Superbee_dad
Newbie
Hello, I have a Sharp 80” TV with an LED backlight error. I have a question about expected voltages in the driver circuit and voltages recorded at the transistors on the return to ground circuit.
> There are 15 LED strings with common input voltage of 192 Vdc, 21 to 24 LEDs per string and each string output is connected to a transistor which the driver IC monitors to control current … the way I understand it.
> The transistors are C4217 NPN and I record 36 volts at the collector pin from each light string.
> The voltage recorded at the emitter pin is also 36 volts … except 4. The voltages are 70 volts on 3 of them and 93 volts on one.
> The gate voltages are all 5.5 volts except these 4. The voltages are 1.5 on 3 of them and 18.4 on one.
Questions:
1) What would cause voltages to be higher at the emitter than the collector?
2) What would cause the gate voltage to drop at 3 of these transistors?
3) What would cause the gate voltage to increase to 18.4 volts on one of them?
Note: There are 4 transistors under each heat sink … 4 heat sinks side by side on top of the board as per attached pic.
Thank you in advance for your replies.
pic.
> There are 15 LED strings with common input voltage of 192 Vdc, 21 to 24 LEDs per string and each string output is connected to a transistor which the driver IC monitors to control current … the way I understand it.
> The transistors are C4217 NPN and I record 36 volts at the collector pin from each light string.
> The voltage recorded at the emitter pin is also 36 volts … except 4. The voltages are 70 volts on 3 of them and 93 volts on one.
> The gate voltages are all 5.5 volts except these 4. The voltages are 1.5 on 3 of them and 18.4 on one.
Questions:
1) What would cause voltages to be higher at the emitter than the collector?
2) What would cause the gate voltage to drop at 3 of these transistors?
3) What would cause the gate voltage to increase to 18.4 volts on one of them?
Note: There are 4 transistors under each heat sink … 4 heat sinks side by side on top of the board as per attached pic.
Thank you in advance for your replies.
pic.
