# 12VDC with LED switching problem.

Discussion in 'The Projects Forum' started by dodbdts, Jul 19, 2007.

1. ### dodbdts Thread Starter New Member

Jul 19, 2007
1
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Hey all, I'm a "beginner" when it comes to electrical designs and would like a little bit of help if someone could lend a hand.

Current Project:
Modifying a computer power supply into a portable test PSU for electrical and computer equipment (using attached molex connectors and added RCA or 'phono' exterior power connectors).

Problem:
Now in a nutshell what i would like to do is wire up the female RCA jacks to emit 12VDC (which is easy considering the PSU already puts out 12VDC 5VDC and 3.3VDC). However, as a nice effect, I would like to have an LED connected to each jack...so that when you plug in your device that needs power, the LED will power on to clue you in to the fact that there is power running through the circuit.

Now....i'm running LEDs that require a min. of 3VDC and a max of 4.5VDC so I require a resistor for each Circuit that the LED is on...which cuts down the power running to the jack....crippleing whatever is on that circuit (besides the LED, that is)

Is there anyway (besides using a relay) that The LED can be hooked up to the jack in the manner that i am looking for?

2. ### John Luciani Active Member

Apr 3, 2007
477
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Adding phono connectors is not a good idea. Phono connectors were designed for
small signals and are customarily used for such. Use banana jacks or some other
power type connector that won't be confused for small signals.

To get the LED to light you need to sense the current out of the supply and use
a circuit to light the LED. Google for current sense circuitry for some ideas.

(* jcl *)

3. ### mrmeval Distinguished Member

Jun 30, 2006
833
2
You'd need some circuitry to do that. There are a few ways to do it. One would be to have a resistor in series with the voltage whose current is to be measured. Across that you'd want to have a current sense circuit that would have a settable trip point. When it's tripped by higher current than an unconnected state it would light an LED. The LED could be driven off any of the supplies mentioned.

A comparator can be used to do that. The output of it would be buffered and used to drive the LED.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/746/

I'm sorry that's as far as I can go. I'm beginning in this myself.