Power on LED

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Dr.EM

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Hi. I am thinking of building this circuit for use as a simple dual polarity power supply, fixed at +/-12v.

**broken link removed**

I was wondering how I would add an LED on indicator, and where. Would it just be the LED and its series resistor between the +15v and gnd rails? Or between +15v and -15v, or somewhere before the regulators? This probably sounds silly, but I know that they can upset the power supply if inproperly installed, but its something i'll really need on this.
 
just conect its to the 15+ or 15- and to ground and add an 1.5 KOhm resistor in seris whith the LED to limit the curent.The LED has to be the right way round to work.

This is realy simple to do
 
make sure the voltage rating of your LED is ok for a P.D of 30v, most LEDs are only rated for use with 3-4V. you dont want a burnt out resistor now do you
 
Misterbenn said:
make sure the voltage rating of your LED is ok for a P.D of 30v, most LEDs are only rated for use with 3-4V. you dont want a burnt out resistor now do you
Click to expand...
Most l.e.ds aren't "rated" for use with a voltage (but see below.) They have a maximum forward current and the forward voltage is usually quoted at this current.
The voltage drop across the diode is largely independent of the current through it, changing very little for a large change in current.
They may be used with any value of voltage higher than the Vf with an appropriate current-limiting resistor.

A few l.e.ds come in a panel-mounting holder and incorporate a resistor which limits the current to a safe value when used with a stated supply voltage.
 
The voltage over the LED will regulate to the LEDs forward breakdown voltage. (like a zener diode)The resistor takes the rest of the volts and converts it to heat.
 
Someone Electro said:
The voltage over the LED will regulate to the LEDs forward breakdown voltage. (like a zener diode)The resistor takes the rest of the volts and converts it to heat.
Click to expand...
"Breakdown voltage" is a term which is used when talking about the reverse biased condition of a diode. In the forward direction it is called simply "forward voltage."
 
Ok, cheers people for the help. Something i'd like to do is have an output at +/-9v too, and mabye a 5v output, not to be used simultaneously I should mention. Is it just a case of putting a 2.7v zener in the +12v and -12v lines to drop it to +/-9v, and a 6.5v zener in the +12v line to derive the 5v output? I'll get a schematic if this doesn't make sense.
 

You can with the following 2 issues:
1. A zener voltage changes a bit with temp and current. If you can deal with less than perfect regulation, maybe this is ok.
2. Check your wattage. Even 500mA through the zener dropping 7v out of 12v is 3.5 watts! That's a big zener, and it is not possible to parallel them without sharing resistors which will mess with the voltage regulation. You can put multiple zeners in series, however.

Generally, it would be more advisable to use another regulator.
 
i request u to find one small project that suits diploma students based on microprocessors if possible pls help me
 
ami said:
i request u to find one small project that suits diploma students based on microprocessors if possible pls help me
Click to expand...

Why have you posted this request in an unrelated thread which is already over one year old?
Diploma students? Kindergarten more like!

JimB
 
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