I am building a voltage controlled current source with +/-20V rails, a desired max current of .5A that supports 3 luxeon star LEDs (equivalent resistance approx. 24 Ohms). The circuit will eventually include switches that bypass the LEDs aka make them selectable so the load will vary from about 8-24 Ohms plus BJT and sense resistor resistance. I have read through the forums looking at different versions and so far have built a simple voltage controlled current source with one op-amp as shown below. The R(power-dis) resistor is just to dissipate some of the power so the BJT doesn't have to handle it. Since I have no power BJT's I am currently using PMV30UN N-channel MOSFETs(multiple in parallel still barely meeting power requirements) in place of the BJT in the circuit. I was wondering the advantages/disadvantages of MOSFETs vs BJTs for this build? Also a recommended BJT to get to replace the MOSFET would be nice, looking at simulations the max power dissipated is 5.2W in the transistor with only one LED on. I realize this is a lot and this has been one of my biggest problems all along.. handling power requirements. I would love suggestions on a more precise current source and component selections to handle power requirements. Thanks in advance for the help! Anything/everything is appreciated. =)

 

National Semiconductor makes a variety of high-power LED driver ICs as do many other companies. Many will sample them for free.

Perhaps you could work one of these into your requirements.

 

Hello marshallf3,
Thanks for the response. Since this is a project for school though buying an IC wont be accepted. I wish it was .

Im starting to think I am going to have buy a heatsink for the BJT in the circuit..