Did you check the current when applying 3V. I don’t think those are current limited themselves and need a circuit that sets the current. I suspect you will find that the draw less than 100mA at 3V.

The specs you gave do not list a yellow option.

 

Did you check the current when applying 3V. I don’t think those are current limited themselves and need a circuit that sets the current. I suspect you will find that the draw less than 100mA at 3V.

The specs you gave do not list a yellow option.

Its warm white. Assume worst case 100ma each when designing. If they draw leas then good. I didnt check with amp meter how much they draw you knkw.

but my focus is not the leds. BJT switch that turns them on or off. Also post #36 questions

 

An AMP meter is no good for measuring LED current when the supply voltage is near the LED forward voltage,
because the meter "steals" some of the circuit voltage resulting in less circuit current.
Usually we measure the accurate voltage across the current-limiting resistor then calculate the current.

 

Just a side note here but as was mentioned a 2N3904 is a 200 mA max collector current. Your load is 15 Ohms @ 3.0 volts so 3.0 / 15 = 0.2 Amp (200 mA) so a 2N3904 is a poor choice. You would do much better using a transistor like a common 2N2222 which can easily handle your 200 mA load.

Your estimated current load is 200 mA so you want a base current of about 20 mA. I suggest you give this a read.

Ron

 

An AMP meter is no good for measuring LED current when the supply voltage is near the LED forward voltage,
because the meter "steals" some of the circuit voltage resulting in less circuit current.
Usually we measure the accurate voltage across the current-limiting resistor then calculate the current.

The trouble is, he is not using a limiting resistor. And warm white LEDs are likely 3.3V or so. So applying 3V directly probably works but gives some random lower current.

An Ammeter on the 10A range should have a low enough burden to measure 100 mA.

 

The simple and accurate solution is to measure the actual load voltage AT THE LOAD, so that whatever drop in the current measuring part is prior to the voltage measurement point.

 

In the LTSPICE, I am using BJT transistor NPN as a switch. I put in the below snapshot vbe to be 0.8V but my voltage from R1 load side where the dash is to the ground is NOT 0V. I get 1.442V. When I put V1 to be 1.0V voltage from R1 load side where the dash is to the ground is NOT 0V. I then get 102.5119mV. If the transistor turns on at VBE of 0.7V, the voltage from the R1 load side where the dashed line is to the ground should see 0V, but it doesn't. What am I doing wrong? Why?

View attachment 322689

View attachment 322690

The emitter voltage Ve will be approximately Vb-Vbe, which ideally is V1-.7. Since your emitter is grounded, and you have no base resistor, you can look at it like the emitter here is a short circuit to ground.