zachtheterrible said:
I cannot find a transformer that you recommend on mouser. The closest I can find is 28VC.T , 3.6A. Those ratings are given in RMS. It is 100VA. By the way, what does VA mean? Would that suffice?
With only 28VAC, if your transistors are weak, you might have a max DC output voltage of 28V at full load. Trying for a higher output voltage would result in high ripple (hum).
VA is volt-amps and is the fully loaded rating of the transformer. 28VAC has a peak voltage of 39.6V then the rectifiers lose about 2V and heat-up due to their current. So the transformer can produce 100VA divided by the peak of 39.6V = 2.53ADC from the supply.
I have an old microwave oven transformer. Perhaps I could wind my own? I would imagine it to be pretty easy because I would be using large gauge wire with not very many turns.
Other people have tried microwave oven transformers. Because they don't have enough turns, their inductance is low so they use a lot of current without much load. It doesn't matter for a 900W oven but it could heat itself with 200W in addition to the power of the supply project.
The project's 2N3055 transistors heat with about 60W each with the supply providing 3A at a very low voltage or shorted. The 2N3055 has a max chip temp of 200 degrees C so the max thermal resistance allowed is calculated like this:
1) Assume a balmy ambient temp of 30 degrees C.
2) 200 - 30 = 170 degrees C for the thermal resistance of the transistor's chip to its case, an insulator and the heatsink.
3) The thermal resistance of a 2N3055 transistor is 1.5 degrees C/W. The thermal resistance of an insulator is 0.7 degrees C. so the total thermal resistance without the heatsink is 2.2 degrees/W.
4) 60W x 2.2 degrees = 132 degrees + 30 degrees ambient = 162 degrees. 200 degrees (transistor's max) - 162 = 38 degrees rise for the heatsink to dissipate 60W. Two transistors dissipate 120W.
5) 38 degrees/120W = a thermal resistance rating for the heatsink of 0.32 degrees C/W. Impossible without a gale-force fan.
6) Try the above calculation without an insulator and insulate the heatsink from the chassis instead. Then the thermal resistance rating of the heatsink will be 1.02 degrees C/W which is available, but it would still be a pretty big heatsink. Check the ratings of smaller heatsinks with a fan.
I would use a heatsink with a thermal resistance rating of 0.8 degrees C or less if I could afford it and had space for it. Then the output transistors would operate cooler than their max rating. Or I would use a smaller heatsink and an automatic fan.
You can't find the transformer nor TI's opamps. Shop at Digikey or Newarkinone since they have everything. :lol: