Hi, am very new to this so i need a lot of help. I wanted to make a 12 LED rig. So my power supply is 48v the blue leds need 3.3v and 20mA. I did the math (i hope correctly) and figured out i needed a 390resistor in order to power 12 leds in series. Now i need help because i only want to pull 1mA to 1.5mA of current from my power supply. I saw that we can achieve this using resistors and transistor but i cant figure out the math. Another thing that i think i could use is a capacitor(s?), however after reading a lot im just more confused than when i began. Can anyone help me figure this project. Its a bit intimidating but i think with some good guidance i can make it work and learn a lot. Thanks!!!!

 

Are you trying to power LEDs from phone line And what you want to do. Can not be done. The 20 mA going through the LEDs must come from the power supply. Hence 20 mA has to be drawn from the power supply.
I have a hunch that the circuit you might think you can use is a joule thief http://en.wikipedia.org/wiki/Joule_thief However in your case it will not work. Unfortunately you will find a lot places on the net that claim to do magic with the joule thief. But very much of this if not all are pure bogus.

 

48V at 20mA is 960mW (the load). 48V at 1.5mA is 72mW (the supply). Where do you propose to get the other missing 888mW from, or did you intend to have the LEDs blink and be lit less than 7.5% of the time?

 

Thank you, i'm trying to use my brothers mixer with phantom power and it says its 48v. Thats y im trying no to use too much current. I read powering the LEDs in series will use about 20mA, i just dont want to use too much current his mics wont work and his mic specs say it uses 1.5 mA to work. This whole phanom power confuses me a bit. Im just truing to light up the mixer good w/o having more cablesaround or using tons of batteries.

 

Im just truing to light up the mixer good w/o having more cablesaround or using tons of batteries.

Believe it or not, an LED acting as an indicator will make a fair amount of light at even 1.5mA.

If you need genuine illumination, for instance so you can read the dials on the mixer, that's going to take quite a lot more power. But I would argue that, say,
even just 5 LEDs directed where you need them, running at 5-10mA, is a LOT of light.

 

http://sound.westhost.com/project96.htm

"The maximum current that can be drawn from a standard phantom circuit (using 6.81k resistors) is 14mA into a short circuit. Since all phantom powered systems need some operating voltage (a typical value being around 10V), the maximum practical current drawn by each powered circuit is around 11mA."

"Output current of this design is rated at 100mA at 48V"

Phantom supplies are still a bit of a mystery to me because I have difficulty finding specifications. Apparently, there aren't any. If you find the definitive answer, I will be watching this thread.

 

So let me see if i get this if i was to use te 12 LEDs in series using a resistor to only pull about 5mA of current i should have no problem wit the phantom power affecting the mixer or the mics. I thought i needed something else like maybe a couple capacitors or a transistor. Thank you im glad i chose to post my ? Here i learn a lot.

 

Your LEDs are not all 3.3V. They have A RANGE of forward voltage so they might be 3.0V or 3.6V. Then with a 390 ohm resistor and a 48V supply (which drops when loaded) the current is from 12mA or less to 31mA or less.

If only 10 or 11 LEDs are in series then the current will not change as much.

An added transistor will not manufacture additional current from the supply.

 

A tutorial on LEDs and more...

LEDs, 555s, Flashers, and Light Chasers

 

Here is a way to try this. Make both the capacitors 1 uf at 63 volts, aluminum electrolytic. Yes, you can use capacitors with a higher voltage rating. You can adjust this from 1ma to 10 ma. You can add LEDs until it doesn't work, then subtract one or two. The weakness with the 317 circuit is that it has a minimum idle current which I tried to account for with a 3.9k resistor.

Another way is to use a jfet as a constant current source. The 2n4416 is in the right current range but it is small and will overheat if you don't use up most of the 48 volts with the LEDs. The advantage is that it requires no current except what goes to the LEDs.

 

Ok. So i think im on to something. I saw that one of my bothers micropones actually light up pretty bright (the fin by heil) and i figured that if i figure out the mic i can figure out my project. I opened up the mic and i noticed that it was a simple dynamic mic mounted on a board that had two 50v capacitors i couldnt really read farads cuz it had glue or something on it. Now it had 4 led lights in it but i couldnt really find any resistiors im thinking they might be hidden somewere else (sucks i cant find the wiring diagram for this mic anywhere).

 

Here is a way to try this. Make both the capacitors 1 uf at 63 volts, aluminum electrolytic. Yes, you can use capacitors with a higher voltage rating. You can adjust this from 1ma to 10 ma. You can add LEDs until it doesn't work, then subtract one or two. The weakness with the 317 circuit is that it has a minimum idle current which I tried to account for with a 3.9k resistor.

Another way is to use a jfet as a constant current source. The 2n4416 is in the right current range but it is small and will overheat if you don't use up most of the 48 volts with the LEDs. The advantage is that it requires no current except what goes to the LEDs.

I really think we are on to someting here. It looks like what i saw when i opened the fin mic. You say we can add as many leds until the leds wont work. I like that but im really afraid of the caps exploding. Sorry if it sounds dumb its just my first time using caps and all the info i have reasearched confuses me a bit.

 

A dynamic mic is a signal generator that uses no power. The 48V phantom power is for a condenser mic that needs external power to operate.
So all the power of the 48V phantom supply can be used to light LEDs when used with a dynamic mic.

A dynamic mic needs at least one series capacitor to isolate it from the phantom voltage. If the polarity of the capacitor is connected backwards then a polarized electrolytic capacitor will be damaged.

 

Ohhh. It still has magic in it (for you).
Well, here's the deal: If you install the capacitors backwards, they will leak, sizzle, smoke, and maybe 'POP' regardless of how many LEDs you have connected. If you install the capacitors correctly, they will sit there like lumps, doing their job for several years, regardless of how many LEDs you have connected.

so...install the capacitors correctly. (read the label)

 

Ok, so the caps are only there to protrct the mic? Which raises more questons like y 2 caps instead if just 1 and also does the phantom power dops across the the whole board or per xlr outputs when u use some phantom power. I dont want to use all the power.thank u

 

The phantom power is designed to power a condenser mic. The available current is fairly low but is enough to light LEDs.
The capacitor or two block the DC phantom voltage from destroying a dynamic mic.

The resistor value in series with the LEDs can be calculated for any amount of current that you want if the current is not more than what is available.

 

It makes sense about the caps protecting the mic. This is just so i undersand caps theory more. Wouldnt caps explode fom the signal sent back to the mixer because they are in a way connected backwards. Just a thought. Im sure it doesnt but y not.

 

Because milliolts of signal isn't enough to reverse the polarity of a 48 volt DC supply.
Millivolts of signal also isn't enough energy to explode a gnat.