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#### georgetwo

##### Member
I builded a dc power suply, and i want to add an indicator with led which turns on when a load is well connected to the power suply. in other words I need to build a load indicator in my power suply. any help on how to do that ?
I have been seeing this feature in some phone chargers.

#### Pommie

##### Well-Known Member
As you built the supply,

Increase the output voltage by 2V.
Add a resistor that will drop two volts at the load current.
Put an LED across the resistor.

If that don't work, try explaining what you are doing better.

Mike.

#### KMoffett

##### Well-Known Member
As you built the supply,

Increase the output voltage by 2V.
Add a resistor that will drop two volts at the load current.
Put an LED across the resistor.

If that don't work, try explaining what you are doing better.

Mike.

If you replace the the resistor with three diodes (rated at > maximum supply current) in series, and increase the supply by 1.8V, and add a 1K resistor in series with the LED, you don't have to be concerned about what the load current is.

Ken

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#### Pommie

##### Well-Known Member
If you replace the the resistor with three diodes (rated at > maximum supply current) in series, and increase the supply by 1.8V, you don't have to be concerned about what the load current is.

Ken

He wants to know that it is "well" connected and therefore drawing the specified current. Your solution will light if more than 20mA flows which could be a "not well" connection. Plus, resistors are cheaper than diodes.

BTW, why 1.8V, you using germanium diodes? And, with 1.8V your LED won't light will it?

Got it, you're a teacher? They're still using germanium diodes.

Mike.
Above all said with tongue firmly placed between molars.

#### georgetwo

##### Member
let me give an instance, I have a 12v power suply with two wires connected as output (vcc and gnd). my question is: if i power my fm radio with that my 12v power supply output, a green LED shuld turn on. how do i go about that? Thanks

#### Pommie

##### Well-Known Member

What are you trying to achieve? Do you not hear the radio when its on?

Mike.

#### georgetwo

##### Member
I want that LED to turn on even if i connected any other circuit to that power suply. I dont need to alter any other circuit apart from my power suply.

#### KMoffett

##### Well-Known Member
He wants to know that it is "well" connected and therefore drawing the specified current. Your solution will light if more than 20mA flows which could be a "not well" connection. Plus, resistors are cheaper than diodes.

BTW, why 1.8V, you using germanium diodes? And, with 1.8V your LED won't light will it?

Got it, you're a teacher? They're still using germanium diodes.

Mike.
Above all said with tongue firmly placed between molars.

Well I am a teacher among other things...but...three forward-biased "silicone" diodes in series...0.6+0.6+0.6=1.8V...a red LED (Vf=~1.7V) plus with a 100Ω series resistor (OK 1K was too high) across them...works...try it. The diodes give a constant voltage drop when almost any load is connected, so the output stays at the desired voltage. A Green one would require more diodes...but they're cheap too)

Ken

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#### MikeMl

##### Well-Known Member
Post a schematic of your supply. It may be easy after we see what you are doing.

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