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### Topic: figuring out resistors for SMA (Read 801 times)previous topic - next topic

#### hilukasz

##### Dec 12, 2013, 09:57 pm
so i keep burning out resistors for some reason, anyone know why? I'm using this data sheet http://www.robotshop.com/media/files/pdf/flexinol-technical-data.pdf

at 6in it should be 7.8ohms resistance on the .006diameter wire
which leads to equation:

(12v/.4a)-7.8ohms = R
which i get to be 22ohm resistor.

but when I connect it the resistor burns up pretty fast.
for(i = 0, i < 820480075, i++){ Design(); Code(); delay(1000); } // hellowoo.com

#### KeithRB

#1
##### Dec 12, 2013, 10:08 pm
.4*.4*22 is 3.5 W, so you need a pretty big resistor to handle the power.

#### hilukasz

#2
##### Dec 12, 2013, 10:10 pm

.4*.4*22 is 3.5 W, so you need a pretty big resistor to handle the power.

ah, I was assuming it might be a watt thing. thank you.
for(i = 0, i < 820480075, i++){ Design(); Code(); delay(1000); } // hellowoo.com

#### CrossRoads

#3
##### Dec 12, 2013, 10:22 pm
Doublechecking:
12V/(7.8 +R) = 0.4
12V = (7.8+R)*.4
12V = 7.8*.4 + R*.4
12v - (7.8*.4) = R *.4
(12-3.12)/.4 = R = 22.2

Power dissipated in resistor = I*I*R = .4*.4*22 = 3.5W.
I suggest a high wattage resistor, like a 5 W part.

http://www.digikey.com/product-search/en?FV=fff40001%2Cfff80482%2C40886%2C80009%2Cc0001&k=5w+resistor&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### KeithRB

#4
##### Dec 12, 2013, 10:35 pm
Don't worry, Hilukasz, it happens to all of us. I was in a meeting where I questioned whether a smallish 50 ohm termination resistor could handle the power required since it had -5 V on it. The reply was "It's OK, it's digital!"

#### MarkT

#5
##### Dec 13, 2013, 02:14 pmLast Edit: Dec 13, 2013, 02:18 pm by MarkT Reason: 1
Ha! Cargo cult engineering.

Concerning the problem at hand, a 3.3V supply would be about right for the SMA wire it seems, so
a DC-DC converter to 3.3V with 0.5A or more current handling would be more power efficient,
perhaps only 150mA needed from the 12V supply then.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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