PWM OPAMP constant current source

[rfm]

Hello,

I have been tinkering with the constant current sources recently and want to drive my 30 W LED using it, I have simulated the circuit below and it seems to give me the correct current.

Basically the current source (1A) is created using the OPAMP circuit, but I also want to pulse the output using the 555 timer, can I please request that somebody look at the attached circuit and advise whether I would have any issues with it before I build it.
I am mostly worried about the pulsing part using the 555 timer where the voltage level is different (1.8V) and I have coupled it to the opamp output (32V) using a 10uF capacitor. Any help and guidance will be much appreciated.

 

[barry]

Not sure what you are trying to do there. If you are trying to turn the current source on and off, I don't think that circuit is the way to go. If you're trying to do something totally different, then you're on the right track.

If you're trying to pulse the current from 0 to 1A, then drive the INPUT of the opamp. In other words, drive the top of the voltage divider on the input. You'll have to verify the output levels of the 555, and the resistors, etc.

 

[rfm]

I was hoping to reduce the power dissipation by pulsing the transistor ON/OFF. Are you suggesting that I should pulse the 36V itself (hence the non-inverting input of the OPAMP)?

 

[crutschow]

If you want to really reduce the power dissipation then you need to go to a switching regulator (that uses an inductor), with a current output. That can give you over 80% efficiency.
Your kludged circuit will only give a partial reduction in power dissipated. ;-)