Picked up a very low power (100w) inverter at Wal-Mart for $18 and did some testing. It produces a nice high voltage from 12v, but at the current I will be drawing, it's only about 40% efficient. As I'd like to run this from battery sometimes, I'd like to get something a little more efficient.
I played around a bit more with a small transformer I had laying around. The secondary actually has two outputs with a shared center tap. I built the circuit below. I switched 12v on half of the lower voltage winding, but took the output from the other half of the higher voltage winding (if that makes any sense). Essentially, I was using one coil with a tap on the lower 1/3rd of the winding.
**broken link removed**
After playing around with various frequencies and duty cycles, the max efficiency I am able to achieve is a little over 72% using a 4.8kHz switching frequency (surprised me since the xformer was for 60Hz). Once the output was rectified, I was able to get a solid 27v at 200mA. The 12v supply was drawing 620mA. I'm pretty happy with the results, but I'd like to get the efficiency a little higher, so I was thinking about using a better transformer. Would this really help? At the very least, it would be smaller and lighter since it wouldn't have the 120v primary winding.
Since I don't have all that much voltage to work with, I decided against using the 317T as a current regulator since it will eat up almost 3v. So, I made the circuit below. I'm using one 317T as a 1.25v regulator to provide a solid base voltage. This then feeds several 2N3904 transistors as a current source. At the emitter, I'm getting a little over 0.5v. And if the transistor saturates, my drop-out voltage is only ~0.7v. Much better! I figured on running each LED chain at about 18mA. Higher current really doesn't emit a noticeably brighter light.
**broken link removed**
So, before I go all out and finish all this, are there any suggestions to improve the design? This is going to be assembled on a perfboard so I don't want to hit much higher frequencies and I'd like to use easily obtainable parts.
Thanks in advance!
I played around a bit more with a small transformer I had laying around. The secondary actually has two outputs with a shared center tap. I built the circuit below. I switched 12v on half of the lower voltage winding, but took the output from the other half of the higher voltage winding (if that makes any sense). Essentially, I was using one coil with a tap on the lower 1/3rd of the winding.
**broken link removed**
After playing around with various frequencies and duty cycles, the max efficiency I am able to achieve is a little over 72% using a 4.8kHz switching frequency (surprised me since the xformer was for 60Hz). Once the output was rectified, I was able to get a solid 27v at 200mA. The 12v supply was drawing 620mA. I'm pretty happy with the results, but I'd like to get the efficiency a little higher, so I was thinking about using a better transformer. Would this really help? At the very least, it would be smaller and lighter since it wouldn't have the 120v primary winding.
Since I don't have all that much voltage to work with, I decided against using the 317T as a current regulator since it will eat up almost 3v. So, I made the circuit below. I'm using one 317T as a 1.25v regulator to provide a solid base voltage. This then feeds several 2N3904 transistors as a current source. At the emitter, I'm getting a little over 0.5v. And if the transistor saturates, my drop-out voltage is only ~0.7v. Much better! I figured on running each LED chain at about 18mA. Higher current really doesn't emit a noticeably brighter light.
**broken link removed**
So, before I go all out and finish all this, are there any suggestions to improve the design? This is going to be assembled on a perfboard so I don't want to hit much higher frequencies and I'd like to use easily obtainable parts.
Thanks in advance!