shut down led on short-circuit

C

carmusic

New Member

• Jan 6, 2011

• #1

Hi i have the very simple circuit with led current source. The led goes on when load draw about 5 to 15 ma (current source is limited to 24V). i must add a function that the led must go off if there is a short-circuit on output (or below 1V approx)
I need a circuit with no ic and less part possible (it is going for commercial so cost is very important)

Thanx for your suggestions

 

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Grossel

Well-Known Member

• Jan 6, 2011

• #2

Here a quick solution. Adjustments is needed if you don't know the led and fet's characteristics in advance.

 

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Last edited: Jan 6, 2011

C

carmusic

New Member

• Jan 6, 2011

• #3

hi i tried your circuit in simulation but was not able to make it work. I design another one, it works ok in simulation but do you think it is good since transistor may have different hfe, if i lower base resistance, it affects current loop.

 

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Grossel

Well-Known Member

• Jan 6, 2011

• #4

Try replacing R2 and R3 with a pot and see if you get it to work with different settings.

I just took a look at your circuit. Your circuit as it is now will cause the transistor to conduct and the led will not light unless you get the voltage divider to make less voltage out.

 

ronv

Well-Known Member

Most Helpful Member

• Jan 6, 2011

• #5

Short Detector

An extra transistor, but better margins I think.

 

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Roff

Well-Known Member

• Jan 6, 2011

• #6

ronv said:

An extra transistor, but better margins I think.

Click to expand...

You may run into base-emitter breakdown on Q1 during normal operation. You could add a diode in series with the emitter to prevent this.

 

C

carmusic

New Member

• Jan 7, 2011

• #7

thanx for schematics, ill test them in real to see which is better (simulations are not always right!)

 

ronv

Well-Known Member

Most Helpful Member

• Jan 7, 2011

• #8

Roff is right. In normal operation Vbeo is to high. Please add the diode as he stated. Real world tests don't always tell the whole story either, never know about the next transistor.