just sanity check transformers

Posted on Feb 7, 2014

Designing a true constant-current AA NiMH charger, and want to select a power supply that will be just right. I want a high enough voltage to that the CC transistor doesn`t saturate, yet low enough voltage to reduce the power dissipation in the transistor. I plan to build a unit that will charge two seperate banks of 2 NiMH cells in series at0. 3A (0. 7A total for both banks), so I will be using two cc transistors in their own circuits,

just sanity check transformers
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one for each 2 cell bank. NiMHs can reach as high as 1. 6v (3. 2v for 2 cells) before they peak, so I figure I want right around 6vdc. Next question: I am a little worried about the higher voltage this will produce. If this transformer provides 6. 3v @ 2A, the smaller load I will be placing on it (~0. 7A total) will cause the output to be higher than 6. 3v. After rectification and at 2A, the output should be 7. 92v (figuring rectifier 0. 7v loss). But since I will only be drawing ~0. 7A total, the output will be closer to 9v DC, if not higher. I know I could regulate the supply and feed the rest of the circuit true 6v, but I thought that splitting the thermal load across the two cc transistors would be preferable (only have to heatsink the two transistors). If I regulate it first, the cc transistor disspation will be lower true, but will still need to be on a heatsink, as well as the regulator. Same thermal load, just split differently. What would you do Incidentally, the circuit I came up with works quite well. It charges at the specified 0. 3A until cell temperature reaches ~100*F (via thermistor). At that point, the circuit cuts the charge and latches "off", so even when the cells cool, the circuit will not start charging again. You reset the latched circuit via a momentary N. C. switch. I`ll provide the schematic once everything is worked out powersupply-wise if anyone cares. I`m using my regulated bench supply right...

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