Can LEDs be used as fuses?

[muashr]

Hi,

I have a question regarding the sub section of a power supply.

Background: I have two similar power supplies one is functioning whereas the other one is not functioning. The functioning one can output variable dc voltage from +(0 - 20)V. The nonfunctioning power supply can only deliver -4V at the output.

The sub circuit about which I have a question is at the output. Both supplies have similar sub circuit. This sub circuit consists of a transistor the base/gate of that transistor is connected in series with a red LED, this red LED is connected to a 1.5k SMD resistor and other end of this 1.5k is connected to a source point of +27V.

Both LEDs were tested in their corresponding sub circuits when the power was off and they appeared to be ok (Vf = +1.7V). Now the problem is that when the power is on the base/gate of the malfunctioning transistor gets only +0.1V whereas the base/gate of the functioning transistor gets +25V. Also the red LED of the malfunctioning power supply is on whereas that of the functioning remains off.

The voltage measured (with respect to ground) on the path/trace which joins LED and 1.5k SMD resistor is as follows.

Functioning Supply: +27v
Malfunctioning Supply: +2.2V

On measuring the voltage drop across these red LEDs following results are obtained.

Functioning Supply: -4V (red lead @ anode & black @ cathode) & +2.3V (black lead @ anode & red lead @ cathode)
Malfunctioning Supply: +2.1V (red lead @ anode & black @ cathode) & -2.1V (black lead @ anode & red lead @ cathode)

My question is whether or not LEDs are also used as some kind of fuses? If not, then what is your opinion about this situation?

 

[JimB]

Instead of trying to describe the circuit using words, just post the schematic.

No, LEDS are not (or should not) be used as fuses.

JimB

 

[tvtech]

Crazy logic...whoever designed this/these PSU's in mind surely would not be that stupid???

I have never heard of an LED being used as a fuse

Suppose it is possible...but why???. I think you have other issues....somehow whatever is driving the working PSU LEDS...has failed...and the faulty one has survived.....somehow.

A little bit of a mind " xxxxxxx" for me

Strange.

Regards,
tvtech

 

[Joe G]

just for future reference, I zapped a string of 3 red leds, before I knew what I did, I was wondering why they didn't glow and the amp reading when through the roof.....they became a dead short...... So I would not use leds as a fuse or other safety disconnect.

 

[4pyros]

LEDS can fail open or closed. They are there for just indicators.

 

[KeepItSimpleStupid]

But, they are wired with a series resistor across fuses sometimes to act as a "blown fuse indicator". Neon lamps were too.

 

[gmphadte]

But then, your unit will not work with these indicators.

Gajanan Phadte

 

[4pyros]

The LED on the unit that is not working is not on.