37 Watts so the 1 Watt loss is just about 3%.  Personally, I try to keep loss from any component no greater than 1% so 20 mOhm fuses would not be a good choice.  I'd want something that gives me a power loss probably half that so I'd look for a fuse with 10 mOhms instead of 20 mOhms.  It's entirely up to you as the designer in how much power you are willing to give up for any component.  If 3% is okay in your book, then 3% is okay.

Power loss also means voltage loss so in some cases that can present a problem for the device under power.  It's not just lower voltage that can give you trouble, but also the higher impedance the device sees from the power source.  Battery internal resistance comes into play there as well.  Any device that uses a switching regulator like the DNA30 can be sensitive to source impedance.

Quote
The minimum hold for the 5A fuses are 25 mOhms, and the not so good resistance is the max- 48 mOhms. What would cause the fuse to go to that max resistance? And how would it affect everything else?

The resistance of a PTC fuse is entirely dependent on its temperature, but it's not a liner relationship, it's exponential.  Once a PTC fuse gets hot enough, it quickly increases to a relatively high resistance.  This in effect shuts down the circuit.  The current flow through the fuse and its resistance is what heats it up.  So, a higher current causes the fuse to increase in temperature which causes an increase in resistance.  Very high currents cause the transition to a high resistance in only a fraction of a second.  Currents less than the hold current have little effect on the temperature of the fuse so there's little increase in resistance.

Quote
I wont be pushing the DNA to its max, this is more about the knowledge and understanding of what would/ could happen if i used the 5 amp on each battery, running 30w @ .5 ohm atty resistance. The only thing that i'm confused about is why would the 5 amp fuses (equalling 10A) trip at 7.7A (30watts @ .5ohms) (just a scenario). Sorry, i'm just trying to get a better understanding for this instead of just following a set of instructions, i want to learn about this stuff!

PTC fuses are unique in that they are specified with both a hold current and trip current.  Neither can really be equated to the rating on a traditional fuse.  The hold current is the current level where the fuse is guaranteed not to trip.  The trip current is where the fuse starts to trip, but it's still not high enough to cause a fast transistion.  You really have to study the trip time charts in the data sheet to get a better understanding of how they work.

Offline fullthrown

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Re: DNA30 and fuses
« Reply #10 on: March 23, 2014, 02:44:52 AM »
Perfect, this is exactly what i was looking for. Thank you!! I decided to go with these: http://www.digikey.co.uk/product-detail/en/RGEF600-2/RGEF600CT-ND/3592635

Im just going to have to study the datasheet, i might even mess around with the different fuses that i have and test em out. Thank you guys, this was a big help.

Offline mamu

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Re: DNA30 and fuses
« Reply #11 on: March 23, 2014, 03:52:12 AM »
http://www.mouser.com/ProductDetail/TE-Connectivity-Raychem/MINISMDC260F-16-2/?qs=k1OHqHwubKfnLivJ0Ae6uA== Is this ok? I would have to run them in parallel but 50 mOhms with 5 amps trip sounds good to me, i think. 2 of these in parallel would be 25 mOhms correct? But the hold current seems kind of low, even if i ran 2 parallel on each wire:/

Your best bet is to always read the datasheet for verification of specs as both mouser and digikey sometimes list inaccurate specs.

Mouser listed the max resistance post trip state - that's misleading to do that as post trip current will always be quite high. 

MINISMDC260F/16-2: http://www.te.com/catalog/pn/en/RF1184-000
    Vmax (V) = 16
    Imax (A) = 100
    IH (Room Temperature) (A) = 2.6
    IT (Room Temperature) (A) = 5.00
    Rmin (ohm) = 0.015
    R1 Max [Post Trip] (ohm) = 0.05


For the DNA30 2x in parallel for each batt = 7.5 mOhm per batt and 4x fuses (2 per batt) = 3.75 mOhm total.

2.6A Ihold x 4 = 10.4A.  5A Itrip x 4 = 20A. 

Ihold is the maximum current that the fuse will allow to pass without tripping (10.4A).  Itrip is the minimum current at which the fuse will trip (20A).

The only issue you may have here, fullthrown, with using a lower Ihold total a bit less than the max input current of the DNA30 is a slight increase in resistance from the fuse IF running the DNA above 10A.

But with 4x fuses you have a current cushion of 10.4A - 20A before the fuse will actually trip. 

I've been using this fuse with the DNA20 this past year and now with the DNA30.  Haven't had any issues with inadvertent tripping from the 2.6A Ihold.  Course, I don't push 10A either.  I'm a weeny at around 5A.  :laughing:

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