There are lots of posts out there dealing with power MOSFET switches and 12VDC lights (and/or motors). I'm experiencing some heating issues and I'm pretty stumped so I'll just describe the circuit and hopefully the mistake will be obvious to the more experienced readers:
Table top power supply (from HP laptop) rated at 19.5VDC and 4.6A output is run through an LM338T regulator (TO-220 package with standard alu heatsink) set up as a current regulator to feed ~2.7A to the circuit (I=Vref/Rref=1.25V/0.47ohm). Three 12V/10W halogen lamps are connected in parallel between the LM338 adj pin and the drain of a IRLZ34N Hexfet power MOSFET, also fitted with a standard heatsink.
With the FET gate set to high the lights come fully on and then fade quickly-- which I believe is caused by the LM338 overheating and turning itself off through protection circuitry. It gets very hot, as does the FET (as in, melt the breadboard hot).
What confuses me is that the LM338 is rated at 5A (7A peak) and the FET is rated at some ungodly source current. I am way below the max for both. There is no official power dissipation rating for the 338 that I can find (the datasheet says "internally limited" in fine print). The power dissipation for the FET is 68W at 25C, which I'm sure is much lower at high temp, but the Rds drops to almost nothing at high current so my expectation was that a sub-3A circuit would be no problemo... What am I missing here?
http://www.national.com/ds/LM/LM138.pdf
http://www.irf.com/product-info/datasheets/data/irlz34n.pdf
Table top power supply (from HP laptop) rated at 19.5VDC and 4.6A output is run through an LM338T regulator (TO-220 package with standard alu heatsink) set up as a current regulator to feed ~2.7A to the circuit (I=Vref/Rref=1.25V/0.47ohm). Three 12V/10W halogen lamps are connected in parallel between the LM338 adj pin and the drain of a IRLZ34N Hexfet power MOSFET, also fitted with a standard heatsink.
With the FET gate set to high the lights come fully on and then fade quickly-- which I believe is caused by the LM338 overheating and turning itself off through protection circuitry. It gets very hot, as does the FET (as in, melt the breadboard hot).
What confuses me is that the LM338 is rated at 5A (7A peak) and the FET is rated at some ungodly source current. I am way below the max for both. There is no official power dissipation rating for the 338 that I can find (the datasheet says "internally limited" in fine print). The power dissipation for the FET is 68W at 25C, which I'm sure is much lower at high temp, but the Rds drops to almost nothing at high current so my expectation was that a sub-3A circuit would be no problemo... What am I missing here?
http://www.national.com/ds/LM/LM138.pdf
http://www.irf.com/product-info/datasheets/data/irlz34n.pdf