sdmuashr
Junior Member level 1
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?
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?
