-
Categories
-
Platforms
-
Content
Ohm's Law plus third-grade arithmetic. In order:
What is the circuit voltage?
What is the voltage drop across an LED (Vf)?
What is the voltage across the resistor?
What is the current through the resistor?
What is the total current through all resistors?
What is the collector current rating of the transistor?
Therefore . . .
ak
What is the circuit voltage?
What is the voltage drop across an LED (Vf)?
What is the voltage across the resistor?
What is the current through the resistor?
What is the total current through all resistors?
What is the collector current rating of the transistor?
Therefore . . .
ak
Liked this ...
But, out of interest, what makes you think 180 Ohms is the correct value? (Probably right, just interested in how it came about)
If this was to set some current, then maybe you already know the current per LED (and resistor)?
.
The analysis above still applies, but that current might also act as a check.
But, out of interest, what makes you think 180 Ohms is the correct value? (Probably right, just interested in how it came about)
If this was to set some current, then maybe you already know the current per LED (and resistor)?
.
The analysis above still applies, but that current might also act as a check.
The OP set the resistor value, not me. We have zero information about the LED or the desired current level, so there is no way to evaluate the appropriateness of the resistor value.
Current levels in the LEDs, the resistors, and the transistor are three different things. The question was about current in the transistor, and that is what I answered.
Note: For improved long-term reliability, do not operate the transistor at more than 50% of its rated continuous collector current spec. (I wanted to add this to my initial response, but this forum has a too-short time limit on editing.)
ak
Current levels in the LEDs, the resistors, and the transistor are three different things. The question was about current in the transistor, and that is what I answered.
Note: For improved long-term reliability, do not operate the transistor at more than 50% of its rated continuous collector current spec. (I wanted to add this to my initial response, but this forum has a too-short time limit on editing.)
ak
Last edited:
No criticism intended. I'll leave it in your capable hands.
If that 2N2222 is being used as a switch, you can safely pass 800mA through its collector.
Not really. I disagree with "safely". 800 mA pushes the device to its absolute limit, and requires that the package be maintained at a relatively low temperature. Yes, it will work - for a short while.
1. 800 mA is its "Absolute Maximum" collector current rating. No device should *ever* be operated continuously under any of its listed "Absolute Maximum" conditions.
2. The condition requires that the device be maintained at a *case* temperature of +25C. This required a heatsink, because ...
3. At 500 mA, the max rated saturation voltage is 1 V, for a device power dissipation of 0.5 W. That is a *lot* of heat for such a small package. The case temperature will literally melt your fingerprint.
The highest collector current mentioned in the parameters tables is 0.5 A. For long-term reliability in uncontrolled conditions, best to operate this part at 300 mA or less.
ak
