[sdiy] Leaving off a pot?

[harry bissell]

Watch out... short circuit protection does not guarantee life...  
Overload on multiple outputs might still exceed
package dissipation.  If I was going to use the opamp ~to~ the current 
limit... I'd recommend a single 741

H^) harry

Dave Manley wrote:
> My point was different *color* LEDs can have different voltage drops.  
> Some people like white or blue LEDs these days and these have a Vf of 
> about 3.5V, while most other colors are around 2V (as low as 1.8 and 
> up to 2.4) .  The other important point is the recommended current for 
> a reasonably bright LED.  This is of course subjective, but could 
> range upward to the typical max spec of 20 (or 40 for white or blue) 
> mA.   I suggested Sam assume 15 mA, calculate the resistor, try it and 
> see if it meets his needs.  If not adjust the resistor up or down with 
> the knowledge that he can't hurt the TL084 because it has short 
> cicruit protection. I also mentioned that the TL084 has a limited 
> current drive (spec'd at 20mA) and too heavy a load will affect the 
> ADSR output voltage, potentially requiring a separate buffer for the 
> LED (as others have suggested).
>
> -Dave
>>
>>>> LEDs have a voltage drop that varies with the color.  So the 
>>>> resistor value depends in part on what color you choose.
>>>
>>> Wow. I had no idea. I actually thought of mentioning in my original 
>>> post that I was thinking of green, but I thought it didn't matter. 
>>> So, I'd need an LED whose forward voltage was rated at at least 10v, 
>>> so this should work, right?:
>>
>> No.  A normal green LED will have a voltage drop of about 2V  That 
>> means that 8V out of 10V will be dropped across the resistor.  This 
>> allows you to work out the current through the LED using simple Ohms 
>> law.
>>
>> I=V/R
>> e.g.
>> I=8V/1K
>> I=8mA
>>
>> Probably a good starting point!  My advice: just try a 1K resistor 
>> and see how you like the LED brightness.  Different LEDs will have 
>> different brightnesses.  The voltage drop shouldn't vary very much 
>> for a green LED - all should be about 2V.
>>
>> Seb
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