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Old 05-09-2009, 02:15 PM #1
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Default DDl circuit 2x10 ohms resistors

why do we need 2 ten ohm resistors when we already have a 100 ohm resistor


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Old 05-09-2009, 02:32 PM #2
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Two 10 ohm resistors (in series or parallel) does not equal 100 ohms... together, they could be either 20 or 5 ohms. Also, it sounds like you don't really understand what they are for either. Current is set by the resistance between the Output and Adjust pins (pins 1 and 2), calculated by the formula:

Iout = Vref / Rset = 1.25 / Rset

For 5 ohms, Iout = 1.25 / 5 = 0.25A or 250mA

For 100 ohms, Iout = 1.25 / 100 = 0.0125A or 12.5mA (not really useful)

If you want a specific current, you can rearrange to calculate the resistance you need and try to find the closest value (or combination of values) of resistor available:

Rset = Vref / Iout = 1.25 / Iout

So, if you want say 200mA, Rset = 1.25 / 0.2A = 6.25 ohms

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Old 05-09-2009, 03:48 PM #3
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I think you may have misunderstood the question rkcstr.

The 2 X 10ohm resistors in parrallel set the maximum current your LD can see, without them you could easily turn the 100ohm pot to zero resistance and wipe out the LD.... It is a safty device usually set to the limit of the LD's capabilities.

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Old 05-09-2009, 04:41 PM #4
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I suppose I misunderstood the question because it wasn't very clear.

When referring to the DDL circuit (which I would guess is to what you are referring), the two 10 ohm resistors act to limit the peak current. Otherwise, as the pot (the 100 ohm "resistor") approached it's minimum, it would be probably be less than 0.5 ohm, which would more than enough to destroy your diode and possibly the driver itself; which you can see if you plug that value into the equations I gave above. You can adjust that resistance to increase or decrease the limit as well.
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Old 05-09-2009, 05:48 PM #5
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thanks rog that cleared it up for me.

Anyway thanks to you too Rckstr your information also helped clear up some other major gaps.
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