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Old 09-09-2010, 03:13 AM #1
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Default Where to get a 2.8 resistor

I spent 30 minutes looking for one at radioshack and found nothing.

Would fries have them? ebay is an option.

2.7 will push my diode more than i want too.

Thanks in advance!


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Old 09-09-2010, 03:14 AM #2
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Default Re: Where to get a 2.8 resistor

Fry's would probably have the resistor you want.
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Old 09-09-2010, 03:16 AM #3
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Default Re: Where to get a 2.8 resistor

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Originally Posted by Coherent Light View Post
Fry's would probably have the resistor you want.
I thought so, Thanks!
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Old 09-09-2010, 03:23 AM #4
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Default Re: Where to get a 2.8 resistor

If the store over at I20 is out have them call the I635 store to see if they have it.
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Old 09-09-2010, 03:29 AM #5
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Default Re: Where to get a 2.8 resistor

Quote:
Originally Posted by Kulhu View Post
I spent 30 minutes looking for one at radioshack and found nothing.

Would fries have them? ebay is an option.

2.7 will push my diode more than i want too.

Thanks in advance!
There are standard values for resistor values.

For 5% tolerance, 2.7 ohms is the closest value.

I don't think you will see any difference between 2.7 & 2.8 ohms.

I have some 1/4 watt 2.7 resistors here in Dallas if that would help.

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Old 09-09-2010, 03:33 AM #6
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Default Re: Where to get a 2.8 resistor

Quote:
Originally Posted by Coherent Light View Post
If the store over at I20 is out have them call the I635 store to see if they have it.
Yea, that store is within jogging distance.
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Old 09-09-2010, 03:34 AM #7
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Default Re: Where to get a 2.8 resistor

But as Larry has some You might want to hit Him with a PM
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Old 09-09-2010, 03:44 AM #8
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Default Re: Where to get a 2.8 resistor

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Originally Posted by Coherent Light View Post
But as Larry has some You might want to hit Him with a PM
If i can't find them there i will buy his, but i have other items i have to buy there aswell.
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Old 09-09-2010, 04:47 AM #9
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Default Re: Where to get a 2.8 resistor

The standard values of resistors is set by the distance between values at each % of accuracy.
If you go for 1% resistors you can find 2.8 Ohm.
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Another solution would be to buy a pack of 100 5% 2.7 Ohms and then go through them with a meter, then verify by testload.
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Old 09-09-2010, 09:15 AM #10
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Wink Re: Where to get a 2.8 resistor

or parallel 2 of them .....

3,3 ohm in parallel with 22 ohm gives 2,86 ohm
3,3 ohm in parallel with 18 ohm gives 2,79 ohm
3,9 ohm in parallel with 10 ohm gives 2,80 ohm

(rounded to 2 decimals)
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Old 09-09-2010, 03:50 PM #11
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Default Re: Where to get a 2.8 resistor

what HIML9 said. An added benefit of this, since I'm thinking you are probably using the lm317-based driver in CC config, is that if you parallel two 1/4W resistors, the resulting equivalent resistor will be able to handle more power, near 1/2W.
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Old 09-24-2010, 09:04 PM #12
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Default Re: Where to get a 2.8 resistor

Quote:
Originally Posted by Kulhu View Post
I spent 30 minutes looking for one at radioshack and found nothing.

Would fries have them? ebay is an option.

2.7 will push my diode more than i want too.

Thanks in advance!
radioshack resistors are always a little off so i have found that
4 "10 ohm" resistors gives me 2.8 ohm exactly because the
resistors are closer to 11.5 ohm each. with my lm17 current regulator
set using 2.8ohm i get 860ma(i think,might be 816ma) and nothing gets
vary hot at this current.
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Old 09-25-2010, 08:24 AM #13
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Default Re: Where to get a 2.8 resistor

Quote:
Originally Posted by bootdsc View Post
..... with my lm17 current regulator
set using 2.8ohm i get 860ma(i think,might be 816ma) and nothing gets
vary hot at this current.
HUH ? .....

An LM317 with a Rset of 2,8 ohm must give you around 450mA, not 860 .....

Rset calculation for current is 1,25 (the reference voltage of the IC), divided for the resistance (in ohm), gives you the current (in ampere) ..... so, 1,25 / 2,8 = 0,446A ..... are you sure that your driver is wired correctly ? ..... and that the resistor is effectively 2,8 ohm ? (by the way, never trust to DMM for low resistance precise measures, if you are checking it with them ..... they are not good for this use, you need a specific instrument)
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Old 09-25-2010, 10:19 AM #14
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Default Re: Where to get a 2.8 resistor

Quote:
Originally Posted by HIMNL9 View Post
HUH ? .....

An LM317 with a Rset of 2,8 ohm must give you around 450mA, not 860 .....

Rset calculation for current is 1,25 (the reference voltage of the IC), divided for the resistance (in ohm), gives you the current (in ampere) ..... so, 1,25 / 2,8 = 0,446A ..... are you sure that your driver is wired correctly ? ..... and that the resistor is effectively 2,8 ohm ? (by the way, never trust to DMM for low resistance precise measures, if you are checking it with them ..... they are not good for this use, you need a specific instrument)
yes you are right, i forgot to mention i use two complete lm317 circuits with the 4 resistors on each one..in parallel. giving me a total over 800ma. so far this is my best driver, stays cool and so does the diode heatsink..unlike when its ran at 1200ma off one lm317. that whole setup stays warm even with a fan.
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Old 09-26-2010, 06:07 AM #15
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Default Re: Where to get a 2.8 resistor

I have chosen a weaker current and the diode are wonderful, burning, and don't get noticeably hot.

Thanks
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