Electronics > Beginners
led control transistor
yalect:
Hi,
--- Quote from: yalect on October 03, 2019, 08:07:08 am ---What you're talking about is usually done for FETs. Resistors between base and emitter are there to turn the transistor off quicker, by helping drain the charge from the base once the control voltage is disconnected.
--- End quote ---
thank you
you mean we can ignor that resistor (base and emitter) for large resistor of the base to limite the current.
greeting
mikerj:
--- Quote from: Dave on October 02, 2019, 09:44:44 pm ---
--- Quote from: Zero999 on October 02, 2019, 09:21:59 am ---It's generally good practise to connect the base to a definite voltage. In reality, the circuit will probably work, without a resistor between the base and emitter, but it makes it more vulnerable to electromagnetic interference. If the wire to the switch is long, it can pick-up stray electromagnetic fields, which can induce a high enough voltage to turn the transistor on. Adding a resistor to pull the base down to ground, reduce the risk of the transistor turning on, due to tiny currents.
--- End quote ---
What you're talking about is usually done for FETs. Resistors between base and emitter are there to turn the transistor off quicker, by helping drain the charge from the base once the control voltage is disconnected.
You'd need a very large EM disturbance to get the transistor conducting sporadically. Darlington pairs, on the other hand, are a different beast.
They can be easily be omitted on circuits like this one.
--- End quote ---
Zero's advice is applicable to both FETS and bipolar transistors. On any well designed circuit with a bipolar transistor used for switching there will be a resistor from base to emitter, in fact you can even buy bipolar transistors designed for switching that have both resistors built into the package.
Zero999:
--- Quote from: mikerj on October 03, 2019, 11:55:20 am ---
--- Quote from: Dave on October 02, 2019, 09:44:44 pm ---
--- Quote from: Zero999 on October 02, 2019, 09:21:59 am ---It's generally good practise to connect the base to a definite voltage. In reality, the circuit will probably work, without a resistor between the base and emitter, but it makes it more vulnerable to electromagnetic interference. If the wire to the switch is long, it can pick-up stray electromagnetic fields, which can induce a high enough voltage to turn the transistor on. Adding a resistor to pull the base down to ground, reduce the risk of the transistor turning on, due to tiny currents.
--- End quote ---
What you're talking about is usually done for FETs. Resistors between base and emitter are there to turn the transistor off quicker, by helping drain the charge from the base once the control voltage is disconnected.
You'd need a very large EM disturbance to get the transistor conducting sporadically. Darlington pairs, on the other hand, are a different beast.
They can be easily be omitted on circuits like this one.
--- End quote ---
Zero's advice is applicable to both FETS and bipolar transistors. On any well designed circuit with a bipolar transistor used for switching there will be a resistor from base to emitter, in fact you can even buy bipolar transistors designed for switching that have both resistors built into the package.
--- End quote ---
To expand further, it depends on what the BJT is driving. Some LEDs can light on their own from stray fields. I have some high efficiency green LEDs which light very dimly when I hold one leg and connect the other to an earthed object, such as a radiator. Add a BJT to the circuit and the LED current will be around 200 times as big, causing it to be much brighter.
SteveyG:
What do you want to achieve? Constant current regardless of input voltage, or just current limiting?
I did a video many years ago which may help, (video #018, Linear LED Constant Current Driver Circuits)
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