- Joined Oct 27, 2015
Here is another way to make a pulsed constant-current LED driver.
I'm assuming that the pulser is the 555. Do not operate the 555 on more than 15V; that is getting close to its rated VccMax. It's output pin will swing from~1V to ~14V, see red trace.
Q1, M1, R1 and R3 constitute a constant-current LED driver. Current through the LEDs is Vbe(Q1)/R1 = 0.67/3.3, or about 0.2A, see yellow trace I(D4), which is in units of mA.
The addition of Q2 gates the CC driver on/off. See V(g), orange trace.
The power dissipation in M1 is a modest ~0.5W, which doesn't require much heatsinking... See green trace, which is in units of Watts.
View attachment 93606
This simulation shows what you can expect for an upper limit of pulsing frequency using this circuit. Note that the turn-on time is dominated by how long it takes to charge the gate capacitance of M1 through R3. Turn-off is much faster. Good enough to pulse at ~200KHz....
Thank you for this circuit, however i really would like to have my initial circuit working and understand whats wrong with it.