48v switch, resistors and cap!

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Spencerleehorton

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Joined
May 12, 2012
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Location
Felixstowe, Suffolk, UK
having trouble with blowing a few leds and to my mind ive got the values right but want to check with you guys.

Im feeding in 48v to DPDT lug, center two lugs are connected and i take a 3.9k off to the + of hte led and off the center lugs i put a 470R resistor which then has a 220uf/63v cap then feeds the two 6.8k resistors to pins 2 and 3.

Why do I keep blowing leds? I've got about 2.9v going to blue led, if i remove the 470R i still keep blowing the resistors?

any diagrams for this? ive taken my arranement form the api-312 48v but just used a 3.9k resistor.
 
> any diagrams for this?

We should be asking you for diagram. Your text makes sense to you but is not instantly clear to others.

However this all sounds very familiar??? Weren't you burning LEDs about 5 months ago?
 
You circuit should give you 12mA through the LED. Do you really need that much? Try 10K resistor instead which should give you 5mA.

Cheers

Ian
 
Check the specs,  leds don't often need much current,  1-2 mA can be good enough.

Also the way you have it you are discharging 48V through 470 in the off position.  That's a lot of peak power.  That resistor may not last long either.
 
In off the cap will still have 48V that will discharge. When you connect the 470 to ground that will be one discharge path.  Along  with the mic and led path.

Edit : on second thought you have the same issue at turn on with the resistor (I'm not a morning person). So you can disregard what I've said.  Just use the right power rating as originally specified.

 
I suppose what I'm really asking is what does the 470R resistor do in this config?
Does it limit the current available for the mic to 102ma?
And is this what's having an impact on the current for the led and why they are going pop?
I'm going to order some leds!! And take away the 470R resistor and see if they don't blow!
 
You need that resistor to limit the in rush current,  otherwise your switch may go pop.  There was another thread recently about this. The value should be lower though so it doesn't drop too much voltage. You also don't want C to be too big.

You can do it as attached.  Some typical values you might see are:
R1 = 200
C1 = 100 uF
R3 = 10k to 20k, check your LED datasheet for its current specs.
R2 can be large, like 100k. It provides a DC path to ground,  not always needed,  but good to include imo.
 

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Spencerleehorton said:
So just to check I got this right!!!
I didn't immediately recognize what that was in bottom right corner.... (my guess is LED).

The 100k across the cap seems redundant as the LED will discharge the cap down to 2V or less.

JR
 
Move the ground off of the switch.

The 100k is redundant,  but that is partially the point.  It ensures the cap will discharge,  even  if other things change,  like perhaps the led path to ground is no longer connected.
 
Hi Spencer, I had a problem in an API 312 type mic pre (access 312) I had all the correct resistor values but was blowing LED's on the 48V section on 2 channels. (4 channels total)

It took me awhile to understand why, I was not getting anywhere.

In the end the problem was the switches used, on those 2 channels the switches I used were different and were make before break (shorting), by the way things were wired in that project that was blowing the LEDs when I activated the 48V.

Changing those for Non-Shorting switches solved the problem.
Might not be your situation, but check it out because that took me a few days to understand what was going on
 

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